title: Crystallographic evidence for substrate ring distortion and protein conformational changes during catalysis in cellobiohydrolase Ce16A from trichoderma reesei.
abstract: BACKGROUND
  Cel6A is one of the two cellobiohydrolases produced by Trichoderma reesei. The catalytic core has a structure that is a variation of the classic TIM barrel. The active site is located inside a tunnel, the roof of which is formed mainly by a pair of loops.
  
  
  RESULTS
  We describe three new ligand complexes. One is the structure of the wild-type enzyme in complex with a nonhydrolysable cello-oligosaccharide, methyl 4-S-beta-cellobiosyl-4-thio-beta-cellobioside (Glc)(2)-S-(Glc)(2), which differs from a cellotetraose in the nature of the central glycosidic linkage where a sulphur atom replaces an oxygen atom. The second structure is a mutant, Y169F, in complex with the same ligand, and the third is the wild-type enzyme in complex with m-iodobenzyl beta-D-glucopyranosyl-beta(1,4)-D-xylopyranoside (IBXG).
  
  
  CONCLUSIONS
  The (Glc)(2)-S-(Glc)(2) ligand binds in the -2 to +2 sites in both the wild-type and mutant enzymes. The glucosyl unit in the -1 site is distorted from the usual chair conformation in both structures. The IBXG ligand binds in the -2 to +1 sites, with the xylosyl unit in the -1 site where it adopts the energetically favourable chair conformation. The -1 site glucosyl of the (Glc)(2)-S-(Glc)(2) ligand is unable to take on this conformation because of steric clashes with the protein. The crystallographic results show that one of the tunnel-forming loops in Cel6A is sensitive to modifications at the active site, and is able to take on a number of different conformations. One of the conformational changes disrupts a set of interactions at the active site that we propose is an integral part of the reaction mechanism.
authors.affiliation: Department of Cell and Molecular Biology Uppsala University BMC Box 596, S-751 24, Uppsala, Sweden.
authors.forename: J y
authors.initials: Jy
authors.lastname: Zou
authors.forename: G J
authors.initials: GJ
authors.lastname: Kleywegt
authors.forename: J
authors.initials: J
authors.lastname: Ståhlberg
authors.forename: H
authors.initials: H
authors.lastname: Driguez
authors.forename: W
authors.initials: W
authors.lastname: Nerinckx
authors.forename: M
authors.initials: M
authors.lastname: Claeyssens
authors.forename: A
authors.initials: A
authors.lastname: Koivula
authors.forename: T T
authors.initials: TT
authors.lastname: Teeri
authors.forename: T A
authors.initials: TA
authors.lastname: Jones
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D002480:Cellulase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D043366:Cellulose 1,4-beta-Cellobiosidase; D018360:Crystallography, X-Ray; D005960:Glucosides / Q000737:chemistry / Q000378:metabolism; D008024:Ligands; D009154:Mutation; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D014242:Trichoderma / Q000201:enzymology*
chemical_list: D005960:Glucosides; D008024:Ligands; C401203:iodobenzyl-glucopyranosyl-(1,4)-xylopyranoside; D002480:Cellulase; D043366:Cellulose 1,4-beta-Cellobiosidase