Abstract

The pH dependence of the second order rate constant of the reaction of 5,5′-dithiobis(2-nitrobenzoate) (DTNB) with CysF9[93]β sulphydryl group of human haemoglobin A at 50 mmol dm^-3 is complex. The removal of the terminal HisHC3[146]β of haemoglobin A by enzymatic cleavage with carboxypeptidase A breaks the salt bridge between HisHC3[146]β and AspFG1[94]β and reduces the strain on CysF9[93]β sulphydryl group. The pH dependence profiles of the second order rate constant for the reaction of DTNB with CysF9[93]β sulphydryl group of the modified haemoglobin A derivatives at 50 mmol dm^-3 became simple with significant reduction in the reaction rates contrary to expectations. The implication is that CysF9[93]β becomes occluded and hence less reactive. The mean pK_as of the ionizable groups linked to the reactivity of CysF9[93]β sulphydryl group of des-HisHC3[146]β human haemoglobin A were 5.52 ± 0.01 and 8.29 ± 0.1. These values are assigned to HisH21[143]β and CysF9[93]β amino acid residues, respectively.

Key words: Haemoglobin, CysF9[93]β, carboxypeptidase A, 5,5′-dithiobis(2-nitrobenzoate), salt bridge.