English: Pathway for the synthesis of amino acids with pyruvate as a precursor. "T" represents transamination. Pyruvate can be transaminated directly to produce alanine. It can also be decarboxylated to produce acetyl-CoA. Due to the kinetic isotope effect associated with this reaction, the red carbons in the resulting acetyl groups are depleted in 13C relative to bulk biomass. Carbons in metabolites derived from acetyl-CoA are also colored red. If pyruvate is acetylated once, it can be transaminated after a rearrangement to produce valine. Further acetylation is required to produce leucine, which is consistently depleted in 13C. Valine is sometimes more or less enriched in 13C compared to alanine. The branch point at alpha-ketoisovalerate could explain this variation. A carbon isotope effect at C-2 (indicated by blue arrows) would be needed for this branch point to affect the isotopic composition of downstream products. This position is the site of transamination to produce valine or acetylation to produce B-isopropylmalate.