Nitric Oxide Production, Damage and Management During Anaerobic Nitrate Reduction to Ammonia
Bacteria encounter the free radical gas, nitric oxide (NO), from various sources. It is an extremely reactive gas that is rapidly oxidised by molecular oxygen, but is far more stable in anaerobic environments. This chapter reviews how bacteria, especially enteric bacteria, generate and protect themselves from NO formed as a side product of anaerobic nitrate reduction to ammonia. Two independent pathways, one in the cytoplasm and the other in the periplasm, reduce nitrate to ammonia. NO is generated from nitrite mainly by the cytoplasmic nitrate reductase, NarG, with smaller contributions from other, less active pathways. Although only sub-micromolar concentrations of NO accumulate in the bacterial cytoplasm, up to 1000-fold higher concentrations were used in most previous laboratory experiments and oxygen was freely available. Chemical damage has been misinterpreted as biochemical repair, or even as physiologically relevant signal transduction. A mechanism is proposed to explain how bacteria limit [NO] to low nM levels. It is based upon our demonstration that the hybrid cluster protein, Hcp, is a physiologically important high-affinity but low-activity NO reductase, and release of NO from nitrosylated proteins by the RIC (YtfE or DnrN protein) repairs iron centres damaged by nitrosative stress.