Chapter 4 - Nitric Oxide Reductase: Structural Variations and Catalytic Mechanism

Abstract

This chapter deals with the structural and mechanistic information on nitric oxide reductases (NORs) from biochemical and biophysical studies (kinetics, electron paramagnetic resonance , resonance raman , Fourier transform infrared , and magnetic circular dichroism) , from sequence comparison between NORs and cytochrome oxidases (CcOs), and from the crystal structures of CcOs. The chapter illustrates the close evolutionary and functional relation between NORs and CcOs by the occurrence in both the branches of cytochrome c- and quinol-linked activities and by the observation that NORs can reduce O2 and CcOs NO. NO reductases (specifically cNOR) are structurally and functionally most closely related to the cbb3-type oxidases. Presently, three different bacterial NORs have been characterized, cNOR, qNOR, and qCuANOR. CcOs reduce O2 to H2O in a highly complex reaction in which protons are pumped across the mitochondrial membrane. Structural variations in NORs cover cNOR, qNOR, and qCuANOR. From the description of the oxidized and reduced state of the enzyme by various spectroscopic techniques, the relative binding affinities of heme b3 and FeB for CO and pre-steady state kinetic analysis, a catalytic cycle is proposed. cNORs are like CcOs, capable of catalyzing the four-electron reduction of O2 to water. The major difference between NORs and CcOs is apparently in the conservation of redox energy by the means of creating an electrochemical proton gradient. NORs do not conserve energy whereas CcOs have evolved to conserve (almost) all. This observation remains enigmatic from a biochemical and an evolutionary point of view.