Carbon monoxide binding properties of hemoglobin M Iwate.

Abstract

The kinetic and equilibrium CO binding properties of hemoglobin (Hb) M Iwate (alpha2 87 His leads to Tyr beta2) have been investigated. The results show that the alpha(Met) beta2 (CO) tetramer of this protein has a low affinity for CO, as indicated by the stopped-flow and flash-photolysis kinetic, as well as the CO binding equilibrium, measurements. However, it has been found that the phosphate-free alpha2(Met)beta2(CO) tetramer does tend to dimerize extensively (K4.2 = 55 muM). The high-affinity forms seen in earlier kinetic measurements may be explained by this fact. When dimers are accounted for in the functional studies, the results show that the tetramer binds CO noncooperatively either with or without the allosteric cofactor, inositol hexaphosphate (IHP). IHP appears to influence the functional properties of a solution of Hb M Iwate by stabilizing the tetrameric state of aggregation, thereby greatly reducing the population of high-affinity dimers. When the CO "off" rate with IHP present (0.23 s-1) and the CO "on" rate to the tetramer either with or without IHP (1.9 X 10(5) M-1 s-1) at 25 degrees C are sued to calculate the equilibrium constant, the value obtained (8.3 X 10(5) M-1 s-1) is similar to that in equilibrium binding measurements on the phosphate-free tetramer (9.5 X 10(5) M-1) estimated from the observed P 1/2 value at 0.48 mM total heme concentration. By showing that dimers account for the high-affinity component seen in earlier kinetic experiments with Hg M Iwate, we can now more strongly suggest that cooperative CO binding to this tetramer is minimal or absent, with both of the active beta-hemes presenting a very low affinity.