Temperature-dependent heme kinetics with nonexponential binding and barrier relaxation in the absence of protein conformational substates

Abstract

We present temperature-dependent kinetic measurements of ultrafast diatomic ligand binding to the “bare” protoheme (L₁-FePPIX-L₂, where L₁ = H₂O or 2-methyl imidazole and L₂ = CO or NO). We found that the binding of CO is temperature-dependent and nonexponential over many decades in time, whereas the binding of NO is exponential and temperature-independent. The nonexponential nature of CO binding to protoheme, as well as its relaxation above the solvent glass transition, mimics the kinetics of CO binding to myoglobin (Mb) but on faster time scales. This demonstrates that the nonexponential kinetic response observed for Mb is not necessarily due to the presence of protein conformational substates but rather is an inherent property of the solvated heme. The nonexponential kinetic data were analyzed by using a linear coupling model with a distribution of enthalpic barriers that fluctuate on slower time scales than the heme–CO recombination time. Below the solvent glass transition (T _g ≈ 180 K), the average enthalpic rebinding barrier for H₂O-PPIX-CO was found to be ≈1 kJ/mol. Above T _g, the barrier relaxes and is ≈6 kJ/mol at 290 K. Values for the first two moments of the heme doming coordinate distribution extracted from the kinetic data suggest significant anharmonicity above T _g. In contrast to Mb, the protoheme shows no indication of the presence of “distal” enthalpic barriers. Moreover, the wide range of Arrhenius prefactors (10⁹ to 10¹¹ s⁻¹) observed for CO binding to heme under differing conditions suggests that entropic barriers may be an important source of control in this class of biochemical reactions.