Asymmetric cyanomet valency hybrid hemoglobin, (alpha+CN-beta+CN-)(alpha beta): the issue of valency exchange.

A new framework for hemoglobin cooperativity was proposed by Ackers and colleagues on the basis of the hyper thermodynamic stability and deoxy (T) quaternary structure of one of diliganded deoxy-cyanomet hybrid hemoglobins, (alpha+CN-beta+CN-)(alpha beta), studied by hybridization of the equimolar mixture of deoxyhemoglobin and cyanomethemoglobin through a long (70-100 h) dimer exchange reaction [Daugherty et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 1110-1114]. Recently, we reported that the published hyperstability of (alpha+CN-beta+CN-)(alpha beta) is incorrect due to the occurrence of valency exchange between the heme sites of both parental hemoglobins during the long deoxy incubation [Shibayama et al. (1997) Biochemistry 36, 4375-4381]. We also noted a difficulty in maintaining both anaerobicity and excess free cyanide of the sample during the long incubation, which led to formation of cyanide-unbound aqometheme in the original deoxyhemoglobin resulting from the electron transfer to cyanometheme. This paper is a response to a recent argument against our work [Ackers et al. (1997) Biochemistry 36, 10822-10829]. Ackers et al. have claimed that no appreciable formation of aqomethemoglobin with their methods ensures their sample integrity, based on a supposition that our observed valency exchange may have occurred via aqometheme. In this paper, however, we demonstrate that appreciable (>27%) valency exchange really occurs between deoxy and cyanometheme sites during 72 h incubation under conditions where both anaerobicity and excess free cyanide of the sample solution are maintained by a continuous flow of humidified N2 with HCN. This confirms our view that previous experimental data on (alpha+CN-beta+CN-)(alpha beta) obtained by the long incubations should be subject to reexamination while our earlier estimation of a lower limit of free energy of (alpha+CN-beta+CN-)(alpha beta) (i.e., >/= -10.1 kcal/mol) by a rapid method (35 min) is still valid. We also suggest a possibility that the T quaternary structure of (alpha+CN-beta+CN-)(alpha beta) assigned by Ackers and colleagues using the long incubations is an artifact arising from the valency exchange. These results suggest that the putative mechanistic picture for hemoglobin cooperativity inferred from studies on deoxy-cyanomet hybrids is without foundation.