BACKGROUND
cathepsin b   is one of the most well-characterized cysteine proteases, belonging to the clan ca .
in humans, its physiological role is implicated in bone resorption, antigen processing and protein turnover  <cit> .
however, catb also participates in pathological processes such as cardiovascular disturbances  <cit> , parasitic infections  <cit> , alzheimer's disease  <cit> , osteoarthritis  <cit> , tumor invasion and metastasis development  <cit> .
its main roles in cancer are i) its activity in directly cleaving extracellular matrix  components, ii) its activation of other ecm degrading proteases, which promotes tumor cell invasion into the surrounding tissue and bloodstream escape  <cit> , and iii) stimulating angiogenesis which provides increased nutrients and oxygen supplies to cancer cells  <cit> .
thus, catb regulates several crucial steps in tumorigenesis and is a promising target for anti-cancer drug design  <cit> .
structurally, catb possesses the regular fold of papain-like enzymes, enclosing two distinct domains stabilized by six disulfide bridges, forming a large polar interface into which project the side chains of a few charged residues such as e <dig> and e <dig> .
this interdomain interface is extremely important to catb overall activity as it comprises the active site residues .
unlike other members of the papain family, catb exhibits both exo- and endo-proteolytic activities.
its exo-activity is dependent on the presence of two adjacent histidine residues  located at an insertion region called the “occluding loop”.
these residues provide the necessary positive charge to anchor the negatively-charged c-terminal carboxylate of exo-substrates  <cit> .
this region is only found in catb within its family, and it controls the access of large substrates to the active site  <cit> .
site-directed mutagenesis studies confirmed the role of the occluding loop since deletion of this entire region impairs exo- but not endo-proteolytic activity  <cit> .
additionally, this region confers thermal stability to catb and resistance against endogenous inhibitors such as cystatin c  <cit> .
currently the most potent and selective structure-based designed compounds available are derived from e- <dig> targeting the unusual occluding loop present in the catb 3d-structure  <cit> .
however, enzymatic assays have shown that these inhibitors are strongly ph dependent as their optimal binding affinities are considerably diminished under neutral/alkaline conditions  <cit> .
since catb can be found in several cellular compartments with distinct ph values, these inhibitors are not effective in vivo.
when catb is within the lysosomal or endosomal vesicles it confronts acidic conditions, in contrast to the neutral/alkaline environment when it is attached to membranes   <cit>  or secreted in the ecm  <cit> .
although catb is rapidly inactivated under alkaline ph conditions, it was reported that membrane-associated forms are resistant to this process  <cit> .
this peculiarity is believed to occur due to its interaction with heparan sulfate glycosaminoglycans  on the cell surface  <cit> .
this polysaccharide, structurally related to heparin, acts on the ecm as a co-receptor for several molecules such as growth factors and proteases  <cit>  .
interaction between catb and heparin-like gags was already shown to prevent the enzyme's inactivation in high ph  <cit> .
the main reported outcome was the maintenance of catb helical content in the presence of heparin at high ph, which was observed for papain as well  <cit> .
nevertheless, it has not been possible to precisely define the catb/heparin interaction sites and the molecular mechanism responsible for this protective effect.
structural and molecular modelling studies can give insight into the molecular events concerning the modification of catb activity by ph changes and heparin binding.
some attempts have already been made towards designing new catb inhibitors using molecular modeling techniques  <cit> , taking into account dynamical aspects of binding.
however, these studies did not evaluate the influence of ph nor that of heparin binding on the modulation of the enzyme intrinsic flexibility.
in order to understand the heparin protective effect over catb structure we performed molecular dynamics  simulations using an approach in which different ph conditions were taken into account by considering different protonation states of titratable groups on the protein surface.
further, docking analyses resulted in the identification of two heparin binding sites on catb structure.
the md calculations confirmed an increase of the overall catb flexibility and the loss of stability of the apo catb inter-domain interface in alkaline ph.
the destabilization and the increased flexibility, notably in the occluding loop, were prevented by interaction with heparin, again in agreement with experimental data.
we observed a role of active site residues in enzyme stabilization and in maintaining the helical content, and we propose an allosteric mechanism for the stabilizing effect promoted by gag interaction.
taken together, our data provides an improved understanding of the molecular mechanisms responsible for both ph-induced inactivation and protection against inactivation by heparin binding.
RESULTS
ph changes result in distinct protonation profiles
prediction of pka's for protein ionizable residues is an important tool for understanding features and catalytic mechanisms of ph-dependent enzymes  <cit> .
we applied the propka program to estimate pka values in the catb structure and to determine the most probable corresponding protonation states of the enzyme under acidic and alkaline conditions.
two different conditions were studied: acidic , and alkaline , which allow comparison to available experimental data  <cit> .
it is known that catalytic residues often present unusual pka values compared to those of free amino acids in solution  <cit> .
accordingly, our results for catb predicted a pka of  <dig>  for the catalytic c <dig>  in contrast to  <dig>  expected in solution.
previous work on the papain catalytic cysteine  showed pka values around  <dig>  to  <dig>   <cit> , in accordance to our estimation for c <dig>
comparing the acidic and alkaline conditions we observed that six key residues are differentially protonated.
table  <dig> gives the pka values for these residues and fig.
<dig> shows their positions in the catb structure.
we note that four of these residues belong to the interdomain interface .
the predicted pka for e <dig> is  <dig> , as carboxyl groups usually exhibit high pka values in buried hydrophobic environments  <cit> .
from our prediction we observed that both e <dig> and e <dig> are likely to be protonated at ph  <dig>  but not in alkaline conditions.
pka values predicted by propka and the state of ionizable groups under different ph conditions are indicated.
the protonation state of c <dig> at ph  <dig> were obtained taking into account deprotonation of h <dig> as previously proposed for papain.
the protonation states of the catalytic residues  are highlighted.
as expected, most of the residues with differing protonation states were histidines, with a pka in solution of  <dig>  and an ionization state that is very susceptible to ph changes in the physiological range.
h <dig> is a key residue for stabilization of the occluding loop and is also crucial for the exo-proteolytic activity of catb, since along with h <dig> it anchors the carboxyl terminus of substrates.
interestingly, we observed an pka of  <dig>  for the catalytic h <dig>  which implies that this residue is protonated only in acidic conditions.
this result is correspondent to the measured pka of h <dig> in papain  <cit> .
in papain, it was previously reported that the ionization of the catalytic c <dig> and h <dig> residues are coupled  <cit> .
there the deprotonation of h <dig> shifts the pka of c <dig> from ~ <dig>  to  <dig> , while the neutralization of c <dig> decreases the pka of h <dig> from ~ <dig>  to  <dig> .
due to the direct relationship between catb and papain we assigned the protonation states of these residues according to this proposed mechanism.
the pka values predicted with propka thus appear to be sensitive to local microenvironment changes around the ionizable residues.
in order to verify if the applied protonation profiles accurately represent the distinct ph conditions simulated, we collected  <dig> snapshots  from each molecular dynamics  trajectory in order to perform pka calculations.
table  <dig> shows the average pka values during the md simulations.
from both ph conditions we obtained values consistent with the protonation states predicted from the crystal structure, thus confirming their validity.
in this table we have also highlighted the pka shift in the catalytic residues c <dig> and h <dig> as observed for papain.
electrostatic potential calculation and docking analyses reveal two heparin binding sites on catb structure
the electrostatic potential at the catb surface revealed the influence of the different protonation states for each of the ph conditions considered.
overall, independent of the conditions, the catb surface is mostly negative, presenting one region of positive potential in each domain .
under alkaline conditions the overall surface is qualitatively more negative.
the positive sites are composed of r <dig>  k <dig>  k <dig>  k <dig> and k <dig> in the l-domain, and the catb n-terminus together with k <dig> and r <dig> in the r-domain.
the protein's total charge changes from - <dig> at ph  <dig> to - <dig> at ph  <dig> , with six key residues protonated exclusively under acidic conditions.
experimental studies have provided knowledge about the affinity and kinetics of the catb-heparin interaction  <cit> .
however, the precise binding site has not been defined.
taking into account that heparin-protein interactions are mainly driven by charge interactions due to the high number of sulfate groups found in the polysaccharide, we visually identified two regions of positive potential at the catb surface as potential heparin binding sites.
blind docking calculations confirmed this prediction, as we found two low energy clusters of docking poses coincident with these positive regions .
almeida and coworkers proposed that heparin mainly interacts with the occluding loop  <cit> .
from a structural view this hypothesis appears unlikely due to the lack of charge and shape complementarity in this region of the enzyme, and is consistent with the absence of low-energy clusters for such a mode of binding in the docking results.
moreover, we carried out another docking calculation verifying the atomic interactions in the heparin binding sites more precisely .
in the lowest energy complex obtained, heparin interacts with catb in the r-domain with an energy of - <dig> kcal/mol .
although the positive potential at this site is smaller than that found in the l-domain, in r-domain site the van der waals interactions were more favorable .
we suggest that for a disaccharide the r-domain site is more important for this interaction; for longer sugar chains the l-domain could be also important for proper heparin accommodation.
catb protonation profile influences protein stability
as flexibility plays a key role in protein biological functions  <cit> , we analyzed the root mean square fluctuation  of catb backbone residues during md for each condition simulated.
a direct influence of protonation state on the overall enzyme flexibility is clearly seen.
figure 4a shows that at ph  <dig>  the fluctuations were around 2Å, in contrast to rmsf values up to 5Å observed at ph  <dig> .
this behavior was reproducible: we performed replicate simulations  for each condition using different initial conditions , with equivalent results .
in addition, we observed that functional regions such as the occluding loop and the active site presented the highest fluctuations at ph  <dig> , revealing that alkaline conditions rapidly affected catb flexibility in regions directly involved with activity, independent of the starting structure .
remarkably, this result shows that changes in protonation of a few residues can alter both local and non-local properties .
local perturbations due to changes in protonation states have already been observed via md simulations .
however, in our simulations both local and global flexibility  of catb were modified as a function of ph, corroborating experimental findings where catb loses its activity in alkaline ph in parallel with large structural changes  <cit> .
heparin binding prevents catb destabilization in alkaline ph
in md simulations of the bound enzyme at ph  <dig> , the rms fluctuation profile was seen to be similar to that of the apo form .
hence, catb is stable under acidic conditions independent of heparin binding.
in contrast, under alkaline conditions heparin-bound catb exhibited much smaller backbone fluctuations  than the very flexible apo form in these conditions.
moreover, we observed similar results when simulations were started using different initial conformations and also with heparin docked at the other binding site  .
functional regions including the occluding loop and the active site were also stabilized, showing that heparin binding at the predicted sites promotes a global stabilizing effect.
in fig.
4c, we show the time-evolution of the number of distinct clusters of catb backbone conformations throughout the different md simulations.
here each cluster contains conformations within an rmsd of  <dig> Å of the cluster center.
if during a simulation the system visits numerous clusters, the rmsd from the starting structure will be very high, indicating conformational instability in the simulation.
in contrast, if a simulation visits only a few  clusters, it can be considered as being more conformationally stable.
indeed, we found few clusters during the simulations of apo catb and the catb-heparin complex  under protonation conditions corresponding to ph  <dig>  .
furthermore, in acidic conditions the backbone rmsd distributions, similar to the rms fluctuation profiles, did not change significantly on binding heparin, presenting deviations in both cases centered at  <dig>  Å. however, in the heparin-bound simulation in acidic conditions, the rmsd distribution began to shift towards a conformational state far from the native structure.
.
considering that catb loses stability and activity in alkaline conditions  <cit> , in the systems simulated under protonation conditions corresponding to high ph we expected to observe a higher number of backbone clusters.
this expectation was confirmed: we observed  <dig> clusters in the apo catb simulation .
in addition, under these conditions the catb structure does not stabilize during the simulation, with a continuous increase of backbone rmsd values throughout the  <dig> ns production period.
a similar profile was observed in the replica simulation and also when starting with another conformation , in which we found  <dig> clusters.
the rms fluctuations of both replica simulations, plus the simulation performed with the alternative catb structure , showed similar results to the apo enzyme in alkaline conditions .
the rmsd distribution plot shows a clear shift dividing into two populations: one around  <dig> Å and the other at higher rmsd values.
.
on the other hand, in the simulation with heparin under alkaline conditions we observed fewer clusters comparable to acidic conditions .
this stabilization afforded by heparin binding is also reflected in the rmsd distribution presenting a narrowed profile centered at small rmsd values  .
heparin prevents the domain separation observed in alkaline ph
it was previously postulated that the loss of inter-domain interface contacts should be a crucial event in catb inactivation in alkaline ph  <cit> .
furthermore, the interface between l and r domains is essential not only for tertiary structure stabilization but also for the enzyme activity, as the catalytic triad is formed with residues from both domains  <cit> .
to address whether this phenomenon is modulated by heparin, we compared the evolution of the distance between the centers of mass of the two domains during the simulations.
in acidic conditions, this distance was stable at approximately  <dig> Å throughout the entire simulations, and was not significantly affected by heparin binding .
in contrast, when the enzyme was simulated under protonation conditions corresponding to alkaline medium, we observed a remarkable domain separation, with domain center-of-mass distances of up to  <dig> Å .
further, we observed that this separation increased progressively throughout the simulation, suggesting that total l and r domain separation could occur at longer time-scales.
this certainly would affect catb catalysis, since the active site is found inserted into a v-shaped cleft between the l and r domains.
a more detailed analysis provided by use of the dyndom software  <cit>  revealed the separation of the l-domain as the principal overall motion of the enzyme in this condition, thus reinforcing this conclusion .
this domain motion was observed in all simulations except those containing heparin, in which it was not possible to identify large domain motions with the dyndom program.
remarkably, in these last systems, the interdomain distance remained at approximately  <dig> Å in alkaline conditions, comparable to acidic conditions.
this shows that heparin binding governs the inter-domain stability even under unfavorable alkaline conditions and would presumably also allow the maintenance of protein activity.
essential dynamics analysis reveals an allosteric role for heparin
the results shown above suggest that catb as a whole and in particular the critical domain interface and occluding loop are stabilized by heparin binding to a relatively distant site in the r domain.
from basic thermodynamic principles, preferential binding of ligand to the native state will generally confer stability to a macromolecule in what has been described as an allosteric mechanism, without necessity for direct interaction between the ligand binding site and the active site residues, and has been extensively described in phenomenological terms  <cit> .
the structural information available for catb, coupled with macromolecular simulations, provides a means for investigating details of such a mechanism directly.
we applied an essential dynamics analysis to examine the most relevant global macromolecular motions occurring during the simulations.
such analyses have been largely applied in the understanding of biological functions in proteins since they provide an evaluation of large-scale movements that are often related to domain motions and conformational changes  <cit> .
we thus diagonalized the atomic positional covariance matrix to obtain the eigenvectors and corresponding eigenvalues.
selecting the first five  principal components, we could recover around 60% of the total motions for apo catb and around 56% for heparin-bound catb.
our analysis focused on the systems simulated under alkaline conditions in order to address how heparin influences catb flexibility and prevents domain separation as shown above.
we point out that certain analyses involving averaged quantities such as the essential dynamics and clustering approaches can be best interpreted for stable systems; the precise results for the apo catb under alkaline conditions will to some extent depend on the starting conformation.
however, even under these conditions the overall results obtained were similar for replicated runs.
we examined the rms fluctuations of each trajectory projected onto its five most representative principal components .
when comparing apo and complexed catb, we observed considerably lower fluctuations in the latter system for all pc analyzed .
further, we identified that the region around c <dig> and the occluding loop presented dramatically higher fluctuations in apo catb , while in the complex these regions were shown to be very stable .
the movements corresponding to the first two components were seen to comprise relevant motions of both the occluding loop and active site.
figure 6b shows the directions of individual residue movements and also their amplitudes, which are proportional to the lengths of the arrows in this representation.
by inspecting other motions corresponding to the first component  in the r-domain complexes, we identified local changes in the binding region.
the residues close to the polysaccharide  adopted a new conformation.
to verify that these motions would not lead to loss of binding interaction between catb and heparin, we measured the distance between the center of mass of heparin and the positive site during md  and found stable behavior: heparin remained bound during the entire simulation.
remarkably, however, this local motion is coupled to movements far from the heparin binding site.
we observed that the residues  <dig> to  <dig> in the l-domain moved towards the interdomain interface .
this motion certainly contributes to the stabilization of the overall structure, thus helping explain the results described above .
we did not find equivalent behavior in the l-domain complexes, which led us to investigate more deeply the correlated motions of catb in the apo form and in complex with heparin.
the analysis of cross correlation coefficients between pairs of residues is a useful method to identify correlated collective motions  <cit> .
the correlation matrix represents the linear correlation between pairs of c-alpha atoms as they move about their average positions during dynamics.
positive correlations are related to motions occurring in the same direction whereas negative correlations indicate motion in opposite directions.
we compared the correlation matrix of apo catb and of the complexes.
in the apo form we observed positive intra-domain correlations but negative inter-domain correlations .
this pattern is not observed in the complexes, in which we identified a more diffuse pattern of correlations regardless of the binding site occupied.
this result shows that heparin acts mainly as a stabilizing element, and its binding seems to restrict the anti-correlated collective motions that would be responsible for domain separation.
concerning the occluding loop, we observed that the opening-closing movement is clearly represented by the two first components in apo catb.
on the other hand, upon heparin binding these motions were not observed, independent of the binding site occupied.
in addition, the first pc reveals the increasing distance between c <dig> and the other catalytic residues in apo catb, while in the complex this motion is absent.
therefore, heparin stabilizes catb motions in its functional regions.
these results are consistent with experimental findings, which, although they revealed the implications of binding on the activity of the enzyme, did not provide a structural view of the phenomenon  <cit> .
heparin binding, despite the distance separating it from active site residues, prevents their disorganization as would otherwise be produced under alkaline conditions.
the principal components obtained from the essential dynamics analysis of our md simulations suggest that global concerted movements in the macromolecule leading to domain separation are suppressed by heparin binding, thus helping explain an allosteric role for heparin in this biological system.
heparin prevents loss of helical content and stabilizes the occluding loop
loss of helical content was reported as the main effect related to alkaline-ph-induced inactivation of catb  <cit> .
in the same study, it was reported that heparin prevented the loss of helical structure, which led to the postulate that the maintenance of activity in the catb-heparin complexes is strictly related to this structural aspect.
in analyzing the time evolution of secondary structural elements throughout the simulations, we observed stability of secondary structure elements in acidic conditions, independent of heparin binding .
in contrast, this same analysis for alkaline conditions revealed differences in the region of the main alpha helix , which is located in the inter-domain interface.
figure 7a shows the phi-psi distribution plot for the first residues of the main helix in alkaline conditions.
it is clear that heparin binding maintained this region in helical form under alkaline conditions, whereas in the apo simulations the same region assumed a random coil profile.
the fact that heparin was bound far form this helical region again emphasizes an allosteric mechanism for this stabilization.
we also believe that the loss of helical content seen here on the  <dig> ns timescale is just a precursor of a major loss of secondary structure at longer timescales.
it is discussed in the literature that papain-like enzymes, which possess a similar domain organization, present structural characteristics that confer some rigidity to the active site region  <cit> .
the most important element supporting this characteristic is the network of interactions in the interdomain interface.
we verified the stability of the interaction between catalytic residues c29-h <dig> and found that disorganization of the main alpha helix in apo catb under alkaline conditions is coupled with the separation of the catalytic residues .
these events are strongly correlated with the overall domain motions observed.
the binding of heparin at a substantial distance form the interface nevertheless stabilizes the interdomain contacts maintaining the structuring of the active site, and would explain the results obtained in biochemical assays  <cit> .
the occluding loop controls access to the active site and also confers the exo-proteolytic activity of the enzyme.
in our simulations, we verified the open-close mechanism of this region and found that in acidic conditions a stable closed state is maintained regardless of heparin binding.
however, at ph  <dig>  the occluding loop exhibited high flexibility in the absence of heparin  as expected after rmsf analysis .
nevertheless, under alkaline conditions, when heparin was bound the occluding loop exhibited a more stable conformation .
in this conformation, besides closing the active site the occluding loop interacts with the r-domain in a distinct fashion from that observed in apo catb .
the occurrence of this conformation was also independent of the positive site occupied by heparin since the rmsd between the different complexes is around  <dig>  Å. on the other hand, the loop conformations observed in the apo form simulations exhibit higher rmsd values  compared to the complexes.
this result allows a discussion of experimental findings in which it was shown that heparin inhibits the exo-proteolytic activity of catb  <cit> .
in that study, the authors proposed a competitive mechanism of inhibition due to the possible interaction of heparin with h <dig> and h <dig>  which are the most important residues involved in accommodation of the negatively-charged c-terminal substrate carboxylate groups.
from our analysis we conclude that the mechanism of inhibition promoted by heparin appears to be related also to the induction of conformational changes in catb, mainly in the occluding loop region, which adopts a distinct stable conformation that may impair proper binding of small substrates.
however, we cannot exclude the possibility that large heparin polymers may also bind to the occluding loop, thus inhibiting the exo-proteolytic activity of the enzyme, especially if the concentration of heparin increases as shown in ref.
<cit> .
a rearrangement of contacts explains heparin-induced stability at alkaline ph
the catb interface is mainly polar and is stabilized by ion pairs and hydrogen bonds between buried residues  <cit> .
since we see that ph changes induce distinct protonation profiles mainly at the interface, and also that heparin binding affects the interface stability, we analyzed the atomic interactions in this region.
in particular, we verified the permanency of interactions between interfacial residues in order to see the influence of heparin binding on their stability .
we first checked certain interactions suggested by experimental studies, such as w30- e <dig>  this interaction was observed only in the acidic ph simulations , and does not occur in alkaline ph  due to deprotonation of the e <dig> carboxyl group which interacts with the backbone of w <dig>  the protonation state of e <dig> also affects the interaction between this residue and w <dig>  since the observed occupancies were higher in acidic ph  than in alkaline conditions .
this interaction was suggested to be important in the overall maintenance of the catb interface  <cit>  and it was significantly stabilized by heparin binding.
other interactions found to be stabilized by heparin binding are detailed in table  <dig>
interactions are broken down into main-chain  and sidechain .
domain definitions were used following papain domain organization.
an important aspect observed through the h-bonding analysis was the network of contacts between q <dig>  e <dig> and s <dig> .
concerning the q <dig> – s <dig> contact, we observed high stability in acidic ph.
we identified two stable h-bonds between these residues in acidic simulations, while in alkaline conditions only one weak interaction was found.
further, the occupancy for each of these contacts was dependent on heparin binding, since their prevalence increasing from 65% to 82% in acidic conditions and 6% to 29% at higher ph.
it appears that the lack of one h-bond between q <dig> and s <dig> in alkaline conditions is compensated by an interaction achieved between e <dig> and s <dig> only in such conditions.
this occurs due to the distinctive protonation state of e <dig>  which only when ionized  is able to interact with s <dig> through an h-bond.
remarkably, the occupancy of the e36-s <dig> interaction increases from 38% to 72% upon heparin binding.
this result may be explained by the global stabilizing effect observed  which is crucial in the establishment of this distinct interdomain contact.
the d22-h <dig> pair is another example of an interaction that was stabilized by the same mechanism.
we measured the occupancy of this h-bonded state during the 40ns md simulation.
while in acidic ph conditions the occupancy of the hydrogen-bonded state was very high , in alkaline conditions heparin binding significantly increased the stability of this interaction, which passed from 6% in apo catb to 67% in the catb-hep complex.
these results are correlated with the distinct loop conformation observed in the heparin bound complexes .
in addition, as shown, heparin binding at the r-domain induces a conformational change that leads to a more compact structure .
further, heparin restricts collective motions involved in domain separation, which allows the stabilization of a distinct pattern of interactions formed in the interface under alkaline conditions.
CONCLUSIONS
heparin-protein interactions are known to regulate several biological processes including protease regulation  <cit> , growth factor activity  <cit> , macromolecular assembly  <cit>  and viral mechanisms  <cit> .
in this paper we unraveled the molecular mechanism of heparin protection against ph-induced inactivation of catb.
this phenomenon was previously demonstrated through biochemical assays without a full understanding of the process because of the lack of structural information concerning heparin binding.
herein we were able to mimic the different ph conditions by performing md simulations of catb with different protonation states according to pka calculations.
we confirmed experimental findings showing that under acidic conditions catb is stable.
in contrast, at alkaline ph six residues are deprotonated, increasing the negative charge of the interdomain interface.
this leads to charge repulsion and subsequent separation of the l and r domain.
in addition, we observed the effect of the alkaline conditions in terms of destabilization of helical content, active-site disruption and increases in the flexibility of the occluding loop.
all these events are closely related to loss of enzyme activity.
heparin binding counteracted these effects by a long-distance, allosteric mechanism which appears to be associated with  conformational changes leading to a more compact interface, and  the restriction of catb flexibility, allowing stabilization and rearrangement of interdomain contacts under neutral/alkaline conditions.
these new findings may be crucial for the design of new catb inhibitors.

 cathepsin b is one of the most well-characterized cysteine proteases, belonging to the clan ca. in humans, its physiological role is implicated in bone resorption, antigen processing and protein turnover. however, catb also participates in pathological processes such as cardiovascular disturbances, parasitic infections, alzheimer's disease, osteoarthritis, tumor invasion and metastasis development. its main roles in cancer are i) its activity in directly cleaving extracellular matrix components, ii) its activation of other ecm degrading proteases, which promotes tumor cell invasion into the surrounding tissue and bloodstream escape, and iii) stimulating angiogenesis which provides increased nutrients and oxygen supplies to cancer cells. thus, catb regulates several crucial steps in tumorigenesis and is a promising target for anti-cancer drug design. structurally, catb possesses the regular fold of papain-like enzymes, enclosing two distinct domains stabilized by six disulfide bridges, forming a large polar interface into which project the side chains of a few charged residues such as e and e. this interdomain interface is extremely important to catb overall activity as it comprises the active site residues. unlike other members of the papain family, catb exhibits both exo- and endo-proteolytic activities. its exo-activity is dependent on the presence of two adjacent histidine residues located at an insertion region called the “occluding loop”. these residues provide the necessary positive charge to anchor the negatively-charged c-terminal carboxylate of exo-substrates. this region is only found in catb within its family, and it controls the access of large substrates to the active site. site-directed mutagenesis studies confirmed the role of the occluding loop since deletion of this entire region impairs exo- but not endo-proteolytic activity. additionally, this region confers thermal stability to catb and resistance against endogenous inhibitors such as cystatin c. currently the most potent and selective structure-based designed compounds available are derived from e- targeting the unusual occluding loop present in the catb 3d-structure. however, enzymatic assays have shown that these inhibitors are strongly ph dependent as their optimal binding affinities are considerably diminished under neutral/alkaline conditions. since catb can be found in several cellular compartments with distinct ph values, these inhibitors are not effective in vivo. when catb is within the lysosomal or endosomal vesicles it confronts acidic conditions, in contrast to the neutral/alkaline environment when it is attached to membranes or secreted in the ecm. although catb is rapidly inactivated under alkaline ph conditions, it was reported that membrane-associated forms are resistant to this process. this peculiarity is believed to occur due to its interaction with heparan sulfate glycosaminoglycans on the cell surface. this polysaccharide, structurally related to heparin, acts on the ecm as a co-receptor for several molecules such as growth factors and proteases. interaction between catb and heparin-like gags was already shown to prevent the enzyme's inactivation in high ph. the main reported outcome was the maintenance of catb helical content in the presence of heparin at high ph, which was observed for papain as well. nevertheless, it has not been possible to precisely define the catb/heparin interaction sites and the molecular mechanism responsible for this protective effect. structural and molecular modelling studies can give insight into the molecular events concerning the modification of catb activity by ph changes and heparin binding. some attempts have already been made towards designing new catb inhibitors using molecular modeling techniques, taking into account dynamical aspects of binding. however, these studies did not evaluate the influence of ph nor that of heparin binding on the modulation of the enzyme intrinsic flexibility. in order to understand the heparin protective effect over catb structure we performed molecular dynamics simulations using an approach in which different ph conditions were taken into account by considering different protonation states of titratable groups on the protein surface. further, docking analyses resulted in the identification of two heparin binding sites on catb structure. the md calculations confirmed an increase of the overall catb flexibility and the loss of stability of the apo catb inter-domain interface in alkaline ph. the destabilization and the increased flexibility, notably in the occluding loop, were prevented by interaction with heparin, again in agreement with experimental data. we observed a role of active site residues in enzyme stabilization and in maintaining the helical content, and we propose an allosteric mechanism for the stabilizing effect promoted by gag interaction. taken together, our data provides an improved understanding of the molecular mechanisms responsible for both ph-induced inactivation and protection against inactivation by heparin binding.  ph changes result in distinct protonation profiles prediction of pka's for protein ionizable residues is an important tool for understanding features and catalytic mechanisms of ph-dependent enzymes. we applied the propka program to estimate pka values in the catb structure and to determine the most probable corresponding protonation states of the enzyme under acidic and alkaline conditions. two different conditions were studied: acidic, and alkaline, which allow comparison to available experimental data. it is known that catalytic residues often present unusual pka values compared to those of free amino acids in solution. accordingly, our results for catb predicted a pka of for the catalytic c in contrast to expected in solution. previous work on the papain catalytic cysteine showed pka values around to, in accordance to our estimation for c comparing the acidic and alkaline conditions we observed that six key residues are differentially protonated. table gives the pka values for these residues and fig. shows their positions in the catb structure. we note that four of these residues belong to the interdomain interface. the predicted pka for e is, as carboxyl groups usually exhibit high pka values in buried hydrophobic environments. from our prediction we observed that both e and e are likely to be protonated at ph but not in alkaline conditions. pka values predicted by propka and the state of ionizable groups under different ph conditions are indicated. the protonation state of c at ph were obtained taking into account deprotonation of h as previously proposed for papain. the protonation states of the catalytic residues are highlighted. as expected, most of the residues with differing protonation states were histidines, with a pka in solution of and an ionization state that is very susceptible to ph changes in the physiological range. h is a key residue for stabilization of the occluding loop and is also crucial for the exo-proteolytic activity of catb, since along with h it anchors the carboxyl terminus of substrates. interestingly, we observed an pka of for the catalytic h which implies that this residue is protonated only in acidic conditions. this result is correspondent to the measured pka of h in papain. in papain, it was previously reported that the ionization of the catalytic c and h residues are coupled. there the deprotonation of h shifts the pka of c from ~ to, while the neutralization of c decreases the pka of h from ~ to. due to the direct relationship between catb and papain we assigned the protonation states of these residues according to this proposed mechanism. the pka values predicted with propka thus appear to be sensitive to local microenvironment changes around the ionizable residues. in order to verify if the applied protonation profiles accurately represent the distinct ph conditions simulated, we collected snapshots from each molecular dynamics trajectory in order to perform pka calculations. table shows the average pka values during the md simulations. from both ph conditions we obtained values consistent with the protonation states predicted from the crystal structure, thus confirming their validity. in this table we have also highlighted the pka shift in the catalytic residues c and h as observed for papain. electrostatic potential calculation and docking analyses reveal two heparin binding sites on catb structure the electrostatic potential at the catb surface revealed the influence of the different protonation states for each of the ph conditions considered. overall, independent of the conditions, the catb surface is mostly negative, presenting one region of positive potential in each domain. under alkaline conditions the overall surface is qualitatively more negative. the positive sites are composed of r k k k and k in the l-domain, and the catb n-terminus together with k and r in the r-domain. the protein's total charge changes from - at ph to - at ph, with six key residues protonated exclusively under acidic conditions. experimental studies have provided knowledge about the affinity and kinetics of the catb-heparin interaction. however, the precise binding site has not been defined. taking into account that heparin-protein interactions are mainly driven by charge interactions due to the high number of sulfate groups found in the polysaccharide, we visually identified two regions of positive potential at the catb surface as potential heparin binding sites. blind docking calculations confirmed this prediction, as we found two low energy clusters of docking poses coincident with these positive regions. almeida and coworkers proposed that heparin mainly interacts with the occluding loop. from a structural view this hypothesis appears unlikely due to the lack of charge and shape complementarity in this region of the enzyme, and is consistent with the absence of low-energy clusters for such a mode of binding in the docking results. moreover, we carried out another docking calculation verifying the atomic interactions in the heparin binding sites more precisely. in the lowest energy complex obtained, heparin interacts with catb in the r-domain with an energy of - kcal/mol. although the positive potential at this site is smaller than that found in the l-domain, in r-domain site the van der waals interactions were more favorable. we suggest that for a disaccharide the r-domain site is more important for this interaction; for longer sugar chains the l-domain could be also important for proper heparin accommodation. catb protonation profile influences protein stability as flexibility plays a key role in protein biological functions, we analyzed the root mean square fluctuation of catb backbone residues during md for each condition simulated. a direct influence of protonation state on the overall enzyme flexibility is clearly seen. figure 4a shows that at ph the fluctuations were around 2Å, in contrast to rmsf values up to 5Å observed at ph. this behavior was reproducible: we performed replicate simulations for each condition using different initial conditions, with equivalent results. in addition, we observed that functional regions such as the occluding loop and the active site presented the highest fluctuations at ph, revealing that alkaline conditions rapidly affected catb flexibility in regions directly involved with activity, independent of the starting structure. remarkably, this result shows that changes in protonation of a few residues can alter both local and non-local properties. local perturbations due to changes in protonation states have already been observed via md simulations. however, in our simulations both local and global flexibility of catb were modified as a function of ph, corroborating experimental findings where catb loses its activity in alkaline ph in parallel with large structural changes. heparin binding prevents catb destabilization in alkaline ph in md simulations of the bound enzyme at ph, the rms fluctuation profile was seen to be similar to that of the apo form. hence, catb is stable under acidic conditions independent of heparin binding. in contrast, under alkaline conditions heparin-bound catb exhibited much smaller backbone fluctuations than the very flexible apo form in these conditions. moreover, we observed similar results when simulations were started using different initial conformations and also with heparin docked at the other binding site. functional regions including the occluding loop and the active site were also stabilized, showing that heparin binding at the predicted sites promotes a global stabilizing effect. in fig. 4c, we show the time-evolution of the number of distinct clusters of catb backbone conformations throughout the different md simulations. here each cluster contains conformations within an rmsd of Å of the cluster center. if during a simulation the system visits numerous clusters, the rmsd from the starting structure will be very high, indicating conformational instability in the simulation. in contrast, if a simulation visits only a few clusters, it can be considered as being more conformationally stable. indeed, we found few clusters during the simulations of apo catb and the catb-heparin complex under protonation conditions corresponding to ph. furthermore, in acidic conditions the backbone rmsd distributions, similar to the rms fluctuation profiles, did not change significantly on binding heparin, presenting deviations in both cases centered at Å. however, in the heparin-bound simulation in acidic conditions, the rmsd distribution began to shift towards a conformational state far from the native structure.. considering that catb loses stability and activity in alkaline conditions, in the systems simulated under protonation conditions corresponding to high ph we expected to observe a higher number of backbone clusters. this expectation was confirmed: we observed clusters in the apo catb simulation. in addition, under these conditions the catb structure does not stabilize during the simulation, with a continuous increase of backbone rmsd values throughout the ns production period. a similar profile was observed in the replica simulation and also when starting with another conformation, in which we found clusters. the rms fluctuations of both replica simulations, plus the simulation performed with the alternative catb structure, showed similar results to the apo enzyme in alkaline conditions. the rmsd distribution plot shows a clear shift dividing into two populations: one around Å and the other at higher rmsd values.. on the other hand, in the simulation with heparin under alkaline conditions we observed fewer clusters comparable to acidic conditions. this stabilization afforded by heparin binding is also reflected in the rmsd distribution presenting a narrowed profile centered at small rmsd values. heparin prevents the domain separation observed in alkaline ph it was previously postulated that the loss of inter-domain interface contacts should be a crucial event in catb inactivation in alkaline ph. furthermore, the interface between l and r domains is essential not only for tertiary structure stabilization but also for the enzyme activity, as the catalytic triad is formed with residues from both domains. to address whether this phenomenon is modulated by heparin, we compared the evolution of the distance between the centers of mass of the two domains during the simulations. in acidic conditions, this distance was stable at approximately Å throughout the entire simulations, and was not significantly affected by heparin binding. in contrast, when the enzyme was simulated under protonation conditions corresponding to alkaline medium, we observed a remarkable domain separation, with domain center-of-mass distances of up to Å. further, we observed that this separation increased progressively throughout the simulation, suggesting that total l and r domain separation could occur at longer time-scales. this certainly would affect catb catalysis, since the active site is found inserted into a v-shaped cleft between the l and r domains. a more detailed analysis provided by use of the dyndom software revealed the separation of the l-domain as the principal overall motion of the enzyme in this condition, thus reinforcing this conclusion. this domain motion was observed in all simulations except those containing heparin, in which it was not possible to identify large domain motions with the dyndom program. remarkably, in these last systems, the interdomain distance remained at approximately Å in alkaline conditions, comparable to acidic conditions. this shows that heparin binding governs the inter-domain stability even under unfavorable alkaline conditions and would presumably also allow the maintenance of protein activity. essential dynamics analysis reveals an allosteric role for heparin the results shown above suggest that catb as a whole and in particular the critical domain interface and occluding loop are stabilized by heparin binding to a relatively distant site in the r domain. from basic thermodynamic principles, preferential binding of ligand to the native state will generally confer stability to a macromolecule in what has been described as an allosteric mechanism, without necessity for direct interaction between the ligand binding site and the active site residues, and has been extensively described in phenomenological terms. the structural information available for catb, coupled with macromolecular simulations, provides a means for investigating details of such a mechanism directly. we applied an essential dynamics analysis to examine the most relevant global macromolecular motions occurring during the simulations. such analyses have been largely applied in the understanding of biological functions in proteins since they provide an evaluation of large-scale movements that are often related to domain motions and conformational changes. we thus diagonalized the atomic positional covariance matrix to obtain the eigenvectors and corresponding eigenvalues. selecting the first five principal components, we could recover around 60% of the total motions for apo catb and around 56% for heparin-bound catb. our analysis focused on the systems simulated under alkaline conditions in order to address how heparin influences catb flexibility and prevents domain separation as shown above. we point out that certain analyses involving averaged quantities such as the essential dynamics and clustering approaches can be best interpreted for stable systems; the precise results for the apo catb under alkaline conditions will to some extent depend on the starting conformation. however, even under these conditions the overall results obtained were similar for replicated runs. we examined the rms fluctuations of each trajectory projected onto its five most representative principal components. when comparing apo and complexed catb, we observed considerably lower fluctuations in the latter system for all pc analyzed. further, we identified that the region around c and the occluding loop presented dramatically higher fluctuations in apo catb, while in the complex these regions were shown to be very stable. the movements corresponding to the first two components were seen to comprise relevant motions of both the occluding loop and active site. figure 6b shows the directions of individual residue movements and also their amplitudes, which are proportional to the lengths of the arrows in this representation. by inspecting other motions corresponding to the first component in the r-domain complexes, we identified local changes in the binding region. the residues close to the polysaccharide adopted a new conformation. to verify that these motions would not lead to loss of binding interaction between catb and heparin, we measured the distance between the center of mass of heparin and the positive site during md and found stable behavior: heparin remained bound during the entire simulation. remarkably, however, this local motion is coupled to movements far from the heparin binding site. we observed that the residues to in the l-domain moved towards the interdomain interface. this motion certainly contributes to the stabilization of the overall structure, thus helping explain the results described above. we did not find equivalent behavior in the l-domain complexes, which led us to investigate more deeply the correlated motions of catb in the apo form and in complex with heparin. the analysis of cross correlation coefficients between pairs of residues is a useful method to identify correlated collective motions. the correlation matrix represents the linear correlation between pairs of c-alpha atoms as they move about their average positions during dynamics. positive correlations are related to motions occurring in the same direction whereas negative correlations indicate motion in opposite directions. we compared the correlation matrix of apo catb and of the complexes. in the apo form we observed positive intra-domain correlations but negative inter-domain correlations. this pattern is not observed in the complexes, in which we identified a more diffuse pattern of correlations regardless of the binding site occupied. this result shows that heparin acts mainly as a stabilizing element, and its binding seems to restrict the anti-correlated collective motions that would be responsible for domain separation. concerning the occluding loop, we observed that the opening-closing movement is clearly represented by the two first components in apo catb. on the other hand, upon heparin binding these motions were not observed, independent of the binding site occupied. in addition, the first pc reveals the increasing distance between c and the other catalytic residues in apo catb, while in the complex this motion is absent. therefore, heparin stabilizes catb motions in its functional regions. these results are consistent with experimental findings, which, although they revealed the implications of binding on the activity of the enzyme, did not provide a structural view of the phenomenon. heparin binding, despite the distance separating it from active site residues, prevents their disorganization as would otherwise be produced under alkaline conditions. the principal components obtained from the essential dynamics analysis of our md simulations suggest that global concerted movements in the macromolecule leading to domain separation are suppressed by heparin binding, thus helping explain an allosteric role for heparin in this biological system. heparin prevents loss of helical content and stabilizes the occluding loop loss of helical content was reported as the main effect related to alkaline-ph-induced inactivation of catb. in the same study, it was reported that heparin prevented the loss of helical structure, which led to the postulate that the maintenance of activity in the catb-heparin complexes is strictly related to this structural aspect. in analyzing the time evolution of secondary structural elements throughout the simulations, we observed stability of secondary structure elements in acidic conditions, independent of heparin binding. in contrast, this same analysis for alkaline conditions revealed differences in the region of the main alpha helix, which is located in the inter-domain interface. figure 7a shows the phi-psi distribution plot for the first residues of the main helix in alkaline conditions. it is clear that heparin binding maintained this region in helical form under alkaline conditions, whereas in the apo simulations the same region assumed a random coil profile. the fact that heparin was bound far form this helical region again emphasizes an allosteric mechanism for this stabilization. we also believe that the loss of helical content seen here on the ns timescale is just a precursor of a major loss of secondary structure at longer timescales. it is discussed in the literature that papain-like enzymes, which possess a similar domain organization, present structural characteristics that confer some rigidity to the active site region. the most important element supporting this characteristic is the network of interactions in the interdomain interface. we verified the stability of the interaction between catalytic residues c29-h and found that disorganization of the main alpha helix in apo catb under alkaline conditions is coupled with the separation of the catalytic residues. these events are strongly correlated with the overall domain motions observed. the binding of heparin at a substantial distance form the interface nevertheless stabilizes the interdomain contacts maintaining the structuring of the active site, and would explain the results obtained in biochemical assays. the occluding loop controls access to the active site and also confers the exo-proteolytic activity of the enzyme. in our simulations, we verified the open-close mechanism of this region and found that in acidic conditions a stable closed state is maintained regardless of heparin binding. however, at ph the occluding loop exhibited high flexibility in the absence of heparin as expected after rmsf analysis. nevertheless, under alkaline conditions, when heparin was bound the occluding loop exhibited a more stable conformation. in this conformation, besides closing the active site the occluding loop interacts with the r-domain in a distinct fashion from that observed in apo catb. the occurrence of this conformation was also independent of the positive site occupied by heparin since the rmsd between the different complexes is around Å. on the other hand, the loop conformations observed in the apo form simulations exhibit higher rmsd values compared to the complexes. this result allows a discussion of experimental findings in which it was shown that heparin inhibits the exo-proteolytic activity of catb. in that study, the authors proposed a competitive mechanism of inhibition due to the possible interaction of heparin with h and h which are the most important residues involved in accommodation of the negatively-charged c-terminal substrate carboxylate groups. from our analysis we conclude that the mechanism of inhibition promoted by heparin appears to be related also to the induction of conformational changes in catb, mainly in the occluding loop region, which adopts a distinct stable conformation that may impair proper binding of small substrates. however, we cannot exclude the possibility that large heparin polymers may also bind to the occluding loop, thus inhibiting the exo-proteolytic activity of the enzyme, especially if the concentration of heparin increases as shown in ref.. a rearrangement of contacts explains heparin-induced stability at alkaline ph the catb interface is mainly polar and is stabilized by ion pairs and hydrogen bonds between buried residues. since we see that ph changes induce distinct protonation profiles mainly at the interface, and also that heparin binding affects the interface stability, we analyzed the atomic interactions in this region. in particular, we verified the permanency of interactions between interfacial residues in order to see the influence of heparin binding on their stability. we first checked certain interactions suggested by experimental studies, such as w30- e this interaction was observed only in the acidic ph simulations, and does not occur in alkaline ph due to deprotonation of the e carboxyl group which interacts with the backbone of w the protonation state of e also affects the interaction between this residue and w since the observed occupancies were higher in acidic ph than in alkaline conditions. this interaction was suggested to be important in the overall maintenance of the catb interface and it was significantly stabilized by heparin binding. other interactions found to be stabilized by heparin binding are detailed in table interactions are broken down into main-chain and sidechain. domain definitions were used following papain domain organization. an important aspect observed through the h-bonding analysis was the network of contacts between q e and s. concerning the q – s contact, we observed high stability in acidic ph. we identified two stable h-bonds between these residues in acidic simulations, while in alkaline conditions only one weak interaction was found. further, the occupancy for each of these contacts was dependent on heparin binding, since their prevalence increasing from 65% to 82% in acidic conditions and 6% to 29% at higher ph. it appears that the lack of one h-bond between q and s in alkaline conditions is compensated by an interaction achieved between e and s only in such conditions. this occurs due to the distinctive protonation state of e which only when ionized is able to interact with s through an h-bond. remarkably, the occupancy of the e36-s interaction increases from 38% to 72% upon heparin binding. this result may be explained by the global stabilizing effect observed which is crucial in the establishment of this distinct interdomain contact. the d22-h pair is another example of an interaction that was stabilized by the same mechanism. we measured the occupancy of this h-bonded state during the 40ns md simulation. while in acidic ph conditions the occupancy of the hydrogen-bonded state was very high, in alkaline conditions heparin binding significantly increased the stability of this interaction, which passed from 6% in apo catb to 67% in the catb-hep complex. these results are correlated with the distinct loop conformation observed in the heparin bound complexes. in addition, as shown, heparin binding at the r-domain induces a conformational change that leads to a more compact structure. further, heparin restricts collective motions involved in domain separation, which allows the stabilization of a distinct pattern of interactions formed in the interface under alkaline conditions.  heparin-protein interactions are known to regulate several biological processes including protease regulation, growth factor activity, macromolecular assembly and viral mechanisms. in this paper we unraveled the molecular mechanism of heparin protection against ph-induced inactivation of catb. this phenomenon was previously demonstrated through biochemical assays without a full understanding of the process because of the lack of structural information concerning heparin binding. herein we were able to mimic the different ph conditions by performing md simulations of catb with different protonation states according to pka calculations. we confirmed experimental findings showing that under acidic conditions catb is stable. in contrast, at alkaline ph six residues are deprotonated, increasing the negative charge of the interdomain interface. this leads to charge repulsion and subsequent separation of the l and r domain. in addition, we observed the effect of the alkaline conditions in terms of destabilization of helical content, active-site disruption and increases in the flexibility of the occluding loop. all these events are closely related to loss of enzyme activity. heparin binding counteracted these effects by a long-distance, allosteric mechanism which appears to be associated with conformational changes leading to a more compact interface, and the restriction of catb flexibility, allowing stabilization and rearrangement of interdomain contacts under neutral/alkaline conditions. these new findings may be crucial for the design of new catb inhibitors. 
our pka calculations of catb titratable residues revealed distinct protonation states under different ph conditions for six key residues, of which four lie in the crucial interdomain interface. molecular dynamics of both apo catb and catb-heparin complexes were performed using protonation states for catb residues corresponding to the relevant acidic or alkaline conditions. cathepsin b is a promising target for anti-cancer drug design due to its implication in several steps of tumorigenesis. moreover, we propose an allosteric role for heparin in the regulation of catb stability in such a manner that the restriction of enzyme flexibility would allow the establishment of stronger contacts and thus the maintenance of overall structure. the results of our study contribute to unraveling the molecular events involved in catb inactivation in alkaline ph, highlighting the fact that protonation changes of few residues can alter the overall dynamics of an enzyme. however, the molecular mechanism of stabilization is not well understood, indicating the need for more detailed structural and dynamic studies in order to clarify the influence of ph and heparin binding on catb stability. in contrast, under alkaline conditions the enzyme's overall flexibility was increased: interactions between active site residues were lost, helical content decreased, and domain separation was observed as well as high-amplitude motions of the occluding loop – a main target of drug design studies. essential dynamics analysis revealed that heparin binding modulates large amplitude motions promoting rearrangement of contacts between catb domains, thus favoring the maintenance of helical content as well as active site stability. this implies changes in the overall charge distribution at the catb surface, as revealed by calculation of the electrostatic potential. catb activity and inhibition are ph-dependent, making it difficult to identify efficient inhibitor candidates for clinical trials. the md of apo catb at ph was very stable, and presented the highest number and occupancy of hydrogen bonds within the inter-domain interface. in addition it is known that heparin binding stabilizes the enzyme in alkaline conditions.
 cathepsin b is a promising target for anti-cancer drug design due to its implication in several steps of tumorigenesis. catb activity and inhibition are ph-dependent, making it difficult to identify efficient inhibitor candidates for clinical trials. in addition it is known that heparin binding stabilizes the enzyme in alkaline conditions. however, the molecular mechanism of stabilization is not well understood, indicating the need for more detailed structural and dynamic studies in order to clarify the influence of ph and heparin binding on catb stability.  our pka calculations of catb titratable residues revealed distinct protonation states under different ph conditions for six key residues, of which four lie in the crucial interdomain interface. this implies changes in the overall charge distribution at the catb surface, as revealed by calculation of the electrostatic potential. we identified two basic surface regions as possible heparin binding sites, which were confirmed by docking calculations. molecular dynamics of both apo catb and catb-heparin complexes were performed using protonation states for catb residues corresponding to the relevant acidic or alkaline conditions. the md of apo catb at ph was very stable, and presented the highest number and occupancy of hydrogen bonds within the inter-domain interface. in contrast, under alkaline conditions the enzyme's overall flexibility was increased: interactions between active site residues were lost, helical content decreased, and domain separation was observed as well as high-amplitude motions of the occluding loop – a main target of drug design studies. essential dynamics analysis revealed that heparin binding modulates large amplitude motions promoting rearrangement of contacts between catb domains, thus favoring the maintenance of helical content as well as active site stability.  the results of our study contribute to unraveling the molecular events involved in catb inactivation in alkaline ph, highlighting the fact that protonation changes of few residues can alter the overall dynamics of an enzyme. moreover, we propose an allosteric role for heparin in the regulation of catb stability in such a manner that the restriction of enzyme flexibility would allow the establishment of stronger contacts and thus the maintenance of overall structure. 18– october 5th international conference of the brazilian association for bioinformatics and computational biology angra dos reis, rj, brazil 