pH Dependence Study of the Kinetic Reaction of Bovine Carbonic Anhydrase with 2,2'-Dithiobispyridine in the Absence and Presence of Surfactants

The pH dependence study reveals that the Cys 206 sulphydryl group of bovine carbonicanhydrase in the native form is not exposed. During the reaction of 2,2'-dithiobispyridine (2-DTP) with the enzyme, there was no absorbance change recorded. In the presence ofsurfactants, the pH dependence profiles of the apparent second order rate constants, kapp, forthe reaction of 2-DTP with bovine carbonic anhydrase assume simple profile forms, whichresemble the diprotic acid titration curve. These simple profiles were quantitatively analyseswith Eq. (1) in the text. Best-fit lines drawn with the parameters shown in Table 1 areobtained. The mean pKi value of 5.1 and the mean pK2 of 8.4 are assigned to His 167 andCys 206, respectively. The pH dependence of the apparent second-order rate constant, kapp, inthe higher concentrations of SDS is higher than that in the lower concentrations. It shows thatthe Cys 206 sulphydryl group of bovine carbonic anhydrase is more exposed in the higherconcentrations of SDS.