Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0016632 (Fox)
 1,461 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

NADPH-sulfite reductase flavoprotein (SiR-FP) was purified from a Salmonella typhimurium cysG strain that does not synthesize the hemoprotein component of the sulfite reductase holoenzyme. cysJ, which codes for SiR-FP, was cloned from S. typhimurium LT7 and Escherichia coli B, and both genes were sequenced. Physicochemical analyses and deduced amino acid sequences indicate that SiR-FP is an octamer of identical 66-kDa peptides and contains 4 FAD and 4 FMN per octamer. Potentiometric titrations of SiR holoenzyme, SiR-FP, and FMN-depleted SiR-FP yielded the following redox potentials for the prosthetic groups at pH 7.7: E'1 (FMNH./FMN) = -152 mV; E'2 (FMNH2/FMNH.) = -327 mV; E'3 (FADH./FAD) = -382 mV; E'4 (FADH2/FADH.) = -322 mV. Microcoulometric titration of SiR-FP at 25 degrees C yielded data which were in full agreement with these potentials. Spectroscopic and catalytic studies of native SiR-FP and of SiR-FP depleted of FMN support the following electron flow sequence: NADPH----FAD----FMN. FMN can then contribute electrons to the hemoprotein component of sulfite reductase, as well as to cytochrome c and various diaphorase acceptors. The FMN is postulated to cycle between the FMNH2 and FMNH. oxidation states during catalysis; in this sense SiR-FP shares a catalytic mechanism with NADPH-cytochrome P-450 oxidoreductase. SiR-FP domains involved in binding FMN, FAD, and NADPH are proposed from amino acid sequence homologies with Desulfovibrio vulgaris flavodoxin (Dubourdieu, M., and Fox, J.L. (1977) J. Biol. Chem. 252, 1453-1463) and spinach ferredoxin-NADP+ oxidoreductase (Karplus, P.A., Walsh, K.A., and Herriott, J. R. (1984) Biochemistry 23, 6576-6583). Comparison of the deduced amino acid sequences of SiR-FP and NADPH-cytochrome P-450 oxidoreductase (Porter, T. D., and Kasper, C.B. (1985) Proc. Natl. Acad. Sci. U. S.A. 82, 973-977) also showed identities that suggest these two proteins are descended from a common precursor, which contained binding regions for both FMN and FAD.
 ...
PMID:Characterization of the flavoprotein moieties of NADPH-sulfite reductase from Salmonella typhimurium and Escherichia coli. Physicochemical and catalytic properties, amino acid sequence deduced from DNA sequence of cysJ, and comparison with NADPH-cytochrome P-450 reductase. 255 Apr 23

Mercuric reductase contains FAD and a redox-active disulfide which is reduced to a thiol/thiolate pair in two-electron reduced enzyme (EH2) (Fox, B. and Walsh, C.T. (1982) J. Biol. Chem. 257, 2498-2503). A charge transfer interaction between the thiolate and oxidized FAD gives EH2 a characteristic absorption spectrum, very similar to that found with other flavoprotein disulfide oxidoreductases. We have examined the reaction of EH2 with HgCl2 (+/- mercaptoethanol) in stopped-flow kinetic and static titration experiments. In the absence of mercaptoethanol, reaction of EH2 with HgCl2 yields a final spectrum which is indistinguishable from that of oxidized enzyme. The nature of the final species was examined by titration of enzyme thiols with 5,5'-dithiobis-2,2'-nitrobenzoic acid under denaturing conditions in the presence of NaI to displace any Hg(II) bound to enzyme thiols. These studies demonstrate that EH2 tightly complexes Hg(II) with its active site thiols, but is incapable of reducing Hg(II) to Hg0. For the latter reaction to occur, additional reducing equivalents are required. In catalysis, the enzyme must first be reduced to EH2 after which it cycles between EH2 and EH2 X NADPH forms. This is in contrast to other flavoprotein disulfide oxidoreductases which cycle between Eox and EH2 forms in catalysis (Williams, C. H., Jr. (1976) in The Enzymes (Boyer, P. D., ed) 3rd Ed., Vol. 13, pp. 89-173, Academic Press, New York). With mercuric reductase, exogenous thiols are required for catalytic reduction of Hg(II) to Hg0. We have shown that this is due to prevention or reversal of formation of an abortive complex of Hg(II) with the thiol/thiolate pair of EH2.
 ...
PMID:Two-electron reduced mercuric reductase binds Hg(II) to the active site dithiol but does not catalyze Hg(II) reduction. 352 63