title: The structure of Mycobacteria 2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase, an essential enzyme, provides a platform for drug discovery.
abstract: BACKGROUND
  The prevalence of tuberculosis, the prolonged and expensive treatment that this disease requires and an increase in drug resistance indicate an urgent need for new treatments. The 1-deoxy-D-xylulose 5-phosphate pathway of isoprenoid precursor biosynthesis is an attractive chemotherapeutic target because it occurs in many pathogens, including Mycobacterium tuberculosis, and is absent from humans. To underpin future drug development it is important to assess which enzymes in this biosynthetic pathway are essential in the actual pathogens and to characterize them.
  
  
  RESULTS
  The fifth enzyme of this pathway, encoded by ispF, is 2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase (IspF). A two-step recombination strategy was used to construct ispF deletion mutants in M. tuberculosis but only wild-type double crossover strains were isolated. The chromosomal copy could be deleted when a second functional copy was provided on an integrating plasmid, demonstrating that ispF is an essential gene under the conditions tested thereby confirming its potential as a drug target. We attempted structure determination of the M. tuberculosis enzyme (MtIspF), but failed to obtain crystals. We instead analyzed the orthologue M. smegmatis IspF (MsIspF), sharing 73% amino acid sequence identity, at 2.2 A resolution. The high level of sequence conservation is particularly pronounced in and around the active site. MsIspF is a trimer with a hydrophobic cavity at its center that contains density consistent with diphosphate-containing isoprenoids. The active site, created by two subunits, comprises a rigid CDP-Zn2+ binding pocket with a flexible loop to position the 2C-methyl-D-erythritol moiety of substrate. Sequence-structure comparisons indicate that the active site and interactions with ligands are highly conserved.
  
  
  CONCLUSIONS
  Our study genetically validates MtIspF as a therapeutic target and provides a model system for structure-based ligand design.
authors.affiliation: Division of Biological Chemistry and Molecular Microbiology, College of Life Sciences, University of Dundee, Dundee DD1 5EH, UK. l.buetow@dundee.ac.uk
authors.forename: Lori
authors.initials: L
authors.lastname: Buetow
authors.forename: Amanda C
authors.initials: AC
authors.lastname: Brown
authors.forename: Tanya
authors.initials: T
authors.lastname: Parish
authors.forename: William N
authors.initials: WN
authors.lastname: Hunter
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D001665:Binding Sites; D003001:Cloning, Molecular; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D004269:DNA, Bacterial / Q000235:genetics / Q000302:isolation & purification; D015195:Drug Design; D018274:Electroporation; D017353:Gene Deletion; D015870:Gene Expression; D005798:Genes, Bacterial; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008024:Ligands; D008274:Magnesium / Q000378:metabolism; D008954:Models, Biological; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D020102:Mycobacterium smegmatis / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D009169:Mycobacterium tuberculosis / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D010957:Plasmids; D018583:Pliability; D011485:Protein Binding; D017433:Protein Structure, Secondary; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D014867:Water / Q000737:chemistry; D015032:Zinc / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D008024:Ligands; D021122:Protein Subunits; D014867:Water; D008274:Magnesium; D015032:Zinc