Reactivity of cyanate with valine-1 (alpha) of hemoglobin. A probe of conformational change and anion binding

Abstract

The 3-fold increase in the carbamylation rate of Val-1 (alpha) of hemoglobin upon deoxygenation described earlier is now shown to be a sensitive probe of conformational change. Thus, whereas this residue in methemoglobin A is carbamylated at the same rate as in liganded hemoglobin, upon addition of inositol hexaphosphate its carbamylation rate is enhanced 30% as much as the total change in the rate between the CO and deoxy states. For CO-hemoglobin Kansas in the presence of the organic phosphate, the relative increase in the carbamylation rate of this residue is about 50%. These results indicate that methemoglobin A and hemoglobin Kansas in the presence of inositol hexaphosphate do not assume a conformation identical with deoxyhemoglobin but rather form either a mixture of R and T states or an intermediate conformation in the region around Val-1 (alpha). Studies on the mechanism for the rate enhancement in deoxyhemoglobin suggest that the cyanate anion binds to groups in the vicinity of Val-1 (alpha) prior to proton transfer and carbamylation of this NH2-terminal residue. Thus, specific removal with carboxypeptidase B of Arg-141 (alpha), which is close to Val-1 (alpha) in deoxyhemoglobin, abolishes the enhancement in carbamylation. Chloride, which has the same valency as cyanate, is a better competitive inhibitor of the carbamylation of deoxyhemoglobin (Ki = 50 mM) compared with liganded hemoglobin. Nitrate and iodide are also effective inhibitors of the carbamylation of Val-1 (alpha) of deoxyhemoglobin (Ki = 35 mM); inorganic phosphate, sulfate, and fluoride are poor competitive inhibitors. The change in pKa of Val-1 (alpha) upon deoxygenation may be due to its differential interaction with chloride.