Alteration of T-state binding properties of naturally glycated hemoglobin, HbA1c

The thermodynamic and kinetic properties of the most abundant glycated hemoglobin in human blood, HbA1c, have been studied in detail. They display significant differences as compared to normal hemoglobin, HbA0, in that (1) the shape of the oxygen binding curve of HbA1c in the Hill plot is markedly asymmetrical, with a lower asymptote extending up to approximately 40% oxygen saturation, and the oxygen affinity of the T state being tenfold higher than in HbA0; (2) oxygen pulse experiments on HbA1c show a slower rate of ligand dissociation (k = 25 s-1) even at low levels of oxygen saturation, where the T state is largely predominant; (3) kinetics of CO combination to deoxy HbA1c followed by means of stopped-flow experiments reveal the presence of a quickly reacting component, whose fraction increases upon dilution of hemoglobin. These results show that in contrast to what has been stated by other authors, HbA1c displays functional properties markedly different from HbA0. Analysis indicates that glycation of human hemoglobin affects the T quaternary structure, bringing about a more "relaxed" T state and leading to preferential binding to one type of chain (which is unaffected by chloride ions).