And an incomplete list of unexpected CO₂ sources:

Process Enzyme Reaction Pentose Phosphate Pathway PGDH Hexose (6C) → pentose (5C) + CO₂ (1C) Fatty acid synthesis KAS* Malonyl (3C) + acetyl (2C) → ketobutyryl (4C) + CO₂ (1C) Acetone synthesis - Acetoacetate (4C) → acetone (3C) + CO₂ (1C) Oxoglutarate oxygenases OGO Oxoglutarate (5C) → succinate (4C) + CO₂ (1C) Pyruvate synthesis from malate ME Malate (4C) → pyruvate (3C) + CO₂ (1C) PEP synthesis PEPCK Oxaloacetate (4C) → pyruvate-enol-phosphate (3C) + CO₂ (1C) Glycine cleavage system GLDC Glycine (2C) + THF (+0C) →→ methylene-THF (+1C) + CO₂ (1C) Coenzime A synthesis PPCD PPC (12C) → P4P (11C) + CO₂ (1C) Aldehyde dehydrogenase ALDH formyl-THF (+1C) → THF (+0C) + CO₂ (1C) Taurine synthesis CSAD CSA/CA (3C) → hTau/Tau (2C) + CO₂ (1C) Cholesterol synthesis PPMD Mevalonate-5-diphosphate (6C) → Isopentenyl diphosphate (5C) + CO₂ (1C) Polyamine synthesis SAMD SAM (–0C) → dcSAM (–1C) + CO₂ (1C) Other biogenic amine syntheses (AA decarboxylations) †

*Preceded by carbon incorporation: gain followed by loss. But remains a reaction that releases carbon dioxide.

†Biochemical and Pharmacological Properties of Biogenic Amines

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