Evidence for two different electron transfer pathways in the same enzyme, nitrate reductase A from Escherichia coli.
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 duroquinone 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.
Giordani
Roger
R
Laboratoire de Chimie Bactérienne, Institut Fédératif 'Biologie Structurale et Microbiologie', Centre National de la Recherche Scientifique, Marseille, France.
Buc
Jean
J
D016227:Benzoquinones / Q000737:chemistry / Q000378:metabolism; D003580:Cytochromes / Q000737:chemistry; D004227:Dithionite / Q000737:chemistry / Q000378:metabolism; D004579:Electron Transport; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism*; D006418:Heme / Q000737:chemistry; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D013057:Spectrum Analysis; D024483:Vitamin K 3 / Q000737:chemistry / Q000378:metabolism
D016227:Benzoquinones; D003580:Cytochromes; D029968:Escherichia coli Proteins; D004227:Dithionite; D006418:Heme; D024483:Vitamin K 3; D009565:Nitrate Reductases; D050901:Nitrate Reductase; C034257:duroquinone