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- W2065656158 abstract "Nitrate reductase from Penicillium chrysogenum catalyzes the FADH2-dependent reduction of nitrate in the absence of excess dithionite. At pH 7.2 (0.1 m potassium phosphate buffer) the FADH2/nitrate reciprocal plot patterns were parallel. Nitrite was competitive with nitrate and an uncompetitive inhibitor with respect to FADH2. FAD was competitive with FADH2 and uncompetitive with nitrate. FMNH2 and riboflavin H2 also served as reducing agents and yielded the same kinetic patterns. The results are consistent with a nonclassical, two-site model in which nitrate and nitrite bind to a site different from the site at which FADH2 and FAD bind. The following results suggest that the site at which FADH2 binds to reduce the enzyme (and at which FAD binds in competition with FADH2 to inhibit the reaction) is not the same high-affinity site to which FAD binds as a cosubstrate in the NADPH + FAD-dependent reaction: (a) The Km for FAD in the NADPH + FAD-dependent reaction at pH 7.2 is only 0.17 μm, while the Ki, for FAD in the FADH2-dependent reaction is 65 μm. (b) FAD is the preferred flavin in the NADPH + oxidized flavin-dependent reaction. (The Km values for FMN and riboflavin are 15 and 41 μm, respectively.) But all three oxidized flavins have about the same Ki in the reduced flavin-dependent reaction (65, 80, and 165 μm, respectively). Furthermore, (c) the Km values for FADH2, FMNH2, and riboflavinH2 in the reduced flavin-dependent reaction are all about the same (ca. 900 μM), (d) V values (specific activities in units per milligram of protein) of the NADPH + oxidized flavin-dependent reaction in 0.1 m phosphate, pH 7.2, are in the order FAD (225) > FMN (150) > riboflavin (60), but the values in the reduced flavin-dependent reaction are in the order FADH2 (100) < FMNH2 (150) < riboflavinH2 (500). Dithionite is known to reduce the cytochrome b component of nitrate reductase. But in the absence of a flavin, the specific activity of the dithionite-dependent reaction is only about 7—extremely low compared to the range of 60–500 for the nucleotide-dependent reactions. This result, together with the intersecting FAD/nitrate reciprocal plot patterns, suggests two possible models for the NADPH + FAD-dependent reactions: (a) FAD binds to two different high-affinity sites. At the “upstream” site, FAD mediates the transfer of electrons from NADPH to the cytochrome b prosthetic group. At the “downstream” site FAD plays either an additional electron transfer role or an activator role promoting the flow of electrons to nitrate. (b) There is only one high-affinity FAD site. FAD must occupy this site (even after the cytochrome is reduced) in order for electrons to flow to nitrate." @default.
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- W2065656158 date "1982-11-01" @default.
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- W2065656158 title "Nitrate reductase from Penicillium chrysogenum: The reduced flavin-adenine dinucleotide-dependent reaction" @default.
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- W2065656158 doi "https://doi.org/10.1016/0003-9861(82)90128-x" @default.
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