The glycine cleavage system: composition, reaction mechanism, and physiological significance.

Abstract

The glycine cleavage system catalyzes the following reversible reaction: Glycine + THF + NAD+ ⇌ 5,10-methylene-THF + + CO2 + NH3 + NADH Reversibility of the overall reaction was established through the studies with the enzymes prepared from liver mitochondria of rat and cock and from extracts ofArthrobacter globiformis grown on glycine. The glycine cleavage system is composed from four protein components. The four proteins were revealed to exist originally as an enzyme complex in the liver mitochondria. Partial reactions of glycine cleavage and glycine synthesis were studied in detail with partially purified individual protein components. Particularly a protein-bound intermediate of glycine metabolism could be isolated and its nature and role were clarified. A tentative scheme was presented to explain the whole process of the reversible glycine cleavage. The glycine cleavage system was shown to represent the major pathway of catabolism of both glycine and serine in vertebrates, including mammals, birds, reptiles, amphibians, and fishes. Serine catabolism in these animals proceeds mainly by way of the cleavage of serine to form methylene-THF and glycine rather than deamination by serine dehydratase. In ureotelic and ammonotelic animals methylene-THF formed from the α-carbon of glycine as well as theβ-carbon of serine could be further oxidized to CO2 in either the mitochondria or the soluble tissue fractions, while in uricotelic animals methylene-THF could hardly be oxidized to CO2 and instead, was utilized mostly for purine synthesis. Glycine synthesis by the glycine cleavage system did not appear to have appreciable physiological significance in animals.