An  Orbital  Control  of  Substrate  Reactivity  on  the  Enzyme  Xanthine Oxidase and  the  Substrate-Enzyme Binding Cofactor Reduction
Creators: name: KIBROM TEKLA
Publisher: name: National Academic Digital Repository of Ethiopia
Year: 2011

description: In  humans,  the  xanthine  oxidase  enzyme  plays  an  important  role  in  physiological 
hydroxylation  of hypoxanthine  to  xanthine,  xanthine  to.  uric  acid  and  it  has  been  a target 
of drugs  against  disease  such  as  gout  and  reperfusion  injury.  For this  reason knowledge of 
detailed  reaction  mechanism  by  which  the  enzyme  functions  is  desirable.  In  order  to 
understand  the  reaction  mechanism  for  the  oxidation  of  lumazine  by  xanthine  oxidase 
particularly  the  spectral  behaviors  associated  with  the  formation  of violapterin  (  oxidation 
product  of  lumazine)  and  the  spectral  behaviors  of  the  reduced  enzyme  bound  to 
products,  EREo-products  such  as;  EREo-uric  acid,  EREo-alloxanthine,  EREo-violapterin, 
EREo-formate,  and  ERE0-C02,  computational  approach  is  applied.  In  addition,  electronic 
structure  calculation  is  carried  out  to  understand  why  the  spectral  band  (  centered  at  650 
nm)  is  a unique  signature  of  EREo-violapterin intermediate.  Besides this,  molecular orbital 
analysis  for  the  constituent  fragments  are  calculated  to  understand  the  unique  spectral 
band  at  650  nm.  The  spectral  band  at  650 nm has  been  assigned  as  metal to  ligand  charge 
transfer  transition  band.  The  metal  to  ligand  charge  transfer  is  due  to  the  equilibrium 
between  structure  (d)  i.e  ((EREo)-[Mo
1
\-SMoH)-Oeq-CviolapterinD  and  structure  (e)  i.e 
v 
((EREo)-[Mo  (-SMoH)-Oeq-CviolapterinD·  The  charge  transfer  is  assigned  from  Mo  dxy 
Vl 
orbital  to  empty  rr*  orbital  of  violapterin.  The  data  from  electronic  structure  calculation 
was  compared  with experimental  data  in  order  to  develop  a plausible  reaction  mechanism 
for  the  product  release  stage.  As  a  result,  a  reaction  mechanism  for  the  product  release 
stage  of the  oxidation  of  lumazine  by  xanthine  oxidase  has  been  proposed.  Our  reaction 
mechanism  for  the  product  release  stage  involved  a  step  wise  mechanism.  Accordingly, 
the  formation  of structure  (d)  i.e  product  bound  or  ((ERrn)-[Mo1v(-SM0H)-Ocq-CviolaptcrinD 
is  energetically  favorable  by  -50.36  K.cal/mol  compared  to  the  structures  proposed  to  be 
formed  during the  product release  stage.  Structure  (d)  is  proposed  to  undergo  one  electron 
oxidation  to  form  structure  (e).  The  formation  of  structure  (e)  is  favorable  by  -14.67 
K.cal/mol.  Structure  (e)  is  proposed  to  undergo  one  electron  oxidation  and  loss  of  a 
proton  simultaneously  to  give the  product,  violapterin  and  the  enzyme  as  the  ligand  water 
enters.  The  energy  for  the  one  electron  oxidation  and  loss  of  a  proton  is  -559.52  K. cal / 
rnol.  The  spectral  behaviors  of  EREo-product  intermediates  such  as  ER1: n-uric  acid ,  ERuY 
alloxanthine,  ER1:o-violapterin,  ERED-formate,  and  ERED-C02  has  been  determined.  It  is 
found  that  the  difference  spectral  behavior  between  the  ERED-products  such  as:  ERrn-uric 
acid,  ERrn-alloxanthine,  ERED-violapterin,  ERrn-formate,  and  ERrn-C02  is  due  to 
difference  in  the  functional  group  of their  substrates  and  environment  of their  interaction site