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-146 beta human hemoglobin have been determined by polarized absorption microspectrophotometry and x-ray diffraction crystallography, In des-His-146 beta, the COOK-terminal histidine residues of the beta 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-146 beta 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-141 alpha hemoglobin showed that removal of the COOH termini of the cu 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-146 beta 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-141 alpha than those associated with His-146 beta.