Role of Bohr group salt bridges in cooperativity in hemoglobin

Abstract

Possible problems in measuring the first Adair constant, K 1, from accurate oxygen equilibrium curves have been investigated. Of these only the presence of small amounts of CO-hemoglobin or hemoglobin dimers had a significant effect. The former can be eliminated by treatment with oxygen, the latter by measuring the concentration-dependence of K 1 or working at high protein concentrations. K 1 values have been measured for normal hemoglobin at pH 7 and 9, hemoglobin specifically reacted with cyanate at Val 1α (α 2 cβ 2) and des-(His 146β) hemoglobin at pH 7. K 1 is equal to K T, the oxygen affinity of the T state of hemoglobin, for all these hemoglobins and was increased in all of them when compared to normal hemoglobin at pH 7. This shows that the breakage of the Bohr group salt bridges by increasing pH or specific modification changes K T. Hence the Bohr group salt bridges break on ligation of the T state and are partially responsible for the free energy of cooperativity.