Purification and Characterization of a Palmityl-CoA Thioesterase from Rhodopseudomonas sphaeroides

Abstract

A palmityl-CoA thioesterase from Rhodopseudomonas sphaeroides has been purified 625-fold. The enzyme was homogenous as judged by both a constant ratio of thioesterase activity to absorbing material at 280 nm in fractions collected during gel filtration, and by the presence of a single major band on SDS polyacrylamide gels. The enzyme molecular weight is estimated at 21,000 D from its electrophoretic mobility on SDS polyacrylamide gels and its relative elution volume on a standardized gel filtration column of G-100. The thioesterase apparently is a single polypeptide and catalyzes the hydrolysis of long-chain fatty acyl thioesters of coenzyme-A whose acyl moiety is 12 to 18 carbons in length. With palmityl-CoA as a substrate, the enzyme exhibited a Km of 4.2 μM, and a Vmax calculated) of 13.9 μ moles of palmityl-CoA hydrolyzed per min per mg protein. The enzyme was one of at least three thioesterases separable by gel filtration and ion exchange chromatography from the soluble protein fraction of R. sphaeroides. Its characteristics with respect to molecular weight, activity on palmityl-CoA, and substrate specificity indicate that it is very similar to the palmityl thioesterase II from Escherichia coli.