Capsaicin Biosynthesis Pathway
Capsaicin biosynthesis proceeds via two parallel metabolic branches that converge at a final amide bond formation step. Branch 1 (phenylpropanoid pathway) produces the vanillyl amine head group from phenylalanine. Branch 2 (branched-chain fatty acid pathway) produces the acyl tail from valine/leucine catabolism. AT3 joins them.
Branch 1: Phenylpropanoid
PAL → C4H → 4CL → HCT → downstream O-methyltransferases → vanillylamine. This pathway is widely conserved across dicots, making tomato and tobacco tractable hosts for Branch 1 without additional modifications. The key final step is DOPA decarboxylase converting an aromatic amino acid intermediate to vanillylamine.
Branch 2: Branched-Chain Acyl
BCAT deaminates leucine to yield the C8 precursor. KAS and KR extend and reduce the acyl chain. pAMT performs a final transamination to produce the acyl amine substrate. The acyl chain length set by KAS determines which capsaicinoid is produced: C8 = capsaicin, C8-saturated = dihydrocapsaicin, C9 = homodihydrocapsaicin.
AT3: The Gate Enzyme
AT3 (acyltransferase 3, also called Capsaicin Synthase) catalyzes the BAHD acyltransferase reaction joining vanillylamine and the acyl-CoA ester via amide bond. Its activity level caps total capsaicinoid output regardless of precursor abundance. A nonfunctional AT3 (the Pun1 locus in bell peppers) gives zero capsaicin regardless of upstream pathway activity.