Ann Lewendon

PURIFICATION AND CHARACTERIZATION OF PHOSPHOPANTETHEINE ADENYLYLTRANSFERASE FROM ESCHERICHIA COLI

Phosphopantetheine adenylyltransferase (PPAT) catalyzes the penultimate step in coenzyme A (CoA) biosynthesis: the reversible adenylation of 4′-phosphopantetheine yielding 3′-dephospho-CoA and pyrophosphate. Wild-type PPAT fromEscherichia coli was purified to homogeneity. N-terminal sequence analysis revealed that the enzyme is encoded by a gene designated kdtB, purported to encode a protein involved in lipopolysaccharide core biosynthesis. The gene, here renamedcoaD, is found in a wide range of microorganisms, indicating that it plays a key role in the synthesis of 3′-dephospho-CoA. Overexpression of coaD yielded highly purified recombinant PPAT, which is a homohexamer of 108 kDa. Not less than 50% of the purified enzyme was found to be associated with CoA, and a method was developed for its removal. A steady state kinetic analysis of the reverse reaction revealed that the mechanism of PPAT involves a ternary complex of enzyme and substrates. Since purified PPAT lacks dephospho-CoA kinase activity, the two final steps of CoA biosynthesis in E. coli must be catalyzed by separate enzymes.

Structural and Mechanistic Studies of Galactoside Acetyltransferase, the Escherichia coli LacA Gene Product

Escherichia coli galactoside acetyltransferase (GAT) is a member of a large family of acetyltransferases that O-acetylate dissimilar substrates but share limited sequence homology. Steady-state kinetic analysis of overexpressed GAT demonstrated that it accepted a range of substrates, including glucosides and lactosides which were acetylated at rates comparable to galactosides. GAT was shown to be a trimeric acetyltransferase by cross-linking with dimethyl suberimidate. Fluorometric analysis of coenzyme A binding showed that there is a fluorescence quench associated with acetyl-CoA binding whereas CoA has no effect. This difference was exploited to measure dissociation rates for both CoA and acetyl-CoA by stopped-flow fluorometry. The rate of dissociation of CoA (2500 s−1) is at least 170-fold faster than kcat for any substrate tested. The fluorescence response to acetyl-CoA binding is entirely due to Trp-139 since replacement by phenylalanine completely abolished the fluorescence quench. Treatment of GAT by [14C]iodoacetamide resulted in complete inactivation of the enzyme and the incorporation of label into histidyl and cysteinyl residues to approximately equal extents. Following replacement of His-115 by alanine, label was incorporated solely into cysteinyl residues. Furthermore, the substitution results in an 1800-fold decrease in kcat suggesting that His-115 has an important catalytic role in GAT.

Inhibitors of phosphopantetheine adenylyltransferase.

Phosphopantetheine adenylyltransferase (PPAT) is an essential enzyme in Coenzyme A biosynthesis. Because bacterial PPAT and mammalian PPAT are dissimilar, this enzyme is an attractive antibacterial target. Based on the structure of the substrate, 4-phosphopantetheine, a dipeptide library was designed, synthesised and tested against Escherichia coli PPAT. The most potent inhibitor PTX040334 was co-crystallised with E. coli PPAT. With this structural information, a rational iterative medicinal chemistry program was initiated, aimed at increasing the number of inhibitor-enzyme interactions. A very potent and specific inhibitor, PTX042695, with an IC(50) of 6 nM against E.coli PPAT, but with no activity against porcine PPAT, was obtained.