Structure and Oxygen Affinity of Crystalline of DesArg141α Human Hemoglobin A in the T State

Abstract

The correlation of a protein structure determined crystallographically to its functional properties determined in solution can be an extremely complex problem due to potential differences of protein conformational flexibility in the two physical states. A more direct approach to the correlation of structure with function is to examine both the structure and the function of a protein in the same crystalline environment. In this paper, the structural and functional properties of T state desArg hemoglobin (human hemoglobin modified by removal of the α-chain COOH-terminal residue, Arg141α) have been studied in the same crystal form by high resolution X-ray diffraction methods and by polarized absorption microspectrophotometry. Specifically, the crystal structure of deoxygenated desArg human hemoglobin has been refined at a 2.1 Å resolution using crystals grown at low salt concentration from solutions of polyethylene glycol. The loss of Arg141α and all of the salt bridges in which it participates is associated with subtle structural perturbations of the α-chains which include an increase in the conformational flexibility of both the NH 2and COOH-terminal peptides. Although the heme pockets appear unchanged and even the side-chain of Tyr140 is oriented nearly as in HbA, the functional characterization by microspectrophotometric measurements indicates that crystals of desArg hemoglobin bind oxygen with an affinity which is roughly 15-fold greater than that of crystals of human hemoglobin A. There is no alkaline Bohr effect or effect of chloride ions, but an acid Bohr effect is observed. The oxygen affinities measured along two principal axes of the crystals differ by 25% indicating heterogeneity in the affinities of the oxygen binding sites. This finding and the measured Hill coefficient of unity suggest significant cooperativity in the binding of oxygen in these crystals. The origins of the observed heterogeneity and the implied cooperativity are unknown.