A proton nuclear magnetic resonance investigation of histidyl residues in sickle hemoglobin

Solution-active structural alterations in liganded hemoglobins C (beta6 Glu --> Lys) and S (beta6 Glu --> Val)

Based upon existing crystallographic evidence, HbS, HbC, and HbA have essentially the same molecular structure. However, important areas of the molecule are not well defined crystallographically (e.g. the N-terminal nonhelical portion of the alpha and beta chains), and conformational constraints differ in solution and in the crystalline state. Over the years, our laboratory and others have provided evidence of conformational changes in HbS and, more recently, in HbC. We now present data based upon allosteric perturbation monitored by front-face fluorescence, ultraviolet resonance Raman spectroscopy, circular dichroism, and oxygen equilibrium studies that confirm and significantly expand previous findings suggesting solution-active structural differences in liganded forms of HbS and HbC distal to the site of mutation and involving the 2,3-diphosphoglycerate binding pocket. The liganded forms of these hemoglobins are of significant interest because HbC crystallizes in the erythrocyte in the oxy form, and oxy HbS exhibits increased mechanical precipitability and a high propensity to oxidize. Specific findings are as follows: 1) differences in the intrinsic fluorescence indicate that the Trp microenvironments are more hydrophobic for HbS > HbC > HbA, 2) ultraviolet resonance Raman spectroscopy detects alterations in Tyr hydrogen bonding, in Trp hydrophobicity at the alpha1beta2 interface (beta37), and in the A-helix (alpha14/beta15) of both chains, 3) displacement by inositol hexaphosphate of the Hb-bound 8-hydroxy-1,3,6-pyrenetrisulfonate (the fluorescent 2,3-diphosphoglycerate analog) follows the order HbA > HbS > HbC, and 4) oxygen equilibria measurements indicate a differential allosteric effect by inositol hexaphosphate for HbC approximately HbS > HbA.

Conformational changes in oxyhemoglobin C (Glu beta 6-->Lys) detected by spectroscopic probing

Hemoglobin C (Glu beta 6-->Lys) shares with hemoglobin S (Glu beta 6-->Val) the site of mutation, but with different consequences: deoxyHbS forms polymers, whereas oxyHbC readily forms crystals. The molecular mechanism for this property of oxyHbC is unknown. Since no detailed oxyHbC crystal structural information exists, spectroscopic probing is used in this study to investigate possible solution-phase conformational changes in HbC compared with HbA. Intrinsic fluorescence combined with UV resonance Raman data demonstrate a weakening of the Trp beta 15-Ser beta 72 hydrogen bond that most likely leads to a displacement of the A helix away from the E helix.

A proton nuclear magnetic resonance investigation of human hemoglobin A2. Implications on the intermolecular contacts in sickle hemoglobin fibers and on the Bohr effect of human normal adult hemoglobin

High-resolution proton nuclear magnetic resonance spectroscopy at 300 and 600 MHz has been used to investigate the conformation of a minor hemoglobin component of human blood, hemoglobin A2 (alpha 2 delta 2), in solution. We have found that (i) the replacement of the beta chains by the delta chains in hemoglobin A2 conserves the alpha 1 delta 2 interface but slightly perturbs the alpha 1 delta 1 interface, and (ii) one surface histidine residue in the deoxy form and one in the carbonmonoxy form of hemoglobin A2 have local conformations and/or electrostatic environments which are different from the corresponding ones in human normal adult hemoglobin. By comparing the proton nuclear magnetic resonance titration of individual histidine residues in hemoglobin A2 and in human normal adult hemoglobin, we can conclude that in human normal adult hemoglobin, both beta 116 and beta 117 histidine residues are titratable in both the deoxy and the carbonmonoxy forms. Thus, these two histidine residues can contribute to the Bohr effect of human normal adult hemoglobin. The present nuclear magnetic resonance data on hemoglobin A2 and those previously obtained in our laboratory on sickle hemoglobin suggest that the antisickling property of hemoglobin A2 does not originate from an alteration of the intermolecular contact site at the beta 6 position, but involves additional amino-acid residues which are different in the beta and delta chains. We have found that the replacement of the beta 116 and beta 117 histidine residues in the delta chains does not play a significant role in the antisickling effect of hemoglobin A2 and, thus, these amino-acid residues do not participate in the intermolecular interactions responsible for the polymerization of sickle hemoglobin.