On the Role of a Palmityl Thioesterase in Fatty Acid Elongation

Elongation of palmityl-CoA to longer chain fatty acids by the high molecular weight fatty acid synthetase from Mycobacterium phlei requires a factor present in boiled mycobacterial extracts in addition to and distinct from the polysaccharides (3 - O - methylmannose - containing polysaccharide (MMP) or lipopolysaccharide that contains 6-O-methylglucose and glucose (MGLP)) which support the de novo synthesis of fatty acids from acetyl-CoA in this system. This elongation factor (EF) has been purified 46-fold and is shown to be a protein with a molecular weight of about 10,500. EF catalyzes the hydrolysis of long chain acyl-CoA derivatives and the following evidence indicates that it functions as a thioesterase in palmityl-CoA elongation. (a) Elongation and thioesterase activities show parallel chain-length specificities for acyl thioesters from C10 to C18. (b) The two activities are inactivated at similar rates on heating at 50°. (c) Elongation and thioesterase activities migrate the same distance on gel electrophoresis at pH 8.5. (d) Thioesterase I from Escherichia coli can replace EF in stimulating palmityl-CoA elongation by the Mycobacterium phlei fatty acid synthetase. (e) Free coenzyme A, in adequate concentrations, stimulates palmityl-CoA elongation as effectively as EF. It is concluded that EF, by virtue of its thioesterase activity, adjusts intracellular concentrations of palmityl-CoA and free CoA to levels which are favorable for chain elongation. The regulatory significance of thioesterase activities is discussed.