Some aspects of glycine metabolism in Arthrobacter globiformis.

Arthrobacter globiformis is able to utilize glycine as sole carbon and energy source for growth. It has been shown that the glycine is first converted to pyruvate by way of the serine pathway. Two of the enzymes in this pathway, namely the tetrahydropteroylglutamate-dependent glycine cleavage system and L-serine dehydratase, increase in activity as the bacterium adapts to growth on glycine. A. globiformis possesses a pyruvate carboxylase and a functioning tricarboxylic acid cycle. Therefore the pyruvate formed from glycine should serve both as a source of energy and of biosynthetic precursors. The pyruvate carboxylase shows an absolute requirement for catalytic amounts of acetyl CoA. It is cold labile and protected from cold inactivation by acetyl CoA. A. globiformis possesses a phosphoenolpyruvate carboxykinase which is considerably more active with inosine or guanosine nucleotides than with adenosine nucleotides. During growth on glycine and other substrates not degraded to phosphoenolpyruvate, the level of this enzyme is relatively high, suggesting that its function is probably to provide the phosphoenolpyruvate required under these growth conditions for gluconeogenesis and other biosynthetic purposes. The L-serine dehydratase extracted from A. globiformis is slowly activated by its substrate, L-serine, for which it exhibits a sigmoid saturation curve. The rate of activation of the enzyme is increased by the presence of Mg2+, and, most probably, by that of any one of a variety of other cations. The enzyme has been shown to indergo a reversible dimerization in the presence of L-serine. Kinetic studies suggest that dimerization of the enzyme is the rate limiting reaction in its activation by L-serine. In contrast to the extracted enzyme, the enzyme in toluene- treated cells shows no evidence of substrate activation. Most of the unusual properties of the extracted enzyme have been explained in terms of an extended form of the Monod-Wyman-Changeux model for allosteric proteins.