RESUMEN
In vivo, 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase catalyzes the reversible, stereospecific retro-aldol cleavage of KDPG to pyruvate and D-glyceraldehyde-3-phosphate. The enzyme is a lysine-dependent (Class I) aldolase that functions through the intermediacy of a Schiff base. Here, we propose a mechanism for this enzyme based on crystallographic studies of wild-type and mutant aldolases. The three dimensional structure of KDPG aldolase from the thermophile Thermotoga maritima was determined to 1.9A. The structure is the standard alpha/beta barrel observed for all Class I aldolases. At the active site Lys we observe clear density for a pyruvate Schiff base. Density for a sulfate ion bound in a conserved cluster of residues close to the Schiff base is also observed. We have also determined the structure of a mutant of Escherichia coli KDPG aldolase in which the proposed general acid/base catalyst has been removed (E45N). One subunit of the trimer contains density suggesting a trapped pyruvate carbinolamine intermediate. All three subunits contain a phosphate ion bound in a location effectively identical to that of the sulfate ion bound in the T. maritima enzyme. The sulfate and phosphate ions experimentally locate the putative phosphate binding site of the aldolase and, together with the position of the bound pyruvate, facilitate construction of a model for the full-length KDPG substrate complex. The model requires only minimal positional adjustments of the experimentally determined covalent intermediate and bound anion to accommodate full-length substrate. The model identifies the key catalytic residues of the protein and suggests important roles for two observable water molecules. The first water molecule remains bound to the enzyme during the entire catalytic cycle, shuttling protons between the catalytic glutamate and the substrate. The second water molecule arises from dehydration of the carbinolamine and serves as the nucleophilic water during hydrolysis of the enzyme-product Schiff base. The second water molecule may also mediate the base-catalyzed enolization required to form the carbon nucleophile, again bridging to the catalytic glutamate. Many aspects of this mechanism are observed in other Class I aldolases and suggest a mechanistically and, perhaps, evolutionarily related family of aldolases distinct from the N-acetylneuraminate lyase (NAL) family.
Asunto(s)
Aldehído-Liasas/química , Aldehído-Liasas/metabolismo , Aldehído-Liasas/clasificación , Aldehído-Liasas/genética , Sitios de Unión , Catálisis , Escherichia coli/enzimología , Escherichia coli/genética , Ácido Glutámico/genética , Ácido Glutámico/metabolismo , Modelos Moleculares , Estructura Cuaternaria de Proteína , Estructura Terciaria de Proteína , Especificidad por Sustrato , Thermotoga maritima/enzimologíaRESUMEN
Vancomycin, the last line of defense antibiotic, depends upon the attachment of the carbohydrate vancosamine to an aglycone skeleton for antibacterial activity. Vancomycin is a naturally occurring secondary metabolite that can be produced by bacterial fermentation. To combat emerging resistance, it has been proposed to genetically engineer bacteria to produce analogues of vancomycin. This requires a detailed understanding of the biochemical steps in the synthesis of vancomycin. Here we report the 1.4 A structure and biochemical characterization of EvaD, an RmlC-like protein that is required for the C-5' epimerization during synthesis of dTDP-epivancosamine. EvaD, although clearly belonging to the RmlC class of enzymes, displays very low activity in the archetypal RmlC reaction (double epimerization of dTDP-6-deoxy-4-keto-D-glucose at C-3' and C-5'). The high resolution structure of EvaD compared with the structures of authentic RmlC enzymes indicates that a subtle change in the enzyme active site repositions a key catalytic Tyr residue. A mutant designed to re-establish the normal position of the Tyr increases the RmlC-like activity of EvaD.
Asunto(s)
Antibacterianos/farmacología , Carbohidrato Epimerasas/química , Tirosina/química , Vancomicina/farmacología , Secuencia de Aminoácidos , Secuencia de Carbohidratos , Carbohidratos , Catálisis , Dicroismo Circular , Cristalografía por Rayos X , Cinética , Espectrometría de Masas , Modelos Químicos , Modelos Moleculares , Datos de Secuencia Molecular , Mutagénesis , Mutagénesis Sitio-Dirigida , Conformación Proteica , Estructura Secundaria de Proteína , Salmonella enterica/metabolismo , Homología de Secuencia de Aminoácido , Relación Estructura-Actividad , Especificidad por Sustrato , Vancomicina/químicaRESUMEN
The vancomycin class of antibiotics is regarded as the last line of defence against Gram-positive bacteria. The compounds used clinically are very complex organic molecules and are made by fermentation. The biosynthesis of these is complex and fascinating. Its study holds out the prospect of utilizing genetic engineering of the enzymes in the pathway in order to produce novel vancomycin analogues. In part, this requires detailed structural insight into substrate specificity as well as the enzyme mechanism. The crystallization of one of the enzymes in the chloroeremomycin biosynthetic pathway (a member of the vancomycin family), dTDP-3-amino-4-keto 2,3,6-trideoxy-3-C-methyl-glucose-5-epimerase (EvaD) from Amycolatopsis orientalis, is reported here. The protein is fourth in the pathway which makes a carbohydrate essential for the activity of chloroeremomycin. The crystals of EvaD diffract to 1.5 A and have unit-cell parameters a = 98.6, b = 72.0, c = 57.1 A with space group P2(1)2(1)2. Data to this resolution were collected at the European Synchrotron Radiation Facility.
