Oxygen-independent decarbonylation of aldehydes by cyanobacterial aldehyde decarbonylase: a new reaction of diiron enzymes.

The search for new biofuels has generated increased interest in biochemical pathways that produce hydrocarbons. Although hydrocarbons are simple molecules, the biosynthesis of molecules that lack any chemical functional groups is surprisingly challenging. Biochemical reactions that remove functionality, such as decarboxylations, dehydrations and reduction of double bonds, invariably rely on the presence of adjacent functional groups to stabilize unfavorable transition states. Enzymes involved in hydrocarbon biosynthesis are therefore of interest both for applications in biofuels production and because of the unusual and chemically difficult reactions they catalyze. One enzyme that has attracted particular interest, is aldehyde decarbonylase (AD), which catalyzes the decarbonylation of long-chain fatty aldehydes, to the corresponding alkanes. AD was originally identified in studies on the biosynthesis of hydrocarbon waxes by higher plants and algae. These enzymes are integral membrane proteins of Mr 70000 Da that convert long-chain aldehydes, derived from fatty acids, to alkanes and carbon monoxide. The enzymes require divalent metal ions for activity; however, the identity of the metal remains unclear. It was reported that the pea enzyme is active with Co, Cu, or Zn whereas the Botryococcus braunii enzyme uses Co. Sequence analysis of cer1, a gene encoding a putative AD from Arabidopsis thaliana, indicates that it is member of the sterol oxidase class of non-heme irondependent oxygenases. More recently, a soluble version of AD was discovered in cyanobacteria (cAD). Serendipitously, a crystal structure for cAD from Prochlorococcus marinus MIT9313 had previously been solved as part of a structural proteomics project, although no function had been assigned. The structure revealed that cAD is member of the non-heme dinuclear iron oxygenase family of enzymes exemplified by methane monoxygenase, type I ribonucleotide reductase, and ferritin. In all these enzymes, the diiron center is contained within an antiparallel four-a-helix bundle in which two histidines and four carboxylates (either from aspartate or glutamate) supply the protein ligands to the metal ions. The dinuclear iron center of cAD superimposes very closely on those of the other enzymes; however in the structure the carboxylate of an adventitiously bound long-chain fatty acid bridges the two iron atoms, displacing one of the glutamate ligands (Figure 1).