PECASE: Mechanistic Studies on Cyclopropane Fatty Acid Synthase

Proposal Title: PECASE: Mechanistic Studies on Cyclopropane Fatty Acid Synthase

Institution: Pennsylvania State Univ University Park

S-adenosyl-L-methionine (AdoMet) is one of the most versatile cellular metabolites used in enzymatic catalysis. Until relatively recently, it has been appreciated primarily as a cellular methylating agent, since it is the primary source of methyl groups for a broad spectrum of biological compounds, including DNA, RNA, proteins, lipids, carbohydrates, and a diverse array of small molecules. AdoMet is also involved in a variety of other interesting transformations. In one class of enzymes, it functions in concert with iron-sulfur clusters to generate enzyme-bound radicals that are intermediates in certain enzymatic reactions. In the project described herein, it functions as a donor of a methylene group rather than a methyl group, in a fascinating reaction catalyzed by the enzyme cyclopropane fatty acid synthase from Escherichia coli. The second substrate is an isolated and unactivated cis olefin present in the unsaturated fatty acid acyl chains of phospholipids. In contrast to methyl transfer, there is very little precedent in the biochemical literature for methylene transfer from AdoMet. All polar mechanisms for the reaction would be expected to involve intermediates that are very high in energy. In addition, since most cellular phospholipids are constituents of phospholipid bilayers, wherein their fatty acid chains are sequestered from the aqueous milieu, it is unclear how this enzyme catalyzes a reaction between two substrates of opposing solubilities. The goal of this project is to address these intriguing questions using a variety of kinetic, mechanistic, and physical techniques, which if successful, will contribute significantly to the general understanding of enzyme reaction mechanisms.

This project was originally funded as a CAREER award, and was converted to a Presidential Early Career Award for Engineers and Scientists (PECASE) award in May 2004.