Flavohemoglobin Hmp affords inducible protection for Escherichia coli respiration, catalyzed by cytochromes bo' or bd, from nitric oxide

Abstract

Respiration of Escherichia coli catalyzed either by cytochrome bo' or bd is sensitive to micromolar extracellular NO; extensive, transient inhibition of respiration increases as dissolved oxygen tension in the medium decreases. At low oxygen concentrations (25-33 μM), the duration of inhibition of respiration by 9 μM NO is increased by mutation of either oxidase. Respiration of an hmp mutant defective in flavohemoglobin (Hmp) synthesis is extremely NO-sensitive (I50 about 0.8 μM); conversely, cells pre-grown with sodium nitroprusside or overexpressing plasmid-borne hmp+ are insensitive to 60 μM NO and have elevated levels of immunologically detectable Hmp. Purified Hmp consumes O2 at a rate that is instantaneously and extensively (> 10-fold) stimulated by NO due to NO oxygenase activity but, in the absence of NO, Hmp does not contribute measurably to cell oxygen consumption. Cyanide binds to Hmp (K(d) 3 μM). Concentrations of KCN (100 μM) that do not significantly inhibit cell respiration markedly suppress the protection of respiration from NO afforded by Hmp and abolish NO oxygenase activity of purified Hmp. The results demonstrate the role of Hmp in protecting respiration from NO stress and are discussed in relation to the energy metabolism of E. coli in natural O2-depleted environments.