Cel7A proceeds along a single cellulose chain in a processive manner, cleaving one cellobiose unit per catalytic event. We have run extensive molecular dynamics simulations of a single cellulose chain inside the Cel7A catalytic tunnel to understand the structural basis for processivity. No large-scale protein motion is involved in the cellulose translocation. Among those residues lining the tunnel, hydrophobic residues are less dynamic than polar residues that are observed to promote the initial entry of the substrate into the tunnel. As the cellulose chain moves through the tunnel, the total number of hydrogen bonds remains constant. So the binding is facilitated by the hydrophobic interactions of the apolar sugar ring with the four Trp residues. The potential of mean force calculation reveals an overall downhill free energy profile for the initial threading, but become progressively flat and rugged toward the active site, highlighting a strong interplay between processivity and catalysis.

Simulating the transport of a cellulose chain through the cellulase catalytic tunnel

243rd National Meeting (2012)

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