CHAPTER 15: Structure and Function of Membrane-bound Bacterial Nitric Oxide Reductases

Just as dioxygen is indispensable for lives, the analogous diatomic gas molecule nitric oxide (NO) also plays essential roles in several biological processes as a signaling molecule. However, NO induces cellular damage through reactions with biomolecules. To minimize the cytotoxic effect of NO in the signaling processes, nature utilizes a very sensitive NO receptor, heme-based soluble guanylate cyclase, to effectively capture NO produced by NO synthase. Nature has also developed NO decomposition systems to eliminate the cytotoxicity of NO. In particular, denitrifying bacteria have an effective NO decomposition system, since nitrite reductase continuously produces NO as a process of denitrification, a form of anaerobic respiration. In this chapter, we focus on the NO decomposition system in microbial denitrification, in which membrane-integrated nitric oxide reductase (NOR) catalyzes NO reduction at the heme/non-heme iron binuclear active center, to learn about heme and NO chemistry. On the basis of the crystal structures of NOR, a possible NO reduction mechanism is described. In addition, the structure of NOR is compared with that of evolutionary related oxygen-reducing cytochrome c oxidase to gain insight into the evolution of these respiratory enzymes.