FIGURE 7.

Hydrolysis of cellulose by processive CBH (A and B) and possible mechanism of synergism with EG (C). A, processive cycle of CBH consists of at least six putative steps as follows (65). a, CBM-mediated binding to the cellulose surface; b, finding and recognition of cellulose chain end; c, threading of cellulose chain into active site tunnel and formation of productive enzyme-substrate complex; d, hydrolysis of glycosidic bond; e, expulsion of product CB; f, threading another cellobiose unit to reform productive complex. Steps d–f are repeated until enzyme happens to dissociate or stops behind an obstacle (55). Transit times were found from the values of rate constants for the corresponding step(s). B, hydrolysis of cellulose by individual CBH. CBH cannot pass through the amorphous regions (wavy lines) and stalls. The length of the crystalline regions between amorphous parts defines the length of obstacle-free path (n_free), which also limits the apparent processivity (P_app) of CBH. The steady state rate of cellobiose formation is governed by the slow dissociation (k_off value was taken from Ref. 57) of stalled CBH. C, synergistic hydrolysis of cellulose. EG accelerates the recruitment of CBH by degrading amorphous regions. P_app of CBH is determined by the DP of EG-fragmented cellulose surface (DP_surface), and the steady state rate of cellobiose formation approaches the limit set by the velocity of processive movement of CBH. Transit times correspond to the steps taken by the CBH depicted to the left of the cellulose. Conventional mechanism of endo-exo synergism is depicted to the right of the cellulose.