The coenzyme A-synthesizing protein complex and its proposed role in CoA biosynthesis in baker's yeast

An improved procedure is described for the recovery and purification of the coenzyme A-synthesizing protein complex (CoA-SPC) of Saccharomyces cerevisiae (bakers' yeast). The molecular mass of the CoA-SPC, determined prior to and following its purification, is estimated by Sephacryl S-300 size exclusion chromatography to be between 375 000–400 000. Two previously unreported catalytic activities attributed to CoA-SPC have been identified. One of these is CoA-hydrolase activity which catalyzes the hydrolysis of CoA to form 3′,5′-ADP and 4′-phosphopantetheine, and the other is dephospho-CoA-pyrophosphorylase activity which catalyzesa reaction between 4′-phosphopantetheine and ATP to form dephospho-CoA. The dephospho-CoA then reacts with ATP, catalyzed by the dephospho-CoA-kinase. to reform CoA. This sequence of reactions, referred to as the CoA/4′-phosphopantethiene cycle, provides a mechanism by which the 4′-phosphopantetheine can be recycled to form CoA. Each turn of the cycle utilizes two mol of ATP and produces one mol of ADP, one mol of PPi, and one mol of 3′,5′-ADP. Other than the hydrolysis of CoA by CoA-SPC, the 4′-phosphopantetheine for the cycle apparently could be supplied by alternate sources. One alternate source may be the conventional pathway of CoA biosynthesis. Intact CoA-SPC has been separated into two segments. One segment is designated apo-CoA-SPC and the other segment is referred to as the 10 000–15 000 M r subunit. The 5′-ADP-4′-pantothenic acid-synthetase, 5′-ADP-4′-pantothenylcysteine-synthetase, 5′-ADP-4′-pantothenylcysteine-decarboxylase, and CoA-hydrolase activities reside in the apo-CoA-SPC segment of CoA-SPC. Whereas the dephospho-CoA-kinase and the dephospho-CoA-pyrophosphorylase activities reside in the 10 000–15 000 M r subunit. Thus, the 10 000–15 000 M r subunit mimics the bifunctional enzyme complex that catalyzes the final two steps in the conventional pathway of CoA biosynthesis.