Structure and oxygen affinity of crystalline des-His-146β human hemoglobin in the T state

To correlate directly structure with function, the oxygen affinity and the three-dimensional structure of crystals of the T quaternary state of des- His-146β human hemoglobin have been determined by polarized absorption microspectrophotometry and x-ray diffraction crystallography. In des-His- 146β, the COOH-terminal histidine residues of the β chains of hemoglobin A have been removed. Oxygen binding to crystalline des-His hemoglobin is non- cooperative and independent of pH. The oxygen affinity is 1.7-fold greater than that of the crystalline state of hemoglobin A. Removal of His-146β results in a small movement of the truncated COOH-terminal peptide and in a very small change in quaternary structure. Previously, similar studies on T state crystals of des-Arg-141α hemoglobin showed that removal of the COOH termini of the α chains results in much larger effects on oxygen affinity and on quaternary structure. Kinetic studies in solution reveal that at pH 7.0, the rates of CO combination with deoxygenated des-His-146β in the absence and presence of inositol hexaphosphate are 2.5- and 1.3-fold, respectively, more rapid than for hemoglobin A. The values for des-Arg are 7.6- and 3.9-fold. The properties of the T state of hemoglobin both in the crystal and in solution are influenced to a greater degree by the interactions associated with Arg-141α than those associated with His-146β.