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Siroheme as an active catalyst in sulfite reduction

Y Seki, N Sogawa, M Ishimoto

    Journal of Biochemistry
    |November 1, 1981
    PubMed
    Summary

    Siroheme, a key component of sulfite reductase, effectively catalyzes sulfite reduction to thiosulfate and sulfide. Its activity is pH-dependent and distinct from other iron-containing porphyrins.

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    Area of Science:

    • Biochemistry
    • Enzymology
    • Metalloporphyrins

    Background:

    • Sulfite reductase (SR) is a crucial enzyme in sulfur metabolism.
    • Siroheme is the prosthetic group responsible for the catalytic activity of SR.
    • Understanding siroheme's catalytic mechanism is vital for sulfur compound transformations.

    Purpose of the Study:

    • To investigate the catalytic activity of purified siroheme in sulfite reduction.
    • To determine the optimal conditions for siroheme-mediated sulfite reduction.
    • To compare the reactivity of siroheme with other iron-containing compounds.

    Main Methods:

    • Siroheme extraction from yeast sulfite reductase using acetone/HCl.
    • Purification of siroheme via column chromatography.
    • Enzymatic assays coupled with hydrogen-hydrogenase-methyl viologen system to measure sulfite reduction.
    • Kinetic analysis to determine apparent Km for sulfite.
    • Comparative studies with sirohydrochlorin and inorganic iron.

    Main Results:

    • Purified siroheme catalyzed the reduction of sulfite to thiosulfate and sulfide.
    • Siroheme activity increased as pH decreased from 7 to 4, with an apparent Km of 50 mM for sulfite.
    • Inorganic iron and 2,2'-bipyridine did not significantly affect siroheme's sulfite-reducing activity.
    • Siroheme reduced hydroxylamine more rapidly than sulfite, with activity increasing from pH 6 to 9.
    • Siroheme from Chromatium vinosum also demonstrated sulfite-reducing activity.

    Conclusions:

    • Siroheme is a potent catalyst for sulfite reduction, with optimal activity at acidic pH.
    • The catalytic mechanism of siroheme in sulfite reduction is distinct from other iron porphyrins.
    • Siroheme's ability to reduce both sulfite and hydroxylamine highlights its versatile role in biological redox reactions.

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