New Functions for the Ancient Globin Family: Bacterial Responses to Nitric Oxide and Nitrosative Stress

Globin-like oxygen-binding proteins occur in bacteria, yeasts and other fungi, and protozoa. The simplest contain protohaem as sole prosthetic group, but show considerable variation in their similarity to the classical animal globins and plant globins. Flavohaemoglobins comprise a haem domain homologous to classical globins and a ferredoxin-NADP+ reductase (FNR)-like domain that converts the globin into an NAD(P)H-oxidizing protein with diverse reductase activities. In Escherichia coli, the prototype flavohaemoglobin (Hmp) is clearly involved in responses to nitric oxide (NO) and nitrosative stress: (i) the structural gene hmp is upregulated by NO and nitrosating agents; (ii) purified Hmp binds NO avidly, but also converts it to nitrate (aerobically) or nitrous oxide (anaerobically); (iii) hmp mutants are hypersensitive to NO and nitrosative stresses. Here, we review recent advances in E. coli and the growing number of microbes in which globins are known, draw particular attention to the essential chemistry of NO and related reactive species and their interactions with globins, and suggest that microbial globins have additional functions unrelated to 'NO' stresses.