Monday, April 19, 2010
LL Conference Facility (Hilton Clearwater Beach)
Markus Alahuhta, Qi Xu, Yannick J. Bomble, William S. Adney, Shi-You Ding, Michael E. Himmel and Vladimir V. Lunin, Bioscience Center, National Renewable Energy Laboratory, Golden, CO
The crystal structure of the CBM4-Ig fused domain from the cellulosomal cellulase, cellobiohydrolase A (CbhA) of Clostridium thermocellum, has been determined, in complex with cellobiose, at 2.11 Å resolution (PDB code 3K4Z). This is the first cellulosomal CBM4 crystal structure reported to date. It has a typical β-sandwich fold with two β-sheets containing 5 antiparallel β-strands each. The two metal ions found in the CBM4 domain were identified as magnesium based on coordination geometry and bonding distances. The CbhA CBM4 structure is similar to the previously solved CBM4 structures from Thermotoga maritima, Cellulomonas fimi, and Rhodothermus marinus which are type B CBMs that bind to soluble oligosaccharides. However, this new structure possesses a significant feature - a peptide loop between Trp114 and Tyr121 with a tryptophan (Trp118) residing midway in the loop. Based on sequence alignment this feature might be common in all cellulosomal Clostridial CBM4s. Molecular dynamics simulations with cellohexaose showed strong stacking interactions between cellohexaose and the side chain of Trp118. This side chain resides on the protein surface ~10 Å away from the binding cleft and would lie parallel to the cellulose surface when CBM4 is in close proximity to cellulose. This observation is in agreement with earlier reports that CbhA CBM4 can bind crystalline cellulose in addition to soluble oligosaccharides. The potential role of Trp118 in binding crystalline cellulose is evaluated, as are the binding mechanisms of CbhA CBM4 to oligodextrins and oligoxylans.
