Structure of MurA (UDP-N-acetylglucosamine enolpyruvyl transferase) from Vibrio fischeri in complex with substrate UDP-N-acetylglucosamine and the drug fosfomycin.

The development of new antibiotics is necessitated by the rapid development of resistance to current therapies. UDP-N-acetylglucosamine enolpyruvyl transferase (MurA), which catalyzes the first committed step of bacterial peptidoglycan biosynthesis, is a prime candidate for therapeutic intervention. MurA is the target of the antibiotic fosfomycin, a natural product produced by Streptomyces. Despite possessing a high degree of sequence conservation with MurA enzymes from fosfomycin-susceptible organisms, recent microbiological studies suggest that MurA from Vibrio fischeri (VfiMurA) may confer fosfomycin resistance via a mechanism that is not yet understood. The crystal structure of VfiMurA in a ternary complex with the substrate UDP-N-acetylglucosamine (UNAG) and fosfomycin has been solved to a resolution of 1.93 Å. Fosfomycin is known to inhibit MurA by covalently binding to a highly conserved cysteine in the active site of the enzyme. A comparison of the title structure with the structure of fosfomycin-susceptible Haemophilus influenzae MurA (PDB entry 2rl2) revealed strikingly similar conformations of the mobile substrate-binding loop and clear electron density for a fosfomycin-cysteine adduct. Based on these results, there are no distinguishing sequence/structural features in VfiMurA that would translate to a diminished sensitivity to fosfomycin. However, VfiMurA is a robust crystallizer and shares high sequence identity with many clinically relevant bacterial pathogens. Thus, it would serve as an ideal system for use in the structure-guided optimization of new antibacterial agents.

Structural biology of bacterial iron uptake systems.

Numerous bacterial proteins are involved in microbial iron uptake and transport and considerable variation has been found in the uptake schemes used by different bacterial species. However, whether extracting iron from host proteins such as transferrin, lactoferrin or hemoglobin or importing low molecular weight iron-chelating compounds such as heme, citrate or siderophores, Gram-negative pathogenic bacteria typically employ a specific outer membrane receptor, a periplasmic binding protein and two inner membrane associated proteins: a transporter coupled with an ATP-hydrolyzing protein. Often, studies have shown that proteins with similar function but little amino acid sequence homology are structurally related. Elucidation of the structures of the Escherichia coli outer membrane siderophore transport proteins FepA and FhuA have provided the first insights into the conformational changes required for ligand transport through the bacterial outer membrane. The variations between the structures of the prototypical periplasmic ferric binding protein FbpA from Neisseria and Haemophilus influenzae and the unusual E coli periplasmic siderophore binding protein FhuD reveal that the different periplasmic ligand binding proteins exercise distinct mechanisms for ligand binding and release. The structure of the hemophore HasA from Serratia marcescens shows how heme may be extracted and utilized by the bacteria. Other biochemical evidence also shows that the proteins that provide energy for iron transport at the outer membrane, such as the TonB-ExbB-ExbD system, are structurally very similar across bacterial species. Likewise, the iron-sensitive gene regulatory protein Fur is found in most bacteria. To date, no structural information is available for Fur, but the structure for the related protein DxtR has been determined. Together, these three-dimensional structures complement our knowledge of iron transport systems from other pathogenic bacteria, including Pseudomonas aeruginosa, which has a number of homologous iron uptake proteins. More importantly, the current structures for iron transport proteins provide rational starting points for design of novel antimicrobial agents.

Escherichia coli TehB requires S-adenosylmethionine as a cofactor to mediate tellurite resistance.

The Escherichia coli chromosomal determinant for tellurite resistance consists of two genes (tehA and tehB) which, when expressed on a multicopy plasmid, confer resistance to K(2)TeO(3) at 128 microg/ml, compared to the MIC of 2 microg/ml for the wild type. TehB is a cytoplasmic protein which possesses three conserved motifs (I, II, and III) found in S-adenosyl-L-methionine (SAM)-dependent non-nucleic acid methyltransferases. Replacement of the conserved aspartate residue in motif I by asparagine or alanine, or of the conserved phenylalanine in motif II by tyrosine or alanine, decreased resistance to background levels. Our results are consistent with motifs I and II in TehB being involved in SAM binding. Additionally, conformational changes in TehB are observed upon binding of both tellurite and SAM. The hydrodynamic radius of TehB measured by dynamic light scattering showed a approximately 20% decrease upon binding of both tellurite and SAM. These data suggest that TehB utilizes a methyltransferase activity in the detoxification of tellurite.

Plant aromatic L-amino acid decarboxylases: evolution, biochemistry, regulation, and metabolic engineering applications.

A comprehensive survey of the extensive literature relevant to the evolution, physiology, biochemistry, regulation, and genetic engineering applications of plant aromatic L-amino acid decarboxylases (AADCs) is presented. AADCs catalyze the pyridoxal-5'-phosphate (PLP)-dependent decarboxylation of select aromatic L-amino acids in plants, mammals, and insects. Two plant AADCs, L-tryptophan decarboxylase (TDC) and L-tyrosine decarboxylase (TYDC), have attracted considerable attention because of their role in the biosynthesis of pharmaceutically important monoterpenoid indole alkaloids and benzylisoquinoline alkaloids, respectively. Although plant and animal AADCs share extensive amino acid homology, the enzymes display striking differences in their substrate specificities. AADCs from mammals and insects accept a broad range of aromatic L-amino acids, whereas TDC and TYDC from plants exhibit exclusive substrate specificity for L-amino acids with either indole or phenol side chains, but not both. Recent biochemical and kinetic studies on animal AADCs support basic features of the classic AADC reaction mechanism. The catalytic mechanism involves the formation of a Schiff base between PLP and an invariable lysine residue, followed by a transaldimination reaction with an aromatic L-amino acid substrate. Both TDC and TYDC are primarily regulated at the transcriptional level by developmental and environmental factors. However, the putative post-translational regulation of TDC via the ubiquitin pathway, by an ATP-dependent proteolytic process, has also been suggested. Isolated TDC and TYDC genes have been used to genetically alter the regulation of secondary metabolic pathways derived from aromatic amino acids in several plant species. The metabolic modifications include increased serotonin levels, reduced indole glucosinolate levels, redirected shikimate metabolism, increased indole alkaloid levels, and increased cell wall-bound tyramine levels.

The structure of the ferric siderophore binding protein FhuD complexed with gallichrome.

Siderophore binding proteins play a key role in the uptake of iron in many gram-positive and gram-negative bacteria. FhuD is a soluble periplasmic binding protein that transports ferrichrome and other hydroxamate siderophores. The crystal structure of FhuD from Escherichia coli in complex with the ferrichrome homolog gallichrome has been determined at 1.9 ¿ resolution, the first structure of a periplasmic binding protein involved in the uptake of siderophores. Gallichrome is held in a shallow pocket lined with aromatic groups; Arg and Tyr side chains interact directly with the hydroxamate moieties of the siderophore. FhuD possesses a novel fold, suggesting that its mechanisms of ligand binding and release are different from other structurally characterized periplasmic ligand binding proteins.

Expression and properties of diacylglycerol acyltransferase from cell-suspension cultures of oilseed rape.

The expression of diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) with predicted molecular mass of 56.9. kDa (BnDGAT1) was examined using microspore-derived cell suspension cultures of oilseed rape (Brassica napus L. cv Jet Neuf). As well, a recombinant histidine-tagged N-terminal fragment of BnDGAT1 [BnDGAT1((1-116))His(6)], which was relatively hydrophilic, was partially characterized. A temporal increase in DGAT activity occurred within a 24 h period following transfer of cells from 6% (w/v) sucrose to 14% (w/v) sucrose. Western blotting indicated that the abundance of BnDGAT1 protein was closely correlated with DGAT activity. BnDGAT1 mRNA also exhibited a temporal increase within the 24 h period following transfer of cells into higher sucrose concentrations, but the transcript level was not closely associated with DGAT activity as BnDGAT1 protein. The fragment BnDGAT1(1-116)His(6) interacted with [1-(14)C]oleoyl-CoA, suggesting that the N-terminal region of BnDGAT1 may have a role in binding cellular acyl-CoA.

M-DNA: A complex between divalent metal ions and DNA which behaves as a molecular wire.

M-DNA is a complex of DNA with divalent metal ions (Zn(2+), Co(2+), or Ni(2+)) which forms at pH conditions above 8. Upon addition of these metal ions to B-DNA at pH 8.5, the pH decreases such that one proton is released per base-pair per metal ion. Together with previous NMR data, this result demonstrated that the imino proton in each base-pair of the duplex was substituted by a metal ion and that M-DNA might possess unusual conductive properties. Duplexes of 20 base-pairs were constructed with fluorescein (donor) at one end and rhodamine (acceptor) at the other. Upon formation of M-DNA (with Zn(2+)) the fluorescence of the donor was 95 % quenched. Fluorescence lifetime measurements showed the presence of a very fast component in the decay kinetics with tau

How do kinases transfer phosphoryl groups?

Understanding how phosphoryl transfer is accomplished by kinases, a ubiquitous group of enzymes, is central to many biochemical processes. Qualitative analysis of the crystal structures of enzyme-substrate complexes of kinases reveals structural features of these enzymes important to phosphoryl transfer. Recently determined crystal structures which mimic the transition state complex have added new insight into the debate as to whether kinases use associative or dissociative mechanisms of catalysis.

Snapshot of an enzyme reaction intermediate in the structure of the ATP-Mg2+-oxalate ternary complex of Escherichia coli PEP carboxykinase.

We report the 1.8 A crystal structure of adenosine triphosphate (ATP)-magnesium-oxalate bound phosphoenolpyruvate carboxykinase (PCK) from Escherichia coli. ATP binding induces a 20 degree hinge-like rotation of the N- and C-terminal domains which closes the active-site cleft. PCK possesses a novel nucleotide-binding fold, particularly in the adenine-binding region, where the formation of a cis backbone torsion angle in a loop glycine residue promotes intimate contacts between the adenine-binding loop and adenine, while stabilizing a syn conformation of the base. This complex represents a reaction intermediate analogue along the pathway of the conversion of oxaloacetate to phosphoenolpyruvate, and provides insight into the mechanistic details of the chemical reaction catalysed by this enzyme.

Base-pair opening and spermine binding--B-DNA features displayed in the crystal structure of a gal operon fragment: implications for protein-DNA recognition.

A sequence that is represented frequently in functionally important sites involving protein-DNA interactions is GTG/CAC, suggesting that the trimer may play a role in regulatory processes. The 2.5 A resolution structure of d(CGGTGG)/d(CCACCG), a part of the interior operator (OI, nucleotides +44 to +49) of the gal operon, co-crystallized with spermine, is described herein. The crystal packing arrangement in this structure is unprecedented in a crystal of B-DNA, revealing a close packing of columns of stacked DNA resembling a 5-stranded twisted wire cable. The final structure contains one hexamer duplex, 17 water molecules and 1.5 spermine molecules per crystallographic asymmetric unit. The hexamer exhibits base-pair opening and shearing at T.A resulting in a novel non-Watson-Crick hydrogen-bonding scheme between adenine and thymine in the GTG region. The ability of this sequence to adopt unusual conformations in its GTG region may be a critical factor conferring sequence selectivity on the binding of Gal repressor. In addition, this is the first conclusive example of a crystal structure of spermine with native B-DNA, providing insight into the mechanics of polyamine-DNA binding, as well as possible explanations for the biological action of spermine.