ABSTRACT
Methylaspartate ammonia lyase (MAL) catalyzes the magnesium-dependent reversible alpha,beta-elimination of ammonia from L-threo-(2S,3S)-3-methylaspartic acid to mesaconic acid. The 1.3 A MAD crystal structure of the dimeric Citrobacter amalonaticus MAL shows that each subunit comprises two domains, one of which adopts the classical TIM barrel fold, with the active site at the C-terminal end of the barrel. Despite very low sequence similarity, the structure of MAL is closely related to those of representative members of the enolase superfamily, indicating that the mechanism of MAL involves the initial abstraction of a proton alpha to the 3-carboxyl of (2S,3S)-3-methylasparic acid to yield an enolic intermediate. This analysis resolves the conflict that had linked MAL to the histidine and phenylalanine ammonia lyase family of enzymes.
Subject(s)
Ammonia-Lyases/chemistry , Citrobacter/enzymology , Protein Structure, Tertiary , Amino Acid Sequence , Ammonia-Lyases/metabolism , Binding Sites , Crystallography, X-Ray , Evolution, Molecular , Models, Molecular , Molecular Sequence Data , Molecular Structure , Protein Folding , Substrate SpecificityABSTRACT
Methylaspartate ammonia lyase (MAL) catalyses the reversible alpha,beta-elimination of ammonia from L-threo-(2S,3S)-3-methylaspartic acid to give mesaconic acid. Crystals of Citrobacter amalonaticus MAL have been obtained by the hanging-drop method of vapour diffusion using ammonium sulfate as the precipitant. Three crystal forms were obtained from identical crystallization conditions, two of which (forms A and B) diffract to high resolution, whilst the third form diffracted poorly. Crystals of form A diffract to beyond 2.1 A and have been characterized as belonging to one of the enantiomorphic space groups P4(1)22 or P4(3)22, with unit-cell parameters a = b = 66.0, c = 233.1 A, alpha = beta = gamma = 90 degrees and a monomer in the asymmetric unit. Crystals of form B diffract to beyond 1.5 A and belong to space group C222, with unit-cell parameters a = 128.3, b = 237.4, c = 65.8 A, alpha = beta = gamma = 90 degrees and a dimer in the asymmetric unit. Determination of the structure of MAL will be an important step in resolving current conflicts concerning the enzyme mechanism which differ between one which places MAL as a member of the superfamily of ammonia lyases whose catalytic activity requires a cofactor formed by post-translational modification of the enzyme and another which links MAL to the enolase superfamily.
Subject(s)
Ammonia-Lyases/chemistry , Citrobacter/enzymology , Crystallization , Crystallography, X-Ray , Protein ConformationABSTRACT
Enoyl acyl carrier protein (ACP) reductase catalyses the last reductive step of fatty acid biosynthesis, reducing the enoyl group of a growing fatty acid chain attached to ACP to its acyl product using NAD(P)H as the cofactor. This enzyme is the target for the diazaborine class of antibacterial agents, the biocide triclosan, and one of the targets for the front-line anti-tuberculosis drug isoniazid. The structures of complexes of Escherichia coli enoyl-ACP reductase (ENR) from crystals grown in the presence of NAD+ and a family of diazaborine compounds have been determined. Analysis of the structures has revealed that a mobile loop in the structure of the binary complex with NAD+ becomes ordered on binding diazaborine/NAD+ but displays a different conformation in the two subunits of the asymmetric unit. The work presented here reveals how, for one of the ordered conformations adopted by the mobile loop, the mode of diazaborine binding correlates well with the activity profiles of the diazaborine family. Additionally, diazaborine binding provides insights into the pocket on the enzyme surface occupied by the growing fatty acid chain.
Subject(s)
Boron Compounds/chemistry , Boron Compounds/pharmacology , Enzyme Inhibitors/chemistry , Enzyme Inhibitors/metabolism , Escherichia coli/enzymology , Oxidoreductases/antagonists & inhibitors , Oxidoreductases/chemistry , Binding Sites , Boron Compounds/metabolism , Crystallography, X-Ray , Enoyl-(Acyl-Carrier-Protein) Reductase (NADH) , Enzyme Inhibitors/pharmacology , Models, Molecular , NAD/metabolism , Oxidoreductases/metabolism , Protein Binding , Protein Conformation , Protein Subunits , Structure-Activity Relationship , Triclosan/chemistry , Triclosan/metabolism , Triclosan/pharmacologyABSTRACT
Molecular genetic studies with strains of Escherichia coli resistant to triclosan, an ingredient of many anti-bacterial household goods, have suggested that this compound works by acting as an inhibitor of enoyl reductase (ENR) and thereby blocking lipid biosynthesis. We present structural analyses correlated with inhibition data, on the complexes of E. coli and Brassica napus ENR with triclosan and NAD(+) which reveal how triclosan acts as a site-directed, picomolar inhibitor of the enzyme by mimicking its natural substrate. Elements of both the protein and the nucleotide cofactor play important roles in triclosan recognition, providing an explanation for the factors controlling its tight binding to the enzyme and for the emergence of triclosan resistance.
