THE ROLE OF ENTROPY IN THE DISCRIMINATION BETWEEN CO AND O-2 IN MYOGLOBIN

Using stopped-flow rapid mixing and flash photolysis techniques, the d issociation rate coefficients of horse carbonmonoxy myoglobin (hMbCO) and oxygenated myoglobin (hMbO(2)) in aqueous solution have been deter mined as a function of temperature between 274 and 342 K. From the Arr henius plot, an activation enthalpy for dissociation of 74 kJ/mol was obtained for both ligands. The pronounced kinetic differences arise fr om markedly different pre-exponentials. We compare the Arrhenius param eters with those of the association reaction, as measured at cryogenic temperatures. In our analysis we conclude that the entropy loss upon binding of O-2 is twice as large as that for CO. Taking reasonable est imates for the frequency factor, the transition state entropy in hMbO( 2) is located roughly half way in between the entropies of the bound a nd unbound states. By contrast, the entropy of the transition state in hMbCO appears to be identical to that of the bound state. Possible st ructural reasons for the different behavior are discussed.