The role of entropy in the discrimination between CO and O 2 in myoglobin

Abstract

Using stopped-flow rapid mixing and flash photolysis techniques, the dissociation rate coefficients of horse carbonmonoxy myoglobin (hMbCO) and oxygenated myoglobin (hMbO2) in aqueous solution have been determined as a function of temperature between 274 and 342 K. From the Arrhenius plot, an activation enthalpy for dissociation of 74 kJ/mol was obtained for both ligands. The pronounced kinetic differences arise from markedly different pre-exponentials. We compare the Arrhenius parameters with those of the association reaction, as measured at cryogenic temperatures. In our analysis we conclude that the entropy loss upon binding of O2 is twice as large as that for CO. Taking reasonable estimates for the frequency factor, the transition state entropy in hMbO2 is located roughly half way in between the entropies of the bound and unbound states. By contrast, the entropy of the transition state in hMbCO appears to be identical to that of the bound state. Possible structural reasons for the different behavior are discussed.

Room Temperature Carbonylation of Iron–Phthalocyanines Adsorbed on a Single Crystal Metal Surface: An in Situ SFG Investigation at Near-Ambient Pressure

“…A lower initial sticking coefficient may be plausible, and pre-exponential factors may vary within a range spanning over several orders of magnitude. Indeed, kinetic modeling of the heme pocket in myoglobin yields a classical attempt frequency for CO of 10 12 Hz that is reduced down to 10 9 Hz when considering the CO motion to be synchronized with the iron displacement from the heme plane (reducing factor of 150), the CO orientation, and the nonadiabaticity of the electronic spin change in Fe (reducing factor of 6). , With these latter assumptions, we obtain a Fe–CO binding energy value of 0.10 ± 0.05 eV, thus confining our estimate in the 0.1–0.3 eV range. Pre-exponential factors and entropy contribute also to the discrimination between preferential adsorption of CO with respect to the O 2 ligand in natural biomolecules .…”

“…Indeed, kinetic modeling of the heme pocket in myoglobin yields a classical attempt frequency for CO of 10 12 Hz that is reduced down to 10 9 Hz when considering the CO motion to be synchronized with the iron displacement from the heme plane (reducing factor of 150), the CO orientation, and the nonadiabaticity of the electronic spin change in Fe (reducing factor of 6). , With these latter assumptions, we obtain a Fe–CO binding energy value of 0.10 ± 0.05 eV, thus confining our estimate in the 0.1–0.3 eV range. Pre-exponential factors and entropy contribute also to the discrimination between preferential adsorption of CO with respect to the O 2 ligand in natural biomolecules . With evident caution due to the different chemical and structural environments, it is noteworthy to mention that while we get a saturation CO partial pressure of 10 mbar, in the case of hemoglobin and myoglobin molecules the working O 2 pressure is of the order of 10 mbar at 300 K, while CO is more strongly bound, yielding saturation partial pressures of the order of 0.1–1 mbar…”