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

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 muM), the duration of inhibition of respiration by 9 muM NO is increased by mutation of either oxidase. Respiration of an hmp mutant defective in flavohemoglobin (Hmp) synthesis is extremely NO-sensitive (I(50) about 0.8 muM); conversely, cells pre-grown with sodium nitroprusside or overexpressing plasmid-borne hmp(+) are insensitive to 60 muM NO and have elevated levels of immunologically detectable Hmp. Purified Hmp consumes O(2), at a rate that,is instantaneously and extensively (>10-fold) stimulated by MO due to NO oxygenase activity but, in the absence of NO, limp does not contribute measurably to cell oxygen consumption. Cyanide binds to Hmp (K(d) 3 muM). Concentrations of KCN (100 muM) 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 limp in protecting respiration from NO stress and are discussed in relation to the energy metabolism off. coli in natural O(2)-depleted environments.