Active site structural changes L-LTA4 system. The overlay of each mutant system with the WT system has allowed observing the structural changes occurred because of the mutations (Figure 9 ). In E271Q system, the loop of G269 was fluctuating and moved into the active site when compared to other systems. This change observed in this loop could be because of the newly formed hydrogen bond between G269 and the tilted carboxylate group of LTA4, which was found to be a response to the metal ion that lost the interaction with mutated E271. The carboxylate group of LTA4 and terminal NH 2 of R563 moved away from each other (2.7 to 8.4 Å ) because of the missing E271. A short beta sheet formed by the residues V306-N308 of HEXXH-(X) 18 -E motif that possess two catalytically conserved histidine residues coordinating with Zn 2+ ion disappeared in E271Q system. Another helix followed by this short beta sheet was extended by four amino acids W311-F314 making the important F314 slightly backward from the active center ( Figure 9A ). Two tyrosine residues Y378 and Y383 located opposite to each other in the active site were highly fluctuating in this system to adjust the binding of the LTA4. The K565 residue present in the loop has become a part of the long helix originally formed by T567-A575 residues during the simulation of E271Q system. This change slightly drew back the K565 residue from the active site. The other important positively charged residue did not show any structural changes during the simulation. The R563A system has shown different structural changes compared to E271Q system. The loop containing G269 has shown slight fluctuation only at the location of G269 because of the hydrogen bonds formed between the carboxylate of LTA4 and G269 but the lower part of the loop was stable unlike E271Q system ( Figure 9B ). The short helix formed by V306-N308 residues was maintained in this system and the helix containing F314 was extended but kept for the same length during the simulation. The region (W311-F314) that turned an extended helix was highly fluctuating in this system because of the moving alkyl part of LTA4. This backward movement buried the alkyl part into the hydrophobic pocket, formed by a mixture of aliphatic and aromatic hydrophobic residues (W311, F362, K364, L365, V366, V367, and V381), was observed because of the missing interactions with R563 residue (not shown in figure). Unlike E271Q system, Y378 residue has shown only slight side chain movement as a response to the moving LTA4 whereas Y383 did not show any fluctuations from its initial position. The same helix extension was observed as in E271Q system and thus K565 was included in helix formed by T567-A575 residues. The missing R563 led to the loss of correct alignment of LTA4 along the catalytic elements and severe instability of the binding mode of LTA4. In the final mutant (K565A) system, the G269 loop was completely stable and no hydrogen bond interaction was observed between LTA4 and G269 residue. The short helix of V306-N308 disappeared during the simulation of K565A system as observed in E271Q system ( Figure 9C ). The helix of F314 was extended as seen in R563A system and thus has shown the mixed characteristics of E271Q and R563A systems. The Y378, one of the oppositely located pair of tyrosine residues, has fluctuated highly in this system. The binding mode of LTA4 was quite similar to that of WT except its carboxylate group, which moved back because of the missing hydrogen bonds from K565 residue. But the hydrogen bonds with R563 were maintained and thus kept the alignment of LTA4 along with the catalytic elements. The overlay of active sites of all the systems have made clear about the structural changes where Y378 was observed to be fluctuating differently in each system maintaining a close distance with the substrate ( Figure 9D ). Thus Y378 residue, along with the carboxylate recognition site residues R563 and K565, can play a key role in aligning the substrate at the active site.