Dynamics of carbon monoxide binding to protoheme and heme c octapeptide

Protoheme and heme a octapeptide rebinding of carbon monoxide after photodissociation have been observed at temperatures from 5 to 340 K for times from 2 ~s to 1 ks. Below 80 K, binding is nonexponentia1 in time and CO-concentration independent, above 230 K exponential and the rate is CO-concentration proportional. A model is proposed in which the carbon monoxide, moving from the solvent to the binding site at the ferrous heme iron, encounters two successive barriers. The outer is formed by the solvent, the inner is a property of the heme and probably connected to the transition of the iron from the spin-2 deoxy to the spin-O carbon monoxide state. The temperature dependence of the two processes yields all activation enthalpies and entropies for the two barriers. The nonexponential rebinding observed at low temperatures implies that the inner barrier possesses distributed activation enthalpy and entropy. The enthalpy spectrum and the entropy spread are determined. The spectrum demonstrates that heme exists in many different conformational states. At low temperatures, these states are frozen; above about 230 K, rapid conformational relaxation renders rebinding exponential. Below about 150 K, a new fast process of 10 us duration is observed for heme a octapeptide; its optical spectrum is different from the one corresponding to the transition from 5=2 to S-O. A possible explanation is that it involves an intermediate state-with 5=1. Below 15 K. quantum-mechanical molecular tunneling dominates. The tunneling rate yields the width of the innermost barrier. Earlier experiments on carbon monoxide bin41ng to myoglobin had provided evidence for four barriers. The present results imply that the innermost barrier in myoglobin is caused" by the heme, the outermost by "the solvent. and the two intermediate ones by the globin.U of I OnlyThesi