Catalytic properties of adenylylsulfate reductase from Desulfovibrio vulgaris Miyazaki

Adenylylsulfate reductase (EC 1.8.99.2) isolated from Desulfovibrio vulgaris Miyazaki catalyzes electron transfer from dihydroflavin coenzyme (FADH2, FMNH2, or dihydroriboflavin) to adenylyl sulfate (APS), and catalyzes flavin-mediated oxidation of ferrocytochrome c3 with APS. The reaction with FAD as an electron mediator was markedly stimulated in the presence of menadione. Km of the enzyme was about 0.015 mM for riboflavin and FAD in the presence of menadione. Free flavin coenzyme was found to be the normal cellular constituent. These observations suggested that free flavin coenzyme may be a physiological electron carrier for APS reductase, and the enzyme may be called AMP, sulfite:flavin oxidoreductase. Km (APS) of this enzyme is lower than 1 microM. The enzyme is not inhibited by ATP and GTP, but was inhibited by AMP and sulfite. Its extremely low Km (APS) enables this enzyme to reduce any traces of cytosolic APS which is present only at micromolar concentration, and inhibition by sulfite makes this organism utilize an energetically favorable electron acceptor, sulfite, preferentially over APS which is produced from sulfate at the cost of ATP.