TAILIEUCHUNG - Báo cáo khoa học: Evidence for two different electron transfer pathways in the same enzyme, nitrate reductase A from Escherichia coli

Inorder to clarify the role of cytochrome innitrate reductase we have performed spectrophotometric and stopped-flow kinetic studies of reduction and oxidation of the cytochrome hemes with analogues of physiological quinones, using menadione as an analogue of menaquinone and duro-quinone as an analogue of ubiquinone, and comparing the results with those obtained with dithionite. The spectropho-tometric studies indicate that reduction of the cytochrome hemes varies according to the analogue of quinoneused, and in no cases is it complete | Eur. J. Biochem. 271 2400-2407 2004 FEBS 2004 doi Evidence for two different electron transfer pathways in the same enzyme nitrate reductase A from Escherichia coli Roger Giordani and Jean Buc Laboratoire de Chimie Bacterienne Institut Federatif Biologie Structurale et Microbiologie Centre National de la Recherche Scientifique Marseille France In order to clarify the role of cytochrome in nitrate reductase we have performed spectrophotometric and stopped-flow kinetic studies of reduction and oxidation of the cytochrome hemes with analogues of physiological quinones using menadione as an analogue of menaquinone and duro-quinone as an analogue of ubiquinone and comparing the results with those obtained with dithionite. The spectrophotometric studies indicate that reduction of the cytochrome hemes varies according to the analogue of quinone used and in no cases is it complete. Stopped-flow kinetics of heme oxidation by potassium nitrate indicates that there are two distinct reactions depending on whether the hemes were previously reduced by menadiol or by duroquinol. These results and those of spectrophotometric studies of a mutant lacking the highest-potential Fe-S cluster allow us to propose a two-pathway electron transfer model for nitrate reductase A from Escherichia coli. Keywords cytochrome b electron transfer Escherichia coli nitrate reductase A quinone. Nitrate can be used as an electron acceptor for anaerobic growth of Escherichia coli 1 2 . This oxidoreduction is catalysed by nitrate reductase A EC . This enzyme is a membrane-bound complex of three subunits designated a b and c coded by three genes narG narH and narJ which form a single operon 3-5 . The a and b subunits are located on the cytoplasmic side of the membrane and form an ab complex that binds to the membrane by interacting with the c subunit a very hydrophobic protein embedded in the membrane 4 . Each subunit carries a different set of redox centers 5 6 .

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