Binding of carbon monoxide to isolated hemoglobin chains

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Abstract
Binding of CO to the separated .alpha. and .beta. chains of Hb [human], with without bound p-mercuribenzoate, was measured at temperatures of 5-340.degree. K for times 2 .mu.s to 1 ks using flash photolysis. All 4 proteins exhibited 2 different rebinding processes. The data were interpreted by a model in which the CO, moving from the solvent to the binding site at the ferrous heme Fe, encounters 3 barriers. The temperature dependences of the 3 processes yielded activation enthalpies and entropies for the 3 barriers for all 4 proteins. Binding at temperatures below about 200.degree. K was nonexponential, implying that the innermost barrier had a distribution of activation enthalpies. The distributions for the 4 proteins were determined. At temperatures below 30.degree. K, the CO binding rates approached finite low-temperature limits; binding proceeded by quantum-mechanical tunneling. Invoking a simple model, the widths of the innermost barriers were extracted from the measured tunneling rates. The experimental parameters were correlated with structural features of the Hb chains and compared with previously published data on myoglobin and protoheme. A correlation was established between the height of the innermost barrier and the equilibrium CO pressure.

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