The crystal structure of a novel bacterial adenylyltransferase reveals half of sites reactivity

Phosphopantetheine adenylyltransferase (PPAT) is an essential enzyme in bac teria that catalyses a rate-limiting step in coenzyme A (CoA) biosynthesis, by transferring an adenylyl group from ATP to 4'-phosphopantetheine, yield ing dephospho-CoA (dPCoA). Each phosphopantetheine adenylyltransferase (PPA T) subunit displays a dinucleotide-binding fold that is structurally simila r to that in class I aminoacyl-tRNA synthetases. Superposition of bound ade nylyl moieties from dPCoA in PPAT and ATP in aminoacyl-tRNA synthetases sug gests nucleophilic attack by the 4'-phosphopantetheine on the a-phosphate o f ATP, The proposed catalytic mechanism implicates transition state stabili zation by PPAT without involving functional groups of the enzyme in a chemi cal sense in the reaction, The crystal structure of the enzyme from Escheri chia coli in complex with dPCoA shows that binding at one site causes a vic e-like movement of active site residues lining the active site surface. The mode of enzyme product formation is highly concerted, with only one trimer of the PPAT hexamer showing evidence of dPCoA binding. The homologous acti ve site attachment of ATP and the structural distribution of predicted sequ ence-binding motifs in PPAT classify the enzyme as belonging to the nucleot idyltransferase superfamily.