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Mechanistic Studies on Aldehyde Decarbonylase from Cyanobacteria: A New Enzyme for Alkane Biosynthesis.

By Debasis Das

Abstract

The challenge posed by global warming has sparked great interest in the development of new, green biofuels. Existing biofuels such as ethanol and biodiesel suffer from several fundamental disadvantages. Hence, there is significant interest in developing superior alkane-based ‘drop-in’ biofuels that could serve as direct substitutes for gasoline. In Nature, alkanes are biosynthesized from fatty aldehyde precursors by enzymes collectively known as aldehyde decarbonylases. Conversion of aldehydes to alkanes is an unusual and chemically challenging reaction. As a consequence the mechanisms of these enzymes are very poorly understood. One such enzyme, aldehyde decarbonylase from cyanobacteria (cAD) is the subject of this dissertation. cAD was over-expressed and purified from E. coli. Spectroscopic characterization established that the enzyme is a metallo-protein and requires only iron for activity. The enzyme converted octadecanal to heptadecane. Formate was formed as the co-product. The enzyme required a reducing system comprising ferredoxin/ferredoxin reductase/NADPH for activity; however, the activity was extremely low. Therefore, an alternate reducing system made of phenazine methosulphate/NADH was employed that resulted in ~100 fold improvement in activity. It was found that the enzyme has broad substrate range and converts aldehydes of chain lengths C18-C7 to the corresponding alkanes. The kinetic properties of the enzyme were investigated using octadecanal as well as a relatively soluble substrate heptanal. Although initial investigation in the reaction suggested cAD may catalyze the reaction in an oxygen-independent manner, further experiments concluded that oxygen is involved in the reaction. The nature of the C1-C2 bond cleavage of aldehydes by cAD has also been investigated. A preliminary investigation using EPR spectroscopy suggested that a radical intermediate might be involved in the cAD-catalyzed reaction. To further probe the mechanism of the cleavage, a ‘radical clock’ cyclopropyl analog of octadecanal was investigated as a substrate for cAD. This substrate underwent rearrangement to an alkene during the reaction implying homolytic cleavage of the C1-C2 bond of the aldehyde. The cyclopropyl aldehyde also inactivated the enzyme by covalent modification. Further evidence of involvement of radical intermediates came from studies using oxiranyl aldehydes that underwent rearrangement resulting in an unusual tandem deformylation to produce Cn-2 alkanes

Topics: Aldehyde Decarbonylase
OAI identifier: oai:deepblue.lib.umich.edu:2027.42/107315

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