Okonjo, K. O. and Adeogun, I. A. and Babalola, J. O,
Transition of haemoglobin between two tertiary conformations: Inositol hexakisphosphate increases the transition constant and the affinity of sheep haemoglobin for 5,5′-dithiobis(2-nitrobenzoate).
Biochimica et Biophysica Acta, 1794.
The equilibrium constant (Kequ) for the reaction of 5,5′-dithiobis(2-nitrobenzoate) — DTNB — with the CysF9
β sulphydryl group of the haemoglobins of the sheep decreases by about two orders of magnitude
between pH≈5.6 and 9.2: from a mean of 7.2±1 to a mean of 0.044±0.01. Calculations from the pH
dependence of Kequ show that in the r⇌t tertiary conformational transition of haemoglobin the t isomer
population is 50.7 and 61.8% for the major and minor haemoglobins, respectively. In the presence of inositol
hexakisphosphate (inositol-P6), Kequ increases for both haemoglobins by about an order of magnitude
through most of the pH range. The t isomer population also increases to 82.1 and 79.6% for the major and
minor haemoglobins, respectively. These results indicate that inositol-P6 increases the affinity of the
sulphydryl for DTNB by increasing the population of the t isomer. It is highly probable that a minimum fourstate
model that includes the r⇌t transition is required for a full understanding of haemoglobin function.
Actions (login required)