FIGURE 2.

Pathways of serine metabolism. In the reaction catalyzed by serine hydroxymethyltransferase (1), serine is converted to glycine and 5,10-methylene-tetrahydrofolate (5,10-CH₂-THF). Glycine is transaminated to glyoxylate by serine: glyoxylate or glutamate: glyoxylate aminotransferase (2). Glyoxylate is reduced to glycolate by glyoxylate reductase (3). 5,10-CH₂-THF is converted to 10-formyl-tetrahydrofolate (10-CHO-THF) by the bifunctional enzyme methylenetetrahydrofolate dehydrogenase – methenyltetrahydrofolate cyclohydrolase (4), the intermediate is 5,10-methenyl-tetrahydrofolate (5,10-CH = THF). 10-CHO-THF is converted to formate and THF by formate-tetrahydrofolate ligase (5), the reaction yields ATP. Formate can be oxidized to CO₂ by formate dehydrogenase (6) or reduced to formaldehyde by formaldehyde dehydrogenase which is also S-nitrosoglutathione reductase (alcohol dehydrogenase type III) (7). Formate can be formed from glyoxylate in a non-enzymatic reaction with hydrogen peroxide (8), glyoxylate in turn can be formed from formate in a putative glyoxylate synthetase reaction (not shown).