title: A prototypical cytidylyltransferase: CTP:glycerol-3-phosphate cytidylyltransferase from bacillus subtilis.
abstract: BACKGROUND
  The formation of critical intermediates in the biosynthesis of lipids and complex carbohydrates is carried out by cytidylyltransferases, which utilize CTP to form activated CDP-alcohols or CMP-acid sugars plus inorganic pyrophosphate. Several cytidylyltransferases are related and constitute a conserved family of enzymes. The eukaryotic members of the family are complex enzymes with multiple regulatory regions or repeated catalytic domains, whereas the bacterial enzyme, CTP:glycerol-3-phosphate cytidylyltransferase (GCT), contains only the catalytic domain. Thus, GCT provides an excellent model for the study of catalysis by the eukaryotic cytidylyltransferases.
  
  
  RESULTS
  The crystal structure of GCT from Bacillus subtilis has been determined by multiwavelength anomalous diffraction using a mercury derivative and refined to 2.0 A resolution (R(factor) 0.196; R(free) 0.255). GCT is a homodimer; each monomer comprises an alpha/beta fold with a central 3-2-1-4-5 parallel beta sheet. Additional helices and loops extending from the alpha/beta core form a bowl that binds substrates. CTP, bound at each active site of the homodimer, interacts with the conserved (14)HXGH and (113)RTXGISTT motifs. The dimer interface incorporates part of a third motif, (63)RYVDEVI, and includes hydrophobic residues adjoining the HXGH sequence.
  
  
  CONCLUSIONS
  Structure superpositions relate GCT to the catalytic domains from class I aminoacyl-tRNA synthetases, and thus expand the tRNA synthetase family of folds to include the catalytic domains of the family of cytidylyltransferases. GCT and aminoacyl-tRNA synthetases catalyze analogous reactions, bind nucleotides in similar U-shaped conformations, and depend on histidines from analogous HXGH motifs for activity. The structural and other similarities support proposals that GCT, like the synthetases, catalyzes nucleotidyl transfer by stabilizing a pentavalent transition state at the alpha-phosphate of CTP.
authors.affiliation: Biophysics Research Division Department of Biological Chemistry University of Michigan Pathology Department University of Michigan Medical School Ann Arbor, MI 48109, USA.
authors.forename: C H
authors.initials: CH
authors.lastname: Weber
authors.forename: Y S
authors.initials: YS
authors.lastname: Park
authors.forename: S
authors.initials: S
authors.lastname: Sanker
authors.forename: C
authors.initials: C
authors.lastname: Kent
authors.forename: M L
authors.initials: ML
authors.lastname: Ludwig
mesh_terms: D000595:Amino Acid Sequence; D000604:Amino Acyl-tRNA Synthetases / Q000737:chemistry / Q000145:classification; D001412:Bacillus subtilis / Q000201:enzymology*; D001665:Binding Sites; D017124:Conserved Sequence; D003461:Crystallography / Q000379:methods; D003570:Cytidine Triphosphate / Q000378:metabolism; D019281:Dimerization; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009713:Nucleotidyltransferases / Q000737:chemistry* / Q000145:classification / Q000378:metabolism*; D011487:Protein Conformation; D017510:Protein Folding
chemical_list: D003570:Cytidine Triphosphate; D009713:Nucleotidyltransferases; C022348:phosphatidate cytidylyltransferase; D000604:Amino Acyl-tRNA Synthetases