A clear understanding of metabolic pathways and enzyme mechanisms is of fundamental importance in obtaining control of biological systems. Precise definition of the processes in normal cells is basic to the detection, understanding and control of processes in abnormal cells. The broad objectives of this research are to define the metabolic pathways, the properties of the enzymes, and the interrelationships of the enzymes and coenzymes in certain bacterial cells which metabolize glycine as an energy source. Elucidation of these properties will be of general significance, not only as pertaining to the world of microorganisms, but also as pertaining to higher plants and animals, including humans. The model systems available in Peptococcus glycinophilus and Clostridium acidiurici, but whose distribution and significance in other organisms, including animals and humans, is becoming more apparent is under investigation. Four enzymes related to the oxidative cleavage of glycine will receive particular emphasis: (a) the pyridoxal phosphate-containing glycine decarboxylase; (b) the lipoic acid-containing electron transfer protein; (c) the FAD-containing dihydrolipoate dehydrogenase; and (d) the tetrahydrofolate-requiring alpha-carbon transferase. The concerted function of these enzymes is to remove the carboxyl group of glycine as CO2, making the methylene carbon available for the condensation with a second glycine molecule to form serine. In the process the electrons removed during decarboxylation are transferred via the lipoate and FAD enzymes to DPN. By reaction with still other enzymes the serine thus formed is converted to pyruvate, thence acetyl-CoA and acetyl-PO4, the major energy source for the cells.