title: Molecular modeling of the reductase domain to elucidate the reaction mechanism of reduction of peptidyl thioester into its corresponding alcohol in non-ribosomal peptide synthetases.
abstract: BACKGROUND
  Nonribosomal peptide synthetases (NRPSs) are multienzymatic, multidomain megasynthases involved in the biosynthesis of pharmaceutically important nonribosomal peptides. The peptaibol synthetase from Trichoderma virens (TPS) is an important member of the NRPS family that exhibits antifungal properties. The majority of the NRPSs terminate peptide synthesis with the thioesterase (TE) domain, which either hydrolyzes the thioester linkage, releasing the free peptic acid, or catalyzes the intramolecular macrocyclization to produce a macrolactone product. TPS is an important NRPS that does not encompass a TE domain, but rather a reductase domain (R domain) to release the mature peptide product reductively with the aid of a NADPH cofactor. However, the catalytic mechanism of the reductase domain has not yet been elucidated.
  
  
  RESULTS
  We present here a three-dimensional (3D) model of the reductase domain based on the crystal structure of vestitone reductase (VR). VR belongs to the short-chain dehydrogenase/reductase (SDR) superfamily and is responsible for the nicotinamide dinucleotide phosphate (NADPH)-dependent reduction of the substrate into its corresponding secondary alcohol product. The binding sites of the probable linear substrates, alamethicin, trichotoxin, antiamoebin I, chrysopermin C and gramicidin, were identified within the modeled R domain using multiple docking approaches. The docking results of the ligand in the active site of the R domain showed that reductase side chains have a high affinity towards ligand binding, while the thioester oxygen of each substrate forms a hydrogen bond with the OH group of Tyr176 and the thiol group of the substrate is closer to the Glu220. The modeling and docking studies revealed the reaction mechanism of reduction of thioester into a primary alcohol.
  
  
  CONCLUSIONS
  Peptaibol biosynthesis incorporates a single R domain, which appears to catalyze the four-electron reduction reaction of a peptidyl carrier protein (PCP)-bound peptide to its corresponding primary alcohol. Analysis of R domains present in the non-redundant (nr) database of the NCBI showed that the R domain always resides in the last NRPS module and is involved in either a two or four-electron reduction reaction.
authors.affiliation: Department of Molecular Science and Technology, Ajou University, Suwon, 443-749, Korea. sangdunchoi@ajou.ac.kr
authors.forename: Balachandran
authors.initials: B
authors.lastname: Manavalan
authors.forename: Senthil K
authors.initials: SK
authors.lastname: Murugapiran
authors.forename: Gwang
authors.initials: G
authors.lastname: Lee
authors.forename: Sangdun
authors.initials: S
authors.lastname: Choi
mesh_terms: D000438:Alcohols / Q000378:metabolism; D000595:Amino Acid Sequence; D001665:Binding Sites; D055162:Biocatalysis; D020134:Catalytic Domain; D003198:Computer Simulation; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry; D010453:Peptide Synthases / Q000737:chemistry* / Q000378:metabolism; D010455:Peptides / Q000737:chemistry / Q000378:metabolism; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012984:Software; D013379:Substrate Specificity; D014242:Trichoderma / Q000201:enzymology
chemical_list: D000438:Alcohols; D010455:Peptides; D009249:NADP; C071072:vestitone reductase; D044925:Oxidoreductases Acting on CH-CH Group Donors; D010453:Peptide Synthases; C104385:non-ribosomal peptide synthase