Evidence for only oxygenative cleavage of aldehydes to alk(a/e)nes and formate by cyanobacterial aldehyde decarbonylases

Cyanobacterial aldehyde decarbonylases (ADs) catalyze the conversion of C_n fatty aldehydes to formate (HCO₂^-) and the corresponding C_n-1 alk(a/e)nes. Previous studies of the Nostoc punctiforme (Np) AD produced in Escherichia coli (Ec) showed that this apparently hydrolytic reaction is actually a cryptically redox oxygenation process, in which one O-atom is incorporated from O₂ into formate and a protein-based reducing system (NADPH, ferredoxin, and ferredoxin reductase; N/F/FR) provides all four electrons needed for the complete reduction of O ₂. Two subsequent publications by Marsh and co-workers [Das, et al. (2011) Angew. Chem. Int. Ed.50, 7148-7152; Eser, et al. (2011) Biochemistry50, 10743-10750] reported that their Ec-expressed Np and Prochlorococcus marinus (Pm) AD preparations transform aldehydes to the same products more rapidly by an O₂-independent, truly hydrolytic process, which they suggested proceeded by transient substrate reduction with obligatory participation by the reducing system (they used a chemical system, NADH and phenazine methosulfate; N/PMS). To resolve this discrepancy, we re-examined our preparations of both AD orthologues by a combination of (i) activity assays in the presence and absence of O₂ and (ii) ¹⁸O₂ and H₂ ¹⁸O isotope-tracer experiments with direct mass-spectrometric detection of the HCO₂^- product. For multiple combinations of the AD orthologue (Np and Pm), reducing system (protein-based and chemical), and substrate (n-heptanal and n-octadecanal), our preparations strictly require O₂ for activity and do not support detectable hydrolytic formate production, despite having catalytic activities similar to or greater than those reported by Marsh and co-workers. Our results, especially of the ¹⁸O-tracer experiments, suggest that the activity observed by Marsh and co-workers could have arisen from contaminating O₂ in their assays. The definitive reaffirmation of the oxygenative nature of the reaction implies that the enzyme, initially designated as aldehyde decarbonylase when the C1-derived coproduct was thought to be carbon monoxide rather than formate, should be redesignated as aldehyde-deformylating oxygenase (ADO).