Selenium-dependent and selenium-independent formate dehydrogenases of Methanococcus vannielii. Separation of the two forms and characterization of the purified selenium-independent form.

Anaerobic oxidation of formate by Methanococcus vannielii is catalyzed by two readily separable formate dehydrogenases. One of these is a 105,000-dalton protein that contains molybdenum, iron, and acid-labile sulfide, but not selenium. The other is a high molecular weight complex composed of selenoporotein and molybdo-iron sulfur protein subunits. Selenium occurs in this selenoenzyme in the chemical form of selenocysteine residues. M. vannielii cells from selenium-deficient media contain the 105,000-dalton formate dehydrogenase. Marked stimulation of growth by selenite supplementation is correlated with the simultaneous appearance in the cells of the high molecular weight selenoprotein . enzyme complex. The latter is the predominant form in cells from media additionally supplemented with tungstate. Under these conditions partial replacement of molybdenum with tungsten appears to occur. Both formate dehydrogenases are maximally active at pH 8.5 to 9.2 and at 60 degrees C and are extremely oxygen-sensitive. They utilize as electron acceptors 8-hydroxy-5-deazaflavin, FMN, FAD, and viologen and tetrazolium dyes.