Category: biosynthesis
Interconverting amino acid axis with major one-carbon donor role. Serine ↔ glycine via SHMT (serine hydroxymethyltransferase) — donates a methylene to THF for the folate cycle (see folate_one_carbon). Glycine is the dominant inhibitory transmitter in spinal cord + brainstem (strychnine antagonist + tetanus toxin). Threonine catabolism produces glycine + acetyl-CoA via TDH (mostly in newborns; minor in adults). PHGDH (phosphoglycerate dehydrogenase) is the rate-limiting step in serine biosynthesis from 3-phosphoglycerate — overexpressed in some breast / melanoma cancers (one-carbon biosynthesis dependency).