NO Reductase from Bacillus azotoformans Is a Bifunctional Enzyme Accepting Electrons from Menaquinol and a Specific Endogenous Membrane-Bound Cytochrome c551
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- 作者：Suharti ; Hendrik A. Heering ; and Simon de Vries
- 刊名：Biochemistry
- 出版年：2004
- 出版时间：October 26, 2004
- 年：2004
- 卷：43
- 期：42
- 页码：13487 - 13495
- 全文大小：160K
- 年卷期：v.43,no.42(October 26, 2004)
- ISSN：1520-4995
文摘
Bacillus azotoformans is a Gram-positive denitrifying soil bacterium, which is capable ofrespiring nitrate, nitrite, nitric oxide, and nitrous oxide under anaerobic conditions. It contains a uniquemenaquinol-dependent nitric oxide reductase (qCuANOR) with a CuA center in its small subunit. TheqCuANOR exhibits menaquinol-dependent NO reductase activity, whereas reduced horse heart cytochromec was inactive. Here we describe the purification of three membrane-bound c cytochromes from B.azotoformans. Their apparent molecular masses on SDS-PAGE are approximately 11 kDa. At neutralpH, these c cytochromes are negatively charged and the Em for all is close to 150 mV. Only one of thesec cytochromes, which exhibits an 
ages/gifchars/alpha.gif" BORDER=0>-band maximum at 551 nm, acts as a direct electron donor to qCuANOR. Further investigation demonstrated that this cytochrome c551 possesses two lipoyl moieties, whichpresumably function to anchor it to the membrane. Steady-state kinetic studies reveal that cytochromec551 is a noncompetitive inhibitor of NO reduction when menaquinol is used as an electron donor. Thisfinding points to the presence of two different electron donation pathways in qCuANOR. The ability ofqCuANOR to accept electrons from both menaquinol and cytochrome c551 might be related to the regulationof the rate of NO reduction especially as a defense mechanism of B. azotoformans against the toxicity ofNO. Growth experiments in batch culture indeed show that B. azotoformans is highly NO tolerant, incontrast to, for example, Paracoccus denitrificans that has a monofunctional cytochrome c-dependentNOR. We propose that the menaquinol pathway, which has a 4-fold greater maximal activity than thepathway via cytochrome c551, is used for NO detoxification, whereas electron donation via the endogenouscytochrome c involves the cytochrome b6f complex serving the bioenergetic needs of the organism.