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-P) on the DTNB affinities of avian and mammalian carbonmonoxyhemoglobins. Inositol-Pdecreases by two orders of magnitude the DTNB affinity of guinea pig hemoglobin, which has His2β and the R2 quaternary structure.