The Effect of Organic Phosphates on the Oxygen Equilibrium Function of Hemoglobin M Iwate

Abstract

Hemoglobin M Iwate is a mutant in which the proximal histidine residue (F8) of the α chain is replaced by tyrosine. One of consequences of this mutation is the oxidation of the iron atom of the α chain, while the β chain of the tetramer remains in the ferrous state, capable of binding oxygen. NMR experiments (1) and X ray analysis (2) have shown that the quaternary structure of this mutant hemoglobin is almost frozen in the T state, no matter whether its normal β hemes are unliganded or liganded, in solution as well as in the crystal. Baldwin has estimated the allosteric equilibrium constant for Hb M Iwate in phosphate buffer as about 10⁵ in favor of the T state (3). It was of interest, therefore, to see the effect of organic phosphate on the oxygen equilibrium function of ferrous β hemes of the mutant.