Kinetics of carbon monoxide binding to Fe(TPP)(Im) and Fe(TPP)(Im-): Evidence regarding protein control of heme reactivity

Protein structure modulation of the reactivity of a heme prosthetic group has been a focus for numerous investigations.¹ For example, ligand binding kinetics of hemoglobin are affected by protein conformation.^1a,2 The CO on-rate for the low-affinity T state is ~20-60-fold less than that for the high-affinity R state.³ One plausible mechanism of this modulation has been advanced by Peisach and collaborators.⁴ It is generally proposed that a protein can induce changes in heme reactivity through structural changes at the proximal imidazole. For hemoglobin, in the low-affinity state the proximal histidine is thought to be in its neutral form with a proton on N-1. The high-affinity form is considered to have a strong hydrogen bond to that proton, which in the limit might be thought of as corresponding to a deprotonated imidazole as the sixth ligand. Such a mechanism also has been invoked to discuss the electronic structure of cytochromes c from different organisms,⁵ and the crystal structures of a number of hemoproteins.⁶.