Kinetic Control of O2 Reactivity in H-Nox Domains

Abstract

Resonance Raman, transient absorption and vibrational coherence spectroscopies are used to investigate the mechanisms of NO and O2 binding to Tt-HNOX and its P115A mutant. Vibrational Coherence spectra of the oxy-complexes provide a clear evidence for the activation of an iron-histidine mode around 217 cm−1 following photoexcitation, indicating that O2 dissociates in both proteins. The quantum yield of O2 photolysis is very low, particularly in the wild type. Geminate recombination of O2 and NO in both proteins is very fast and highly efficient. This indicates that the distal heme pocket in these proteins is tightly packed, and forms an efficient trap, preventing the bound ligand from escaping into the solvent upon thermal dissociation. This, along with the the stabilization of the Fe-O2 bond, explains the unusually high O2 affinity in Tt H-NOX and its P115A mutant.