Siroheme sulfite reductase isolated from Chromatium vinosum. Purification and investigation of some of its molecular and catalytic properties

Abstract

Cells of the phototrophic bacterium Chromatium vinosum strain D were shown to contain a siroheme sulfite reductase after autotrophic growth in a sulfide/bicarbonate medium. The enzyme could not be detected in cells grown heterotrophically in a malate/sulfate medium. Siroheme sulfite reductase was isolated from autotrophic cells and obtained in an about 80% pure preparation which was used to investigate some molecular and catalytic properties of the enzyme. It was shown to consist of two different types of subunits with molecular weights of 37,000 and 42,000, most probably arranged in an α₄β₄-structure. The molecular weight of the native enzyme was determined to 280,000, 51 atoms of iron and 47 atoms of acid-labile sulfur were found per enzyme molecule. The absorption spectrum indicated siroheme as prosthetic group; it had maxima at 280 nm, 392 nm, 595 nm, and 724 nm. The molar extinction coefficients were determined as 302×10³ cm²xmmol^-1 at 392 nm, 98×10³ cm² xmmol^-1 at 595 nm and 22×10³ cm²x-mmol^-1 at 724 nm. With reduced viologen dyes as electron donor the enzyme reduced sulfite to sulfide, thiosulfate, and trithionate. The turnover number with 59 (2 e^-/enzyme moleculexmin) was low. The pH-optimum was at 6.0. C. vinosum sulfite reductase closely resembled the corresponding enzyme from Thiobacillus denitrificans and also desulfoviridin, the dismilatory sulfite reductase from Desulfovibrio species. It is proposed that C. vinosum catalyses anaerobic oxidation of sulfide and/or elemental sulfur to sulfite in the course of dissimilatory oxidation of reduced sulfur compounds to sulfate.