Various functions of selenols and thiols in anaerobic Gram‐positive, amino acids‐utilizing bacteria

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Abstract
Electron transfer reactions for the reduction of glycine in Eubacterium acidaminophilum involve many selenocysteine (U)‐ and thiol‐containing proteins, as shown by biochemical and molecular analysis. These include an unusual thioredoxin system (‐CXXC‐), protein A (‐CXXU‐) and the substrate‐specific protein B of glycine reductase (‐UXXCXXC‐). Most probably a selenoether is formed at protein B by splitting the C‐N‐bond after binding of the substrate. The carboxymethyl group is then transferred to the selenocysteine of protein A containing a conserved motif. The latter protein acts as a carbon and electron donor by giving rise to a protein C‐bound acetyl‐thioester and a mixed selenide‐sulfide bond at protein A that will be reduced by the thioredoxin system. The dithiothreitol‐dependent D‐proline reductase of Clostridium sticklandii exhibits many similarities to protein B of glycine reductase including the motif containing selenocysteine. In both cases proprotein processing at a cysteine residue gives rise to a blocked N‐terminus, most probably a pyruvoyl group. Formate dehydrogenase and some other proteins from E. acidaminophilum contain selenocysteine, e.g., a 22 kDa protein showing an extensive homology to peroxiredoxins involved in the detoxification of peroxides.