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]β sulphydryl group of the horse haemoglobins were studied at neutral and physiological pH (6.8 < pH > 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]β by changing the relative distribution of two protein conformations.