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

Abstract

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 Ct Cel124. 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 Ct Cel124 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.

Novel Clostridium thermocellum Type I Cohesin-Dockerin Complexes Reveal a Single Binding Mode

“…6 and Table 5). As described above, the Doc124A dockerin presents a symmetry-related pair at helix-1, Ile 28 -Leu 29 , which could be involved in a similar interaction if binding was mediated by the helix-3 lower affinity interface. The importance of this pair was explored in m6.…”

A seawater-based open and continuous process for polyhydroxyalkanoates production by recombinant Halomonas campaniensis LS21 grown in mixed substrates

“…Industrial biotechnology aims to produce chemicals, materials and biofuels on a large scale using sustainable resources from agriculture [1][2][3]. By partially replacing depleting petrochemicals, industrial biotechnology should help sustainable development of human society [4][5][6][7][8][9].…”

Structural basis for entropy-driven cellulose binding by a type-A cellulose-binding module (CBM) and bacterial expansin

“…S7), may preferentially direct binding to cellulose surfaces with a similar twist. These sites might be junctions between crystalline and disordered cellulose, perhaps similar to the proposed targets of endoglucanase CtCel124, which has distinct binding surfaces for crystalline and noncrystalline cellulose (46). One speculative scenario is that EXLX1 binds to the slightly twisted region of a glucan chain that bridges two cellulose microfibrils.…”