Structure of CBM4 from Clostridium thermocellum cellulase K

Abstract

Here, a 2.0 Å resolution X-ray structure of Clostridium thermocellum cellulase K family 4 carbohydrate-binding module (CelK CBM4) is reported. The resulting structure was refined to an R factor of 0.212 and an R(free) of 0.274. Structural analysis shows that this new structure is very similar to the previously solved structure of C. thermocellum CbhA CBM4. Most importantly, these data support the previously proposed notion of an extended binding pocket using a novel tryptophan-containing loop that may be highly conserved in clostridial CBM4 proteins.

Figure 1

The overall structure of C. thermocellum CelK CBM4 with two molecules in the asymmetric unit. β-Strands are shown in yellow, α-helices in red and loops in green.

Figure 2

The electron-density map of the Trp138 loop between the anchoring residues Trp134 and Phe141. This 2F _o − F _c map was calculated at 1.5σ after one cycle of REFMAC5 (Murshudov et al., 1997 ▶). The residues are shown in stick representation, with red O atoms, blue N atoms and gray C atoms.

Figure 3

The conserved binding-pocket features of clostridial CBM4 modules. The residues and the cellobiose of CbhA CBM4 are shown in stick representation, with red O atoms, blue N atoms and magenta C atoms for the CelK CBM4 structure and gray C atoms for the CbhA CBM4 module.