Characterization and Kinetics of Phosphopantothenoylcysteine Synthetase from Enterococcus faecalis

The enzyme phosphopantothenoylcysteine synthetase (PPCS) catalyzes the nucleotide-dependent formation of phosphopantothenoylcysteine from (R)-phosphopantothenate and L-Cysteine in the biosynthetic pathway leading to the formation of the essential biomolecule, coenzyme A. The Enterococcus faecalis gene coaB encodes a novel monofunctional PPCS which has been cloned into pET23a and expressed in Escherichia coli BL21 AI. The heterologous expression system yielded 30 mg of purified PPCS per liter of cell culture. The purified enzyme chromatographed as a homodimer of 28 kDa subunits on Superdex HR 200 gel filtration resin. The monofunctional protein displayed a nucleotide specificity for cytidine 5'-triphosphate (CTP) analogous to that seen for bifunctional PPCS expressed by most prokaryotes. Kinetic characterization, utilizing initial velocity and product inhibition studies, found the mechanism of PPCS to be Bi Uni Uni Bi Ping-Pong, with the nucleotide CTP binding first and CMP released last. Michaelis constants were 156, 17, and 86 mu M for CTP, (R)-phosphopantothenate, and L-Cysteine, respectively, and the k(cat) was 2.9 s(-1). [carboxyl-O-18]Phosphopantothenate was prepared by hydrolysis of methyl pantothenate with (NaOH)-O-18, followed by enzymatic phosphorylation with E. faecalis pantothenate kinase (PanK). The fate of the carboxylate oxygen of labeled phosphopantothenate, during the course of the PPCS-catalyzed reaction with CTP and L-cysteine, was monitored by P-31 NMR spectroscopy. The results show that the carboxylate oxygen of the phosphopantothenate is recovered with the CMP formed during the reaction, indicative of the formation of a phosphopantothenoyl cytidylate catalytic intermediate, which is consistent with the kinetic mechanism.