Reactivities of the sulphydryl groups of horse (Equus caballus) haemoglobin (1013.11)

The kinetics of the reaction of Ellman’s reagent (DTNB) with CysF9[93]5 sulphydryl group of the horse haemoglobins were studied at neutral and physiological pH (6.8 7.6) ranges under pseudo first order conditions. The reaction is of first order with respect to the DTNB concentration. The reactions are pH dependent of the observed rate constant gave a complex trend. The observed rate shows that at neutral pH, the presence of inositol hexakisphosphate (inositol-P 6 ) increases the pseudo first order rate constant. For the first time, inositol-P 6 increases rate of forward reaction, kF at neutral pH values by increasing the mean value of the transition constant, K rt3 . The K rt3 for the haemoglobin without inositol-P 6 gave the value of 0.138 ± 0.1 while the haemoglobin in the presence of inositol-P 6 gave the K rt3 value of 0.325 ± 0.2. The results show that inositol-P 6 increases the relative population of the t tertiary conformation. So, it increases the reactivity of CysF9[93]5 by changing the relative distribution of two protein conformation