Carbon–oxygen functional groups are the most common in organic metabolites. Alcohols, aldehydes and ketones, and acyl groups are related and interconverted by two-electron redox steps. Hydride transfers to and from nicotinamide coenzymes are the most common mechanistic route for these redox interconversions. Molecule at the acyl oxidation state include resonance-stabilized carboxylate anions, amides, oxoesters, thioesters and mixed acyl phosphoric anhydrides in ascending order of thermodynamic destabilization and tendency to undergo electrophilic acyl group transfers. Carbon dioxide is generated by decarboxylases acting on acyl groups by a number of low energy routes. In the opposite direction, CO₂ is fixed in amounts of 250 billion tonnes globally each year by plant ribulose-bis-phosphate carboxylase.