CHAPTER 20: 1-Aminocyclopropane-1-Carboxylic Acid Oxidase¹

1-Aminocyclopropane carboxylic acid (ACC) oxidase (ACCO) catalyses the final step in ethylene biosynthesis, a key hormone in plant development, stress responses and defence. The substrate, ACC, is converted into ethylene in the presence of dioxygen and a reductant. ACCO also requires carbon dioxide (or bicarbonate) for activity. The crystal structure of ACCO from Petunia hybrida reveals a core folded into a distorted jelly-roll motif (double-stranded β-helix fold) and an active site composed of a single Fe(ii) coordinated by the side chains of two histidines and one aspartate in a 2-His, 1-Asp facial triad. ACCO is related to the 2-oxoglutarate (2OG)-dependent oxygenases although using carbon dioxide and ascorbate rather than 2OG for activity. The present chapter summarizes the advances towards understanding this intriguing enzyme and current hypotheses on the active conformation of the protein, its interaction with substrates and cofactors, the role of CO₂/bicarbonate as a cofactor, and its catalytic mechanism.