Covering: up to 2019

Metabolic production of CO₂ is natural product chemistry on a mammoth scale. Just counting humans, among all other respiring organisms, the seven billion people on the planet exhale about 3 billion tons of CO₂ per year. Essentially all of the biogenic CO₂ arises by action of discrete families of decarboxylases. The mechanistic routes to CO₂ release from carboxylic acid metabolites vary with the electronic demands and structures of specific substrates and illustrate the breadth of chemistry employed for C–COO (C–C bond) disconnections. Most commonly decarboxylated are α-keto acid and β-keto acid substrates, the former requiring thiamin-PP as cofactor, the latter typically cofactor-free. The extensive decarboxylation of amino acids, e.g. to neurotransmitter amines, is synonymous with the coenzyme form of vitamin B₆, pyridoxal-phosphate, although covalent N-terminal pyruvamide residues serve in some amino acid decarboxylases. All told, five B vitamins (B₁, B₂, B₃, B₆, B₇), ATP, S-adenosylmethionine, manganese and zinc ions are pressed into service for specific decarboxylase catalyses. There are additional cofactor-independent decarboxylases that operate by distinct chemical routes. Finally, while most decarboxylases use heterolytic ionic mechanisms, a small number of decarboxylases carry out radical pathways.