FIGURE 1.

Pathways of serine metabolism in mammals. The pathways for the synthesis and metabolic fate of serine are shown. The intermediates in the pathways shown in blue are involved in the synthesis of serine, either from glucose via glycolysis or from the triose phosphate pool, where carbon is generated from citric acid cycle intermediates. The latter pathway involves PEPCK (step 1), which is a major cataplerotic enzyme in the synthesis of serine. The conversion of 3-phosphoglycerate to phosphohydroxypyruvate is catalyzed by the enzyme 3-phosphoglycerate dehydrogenase (step 2). The final steps in the synthesis of serine involve the transamination of phosphohydroxypyruvate to l-phosphoserine (step 3) and the conversion of l-phosphoserine into serine by phosphoserine phosphatase (step 4). The pathway involved in methyl group transfer is shown by the red arrows, and the transsulfuration pathway is shown in orange. OAA, oxalacetate; PEP, phosphoenolpyruvate; MS, methionine synthase; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine; CβS, cystathionine β-synthase; CγL, cystathionine γ-lyase.