Cyclopropane fatty acid synthase of Escherichia coli. Stabilization, purification, and interaction with phospholipid vesicles

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Abstract
The cyclopropane fatty acid (CFA) synthase of E. coli catalyzes the methylenation of the unsaturated moieties of phospholipids in a phospholipid bilayer. The methylene donor is S-adenosyl-L-methionine. The enzyme is loosely associated with the inner membrane of the bacterium and binds to and is stabilized by phospholipid vesicles. The enzyme was purified over 500-fold by flotation with phospholipid vesicles and appears to be a monomeric protein having a MW of about 90,000. The enzyme binds only to vesicles of phospholipids which contain unsaturated or cyclopropane fatty acid moieties. CFA synthase is active on phosphatidylglycerol, phosphatidylethanolamine and cardiolipin, the major phospholipids of E. coli, and also has some activity on phosphatidylcholine. The enzyme is equally active on phospholipid vesicles in the ordered or the disordered states of the lipid phase transition. Studies with a reagent that reacts only with the phosphatidylethanolamine molecules of the outer leaflet of a phospholipid bilayer indicate that CFA synthase reacts with phosphatidylethanolamine molecules of the outer and the inner leaflets of phospholipid vesicles.

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