Temperature Dependence Study of the Reaction of Human Oxyhemoglobin "A" with 5,5' -Dithiobis (2-Nitrobenzoic Acid).

The      temperature-dependence study   of  the  reaction  of  human   oxyhemoglobin A(O2HbA)    with   55'-,  dithiobis(2-nigrobemaio acid),   DTNB,   as    a  function  of pH    has    been studied.    The quantitative  analysis  of  the pH   dependence  of  the  apparent second  order  rate constant  shows that two  ionizable groups  are electrostatically linked to  the reaction. Their pKa values  are  5.5 and 8.7.  These values are assigned to His HC3 (146) ß and to the Cys F9 (93)     ß sulphydryl   The intrinsic activation parameters  for  these ionizable  groups  have been  determined using the   Arrhenius and Eyring  theories.  The intrinsic activation entropies or His HC3 ( 146) ß and Cys    F9  (93) ß   have  confirmed that the two  ionizable  groups  are electrostatically  linked. Negative  value  of  activation entropy for Cys  F9 (93) ß   shows that there is  an  increase in  the polarity  around  the  reactive center. Large  and positive  intrinsic activation entropy of His  HC (146) ß further confirm that the presence of salt bridge between His HC3 (146) ß and Asp FG 1 (94) ß plays    a major  role  in the  reaction. The variation of apparent activation parameters as a function   of pH    could   be   attributed  to  the  effect  of  salt   bridges,   hydrogen  bonding,  van derWaals forces and hydrophobicity of oxyhemoglobin A.

A A MOOSAVI MNOHAVAHEDI ,MUHAMMAD R DAYER ,A K BORDBAR ,C O ABOLUWOYE ,