The oxygen-linked zinc-binding site of human haemoglobin

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Abstract
Zn2+ is known to increase the 02 affinity of human haemoglobin. Previous data suggested that Zn2+ exerts its effect by directly binding to haemoglobin, rather than by competing with or binding to 2,3-bisphosphoglycerate. It was also shown that there are two 02-linked zinc-binding sites in haemoglobin, and that Zn2+ does not significantly alter haemoglobin co-operativity or the alkaline Bohr effect. The effect of Zn2+ on 02 affinity of haemoglobin can also be observed for other haemoglobins as diverse as those of cow and chicken. This paper presents new data on the haemoglobin-zinc interaction for normal haemoglobin, des-His146beta-haemoglobin and N-ethylsuccinimide-haemoglobin of humans. For normal haemoglobin (0.05 mM in tetramers), at 20 degrees C in buffer containing 0.1 M-Cl-, 02-dissociation-curve experiments showed that the addition of 0.4-0.5 mM-ZnS04 did not change the Bohr effect between pH 6.71 and 7.29. Similar experiments, with “zinc-ion buffers”, showed that the value of the Hill coefficient, h, decreased only slightly if the concentration of free Zn2+ was held constant. For N-ethylsuccinimide-haemoglobin, Zn2+ caused less increase in O2 affinity than for normal haemoglobin. These studies, together with data on the equilibrium binding of Zn2+ to oxy-, deoxy- and des-His146beta-haemoglobins, suggest that zinc is chelated in oxyhaemoglobin by at least three amino acids, two of which are histidine-146beta and cysteine-93beta.