Oxygen-Independent Alkane Formation by Non-Heme Iron-Dependent Cyanobacterial Aldehyde Decarbonylase: Investigation of Kinetics and Requirement for an External Electron Donor

Abstract

Cyanobacterial aldehyde decarbonylase (cAD) is, structurally, a member of the di-iron carboxylate family of oxygenases. We previously reported that cAD from Prochlorococcus marinus catalyzes the unusual hydrolysis of aldehydes to produce alkanes and formate in a reaction that requires an external reducing system but does not require oxygen [Das et al. (2011) Angew. Chem. 50, 7148-7152]. Here we demonstrate that cADs from divergent cyanobacterial classes, including the enzyme from N. puntiformes that was reported to be oxygen dependent, catalyze aldehyde decarbonylation at a much faster rate under anaerobic conditions and that the oxygen in formate derives from water. The very low activity (<1 turnover/h) of cAD appears to result from inhibition by the ferredoxin reducing system used in the assay and the low solubility of the substrate. Replacing ferredoxin with the electron mediator phenazine methosulfate allowed the enzyme to function with various chemical reductants, with NADH giving the highest activity. NADH is not consumed during turnover, in accord with the proposed catalytic role for the reducing system in the reaction. With octadecanal, a burst phase of product formation, k(prod) = 3.4 ± 0.5 min(-1), is observed, indicating that chemistry is not rate-determining under the conditions of the assay. With the more soluble substrate, heptanal, k(cat) = 0.17 ± 0.01 min(-1) and no burst phase is observed, suggesting that a chemical step is limiting in the reaction of this substrate.