Contribution of arginine (HC3) 141α to the Bohr effect of the fourth binding step in the reaction of ligand with human hemoglobin

A few years ago we reported that histidine (HC3) 146β plays a major role in the pH‐dependent propeties of the R‐state of human hemoglobin, accounting for close to 50% of the R‐state Bohr effect. We have extended these studies by examining the role of arginine 141α, another group known to affect the overall Bohr effect. We have compared the pH dependencies of the rate constants for the dissociation and combination of the fourth carbon monoxide molecule, I₄ and I′₄, respectively, for native hemoglobin A (HbA) and a control reconstututed HbA, and des‐(Arg 141α) HbA, the hemoglobin molecule resulting from the enzymatic removal of the C–terminal arginine of the α‐chain of human Hb. From these kinetic contants the pH dependence of L₄, by about 80% between pH 6 and 8, where the aldkaline Bohr effect normally occurs, The sum of the effects of the removal of His 146β and of Arg 141α is grater than 100%. This suggests that at least one of these modifications altrs the contrubutions of other residues of this Bohr effect.