Hydrogen exchange kinetics of human hemoglobins. The pH dependence of solvent accessibility in cyanomet-, oxy-, and deoxyhemoglobin.

B.E Hedlund,P.E Hallaway,B.E Hallaway,E.S Benson,A Rosenberg

doi:10.1016/s0021-9258(17)34858-5

Abstract

The hydrogen exchange kinetics of human oxy-, deoxy-, and cyanomethemoglobin have been measured as a function of pH by the tritium tracer method. At 5 degrees C and in phosphate buffer both liganded and unliganded forms of ferrohemoglobin exhibit deviations from the regular pH dependence of exchange that is characteristic of cyanomethemoglobin. In oxyhemoglobin, the deviation from the normal exchange pattern is centered at pH 7.4 and is in the direction of increased exchange or solvent accessibility. The effect in deoxyhemogloin, while occurring at the same pH and being of the same order of magnitude, is in the opposite direction, thus suggesting a pH-induced conformational transition leading to a less accessible structure. The width of these pH-induced deviations in solvent accessibility is approximately 1 pH unit in both cases. We propose a model in which specific interactions between charged groups in both froms of ferrohemoglobin account for these deviations.

Highlights

The hydrogen exchange kinetics of human oxy, deoxy, and cyanomethemoglobin have been measured as a function of pH by the tritium tracer method
The course of acquiring knowledge of the functional properties of the hemoglobin molecule has been dominated by investigations relating to structural and energetic features of ligation, homo- and heterotropic effects, the oxidative state and position of the iron atom within the plane of the heme, and other conformational parameters
A major difficulty in illustrating experimental data points obtained from hydrogen exchange kinetics is the fact that rather extended time periods must be used in order to follow a reasonable fraction of exchangeable hydrogens

Summary

Introduction

The hydrogen exchange kinetics of human oxy-, deoxy-, and cyanomethemoglobin have been measured as a function of pH by the tritium tracer method. The effect in deoxyhemoglobin, while occurring at the same pH and being of the same order of magnitude, is in the opposite direction, suggesting a pa-induced conformational transition leading to a less accessible structure. A very large fraction of hydrogen exchange data on a variety of proteins has been comparative, and theoretical efforts have been aimed primarily toward an understanding of the kinetic parameters as such, rather than the dynamic behavior of the protein molecule. These studies include the deuterium exchange ex-.

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