The ability of some anaerobic bacteria to conserve energy via a soluble substrate level phosphorylation system by reducing glycine to acetyl-phosphate has been an intriguing mechanism for about half a century. The genes implicated in this system have been sequenced and form an operon structure with those of the thioredoxin system. The deduced proteins exhibit high degrees of similarity with glycine reductase from other bacteria. Faster progress in understanding the exact mechanisms is hampered, for example, by some unique reactions involving selenoethers and redox active selenocysteines, which do not allow an easy heterologous formation in Escherichia coli. Further major obstacles are the processing of a substrate-specific pro-protein to a new carbonyl/pyruvoyl group in one of the two peptides formed that stabilize the substrate-binding selenoprotein, which contains an additional rather unstable carbonyl group.