Transition of hemoglobin between two tertiary conformations: determination of equilibrium and thermodynamic parameters from the reaction of 5,5'-dithiobis(2-nitrobenzoate) with the CysF9[93]beta sulfhydryl group

R and R2 quaternary structures of carbonmonoxyhemoglobins: Differential effect of inositol hexakisphosphate on their affinity for Ellman's reagent

The reaction of 5,5'-dithiobis(2-nitrobenzoate), DTNB, with hemoglobin sulfhydryl groups is linked to three negatively contributing Bohr effect groups: His2β is present in all avian hemoglobins but absent in some mammalian hemoglobins; His77β and His143β are absent in avian but present in nearly all mammalian hemoglobins. To probe the consequences of these differences, we determined the influence of inositol hexakisphosphate (inositol-P₆) on the DTNB affinities of avian and mammalian carbonmonoxyhemoglobins. Inositol-P₆decreases by two orders of magnitude the DTNB affinity of guinea pig hemoglobin, which has His2β and the R2 quaternary structure. It decreases, or has no effect on, the DTNB affinities of hemoglobins that have His2β and whose structures lie along the R2 ⇌ R quaternary equilibrium. Finally, inositol-P₆increases by one to two orders of magnitude the DTNB affinities of hemoglobins that lack His2β. Thus His2β, DTNB and inositol-P₆, in combination, distinguish the R2 from the R quaternary structure.

Tertiary conformational transition in sheep hemoglobins induced by reaction with 5,5 -dithiobis(2-nitrobenzoate) and by binding of inositol hexakisphosphate

We have determined the second-order reverse rate constant, k(R), for the reaction of 5,5 -dithiobis(2-nitrobenzoate) - DTNB - with sheep hemoglobins as a function of pH from values of the second-order forward rate constant, k(F), and the equilibrium constant, K(equ), at 25 degrees C: k(R)=k(F)K(equ). We demonstrate that (i) inositol hexakisphosphate (inositol-P(6)) decreases k(F) and k(R) by increasing K(rt), the rright harpoon over left harpoont tertiary conformation transition constant; (ii) the conformation favored for both the forward and reverse reactions is the r conformation. For stripped hemoglobin we obtain from the k(F) data a t isomer population of 34.6% (+/-14) prior to reaction with DTNB; from the k(R) data we calculate a t isomer population of 44.8% (+/-4) following reaction with DTNB. In the presence of inositol-P(6) the latter value is increased to 79.5% (+/-2). These results demonstrate that an allosteric transition occurs on reaction with DTNB and on inositol-P(6) binding.

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)

The equilibrium constant (K(equ)) for the reaction of 5,5'-dithiobis(2-nitrobenzoate) - DTNB - with the CysF9[93]beta sulphydryl group of the haemoglobins of the sheep decreases by about two orders of magnitude between pH approximately 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 K(equ) 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-P(6)), K(equ) 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-P(6) increases the affinity of the sulphydryl for DTNB by increasing the population of the t isomer. It is highly probable that a minimum four-state model that includes the r-t transition is required for a full understanding of haemoglobin function.

Transition of hemoglobin between two tertiary conformations: The transition constant differs significantly for the major and minor hemoglobins of the Japanese quail (Cortunix cortunix japonica)

We demonstrate that 5,5'-dithiobis(2-nitrobenzoate) - DTNB - reacts with only CysF9[93]beta and CysB5[23]beta among the multiple sulfhydryl groups of the major and minor hemoglobins of the Japanese quail (Cortunix cortunix japonica). K(equ), the equilibrium constant for the reaction, does not differ very significantly between the two hemoglobins. It decreases 430-fold between pH approximately 5.6 and pH approximately 9: from a mean of 7+/-1 to a mean of 0.016+/-0.003. Quantitative analyses of the K(equ) data based on published X-ray and temperature-jump evidence for a tertiary structure transition in liganded hemoglobin enable the calculation of K(rt), the equilibrium constant for the r<---->t tertiary structure transition. K(rt) differs significantly between the two hemoglobins: 0.744+/-0.04 for the major, 0.401+/-0.01 for the minor hemoglobin. The mean pK(a)s of the two groups whose ionizations are coupled to the DTNB reaction are about the same as previously reported for mammalian hemoglobins.