title: Structure and mechanism of endo/exocellulase E4 from Thermomonospora fusca.
abstract: Cellulase E4 from Thermomonospora fusca is unusual in that it has characteristics of both exo- and endo-cellulases. Here we report the crystal structure of a 68K M(r) fragment of E4 (E4-68) at 1.9 A resolution. E4-68 contains both a family 9 catalytic domain, exhibiting an (alpha/alpha)6 barrel fold, and a family III cellulose binding domain, having an antiparallel beta-sandwich fold. While neither of these folds is novel, E4-68 provides the first cellulase structure having interacting catalytic and cellulose binding domains. The complexes of E4-68 with cellopentaose, cellotriose and cellobiose reveal conformational changes associated with ligand binding and allow us to propose a catalytic mechanism for family 9 enzymes. We also provide evidence that E4 has two novel characteristics: first it combines exo- and endo-activities and second, when it functions as an exo-cellulase, it cleaves off cellotetraose units.
authors.affiliation: Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, New York 14853, USA.
authors.forename: J
authors.initials: J
authors.lastname: Sakon
authors.forename: D
authors.initials: D
authors.lastname: Irwin
authors.forename: D B
authors.initials: DB
authors.lastname: Wilson
authors.forename: P A
authors.initials: PA
authors.lastname: Karplus
mesh_terms: D000192:Actinomycetales / Q000201:enzymology*; D000595:Amino Acid Sequence; D001665:Binding Sites; D002480:Cellulase / Q000737:chemistry* / Q000378:metabolism*; D002482:Cellulose / Q000378:metabolism; D003001:Cloning, Molecular; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008961:Models, Structural; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013302:Streptomyces
chemical_list: D010446:Peptide Fragments; D011994:Recombinant Proteins; D002482:Cellulose; D002480:Cellulase