Structural insights into a unique cellulase fold and mechanism of cellulose hydrolysis

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 108; no. 13; pp. 5237 - 5242
• Main Authors: Brás, Joana L.A, Cartmell, Alan, Carvalho, Ana Luísa M, Verzé, Genny, Bayer, Edward A, Vazana, Yael, Correia, Márcia A.S, Prates, José A.M, Ratnaparkhe, Supriya, Boraston, Alisdair B, Romão, Maria J, Fontes, Carlos M.G.A, Gilbert, Harry J
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 29-03-2011; National Acad Sciences
• Subjects: Active sites; Biological Sciences; Carbohydrate Sequence; Catalytic Domain; Cells; Cellulase - chemistry; Cellulase - genetics; Cellulase - metabolism; Cellulose; Cellulose - metabolism; Cellulosomes; Clostridium thermocellum; Clostridium thermocellum - enzymology; Clostridium thermocellum - genetics; Crystal structure; Crystallography, X-Ray; endo-1,4-beta-glucanase; Enzyme substrates; Enzymes; Hydrolysis; Models, Molecular; Molecular Sequence Data; Molecular Structure; Molecules; nucleotide sequences; Oligosaccharides - chemistry; Oligosaccharides - metabolism; Plant cells; Polysaccharides; Protein Structure, Secondary; Protein Structure, Tertiary; Proteins; sequence homology
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1015006108

Clostridium thermocellum is a well-characterized cellulose-degrading microorganism. The genome sequence of C. thermocellum encodes a number of proteins that contain type I dockerin domains, which implies that they are components of the cellulose-degrading apparatus, but display no significant sequence similarity to known plant cell wall-degrading enzymes. Here, we report the biochemical properties and crystal structure of one of these proteins, designated CtCel124. The protein was shown to be an endo-acting cellulase that displays a single displacement mechanism and acts in synergy with Cel48S, the major cellulosomal exo-cellulase. The crystal structure of CtCel124 in complex with two cellotriose molecules, determined to 1.5 Å, displays a superhelical fold in which a constellation of α-helices encircle a central helix that houses the catalytic apparatus. The catalytic acid, Glu96, is located at the C-terminus of the central helix, but there is no candidate catalytic base. The substrate-binding cleft can be divided into two discrete topographical domains in which the bound cellotriose molecules display twisted and linear conformations, respectively, suggesting that the enzyme may target the interface between crystalline and disordered regions of cellulose.