S153: Cyanobacterial Aldehyde Decarbonylase: An Unexpected New Catalytic Capability for the Ferritin-Like Dimetal-Carboxylate Proteins

Aldehyde decarbonylase (AD) catalyzes the last of three biochemical steps in cyanobacteria through which fatty acids of chain-length n are converted to alka(e)nes of length n–1. This pathway, combined with the photosynthetic capability and genetic addressability of the host organisms, affords the opportunity for the engineering of a bioprocess for conversion of carbon dioxide and sunlight to truly fungible alka(e)ne fuels. The initial discovery of the AD revealed it to have a ferritin-like four-helix-bundle protein architecture and a dinuclear metal cofactor generally associated with the O₂-dependent oxidation of organic substrates, but our identification of formate as the alka(e)ne co-product showed the reaction to be formally hydrolytic. Subsequent oxygen-isotope-tracer experiments revealed that AD catalyzes a cryptically redox oxygenation reaction that is, to the best of our knowledge, unprecedented in biology. We are currently addressing the structure of the AD dimetal cofactor, including the identities of the metal ions, and the mechanism of this surprising reaction.