serine

Serine is synthesized from the glycolytic intermediate 3-phosphoglycerate through a three-step pathway (oxidation, transamination, dephosphorylation). It is metabolically versatile: (1) Serine and glycine are interconverted by serine hydroxymethyltransferase, a reaction that transfers one-carbon units to/from tetrahydrofolate (THF), linking serine metabolism to nucleotide synthesis and the folate cycle. (2) Serine donates a carbon to form cysteine when combined with homocysteine via cystathionine synthase (transsulfuration pathway), making it essential for cysteine/glutathione production. (3) Serine is a precursor for phosphatidylserine, an important membrane phospholipid, and for sphingosine synthesis (via serine palmitoyltransferase, combining serine with palmitoyl-CoA), which is the backbone of sphingolipids and myelin components—clinically relevant in neurodegenerative and lysosomal storage diseases. (4) Serine is a critical catalytic residue in serine proteases (e.g., trypsin, chymotrypsin, elastase, plasmin, thrombin), where it participates in the classic catalytic triad (Ser-His-Asp) to hydrolyze peptide bonds via a covalent acyl-enzyme intermediate—a mechanism heavily tested on USMLE Step 1 for enzyme kinetics and protease inhibitor pharmacology (e.g., organophosphates irreversibly inhibit serine hydrolases like acetylcholinesterase by phosphorylating the active-site serine). Clinically, defects in serine synthesis enzymes cause rare neurometabolic disorders with microcephaly and seizures, and serine metabolism is also relevant to psychiatric neurotransmission since D-serine acts as a co-agonist at NMDA receptors.