RESUMO
Pantothenate (vitamin B5) is the precursor for the biosynthesis of the phosphopantetheine moiety of coenzyme A and acyl carrier protein, and is synthesised in Escherichia coli by four enzymic reactions. Ketopantoate hydroxymethyltransferase (KPHMT) and pantothenate synthetase (PtS) catalyse the first and last steps, respectively. Two genes encoding KPHMT and one for PtS were identified in the Arabidopsis thaliana genome, and cDNAs for all three genes were amplified by PCR. The cDNAs were able to complement their respective E. coli auxotrophs, demonstrating that they encoded functional enzymes. Subcellular localisation of the proteins was investigated using green fluorescent protein (GFP) fusions and confocal microscopy. The two KPHMT-GFP fusion proteins were targeted exclusively to mitochondria, whereas PtS-GFP was found in the cytosol. This implies that there must be transporters for pathway intermediates. KPHMT enzyme activity could be measured in purified mitochondria from both pea leaves and Arabidopsis suspension cultures. We investigated whether Arabidopsis encoded homologues of the remaining two pantothenate biosynthesis enzymes from E. coli, l-aspartate-alpha-decarboxylase (ADC) and ketopantoate reductase (KPR). No homologue of ADC could be identified using either conventional blast or searches with the program fugue in which the structure of the E. coli ADC was compared to all the annotated proteins in Arabidopsis. ADC also appears to be absent from the genome of the yeast, Saccharomyces cerevisiae, by the same criteria. In contrast, a putative Arabidopsis oxidoreductase with some similarity to KPR was identified with fugue.
Assuntos
Proteínas de Arabidopsis/genética , Arabidopsis/genética , Hidroximetil e Formil Transferases/genética , Hidroximetil e Formil Transferases/metabolismo , Ácido Pantotênico/biossíntese , Peptídeo Sintases/genética , Sequência de Aminoácidos , Arabidopsis/química , Arabidopsis/enzimologia , Proteínas de Arabidopsis/metabolismo , Clonagem Molecular , Citosol/metabolismo , DNA Complementar/química , DNA Complementar/genética , Proteínas de Fluorescência Verde , Proteínas Luminescentes/genética , Proteínas Luminescentes/metabolismo , Mitocôndrias/genética , Mitocôndrias/metabolismo , Dados de Sequência Molecular , Peptídeo Sintases/metabolismo , Proteínas Recombinantes de Fusão/genética , Proteínas Recombinantes de Fusão/metabolismo , Análise de Sequência de DNA , Homologia de Sequência de AminoácidosRESUMO
We report the crystal structure of E. coli ketopantoate hydroxymethyltransferase (KPHMT) at 1.9 A resolution, in complex with its product, ketopantoate. KPHMT catalyzes the first step in the biosynthesis of pantothenate (vitamin B(5)), the precursor of coenzyme A and the acyl carrier protein cofactor. The structure of the decameric enzyme was solved by multiwavelength anomalous dispersion to locate 160 selenomethionine sites and phase 560 kDa of protein, making it the largest structure solved by this approach. KPHMT adopts the (betaalpha)(8) barrel fold and is a member of the phosphoenolpyruvate/pyruvate superfamily. The active site contains a ketopantoate bidentately coordinated to Mg(2+). Similar binding is likely for the substrate, alpha-ketoisovalerate, orienting the C3 for deprotonation.
