Reactions of glycine synthesis and glycine cleavage catalyzed by extracts of Arthrobacter globiformis grown on glycine

Abstract

Cell-free extracts of Arthrobacter globiformis grown on glycine as the sole source of of carbon and nitrogen were revealed to catalyze the following reactions: 1. (a) l-Serine + THF ⇌ glycine + methylene-THF, 1 1 Abbreviations used are: A, Arthrobacter: THF, tetrahydrofolate; DTT, dithiothreitol; GSH, reduced glutathione. 2. (b) l-Serine + CO 2 + NH 3 + 2H ⇌ 2 glycine, 3. (c) Methylene-THF + CO 2 + NH 3 + 2H ⇌ glycine + THF, 4. (d) Glycine + 14CO 2 ⇌ 14C-glycine + CO 2. All these reactions could proceed reversibly. Reaction (d) was strongly suppressed by the addition of THF, reflecting that Reaction (d) is a partial reaction of Reaction (c). Reaction (b) is apparently the sum of Reactions (a) and (c) and provides a new pathway of glycine synthesis from l-serine, CO 2 and ammonia which had been shown to occur in liver mitochondria from various animals. Evidence was obtained that a pyridoxal phosphate enzyme plays a central role in Reactions (c) and (d) and therefore in Reaction (b). Reactions (b), (c), and (d) were also highly dependent upon the presence of thiol compounds such as dithiothreitol and GSH, indicating the involvement of thiol enzyme(s) in these reactions. The glycine cleavage reaction was also strongly stimulated by NAD +, while NADP + had no effect.