There is considerable current interest in the economical production of ethanol from cellulosic biomass on commercial scale.  A key step in the process is the enzymatic hydrolysis of the polysaccharide chains that are tightly bound to the cellulose crystal.   We develop a mechanochemical model for the hydrolysis kinetics by the cellulase, a two-domain enzyme connected by a peptide linker, as it extracts and breaks down the polyscchraide chain from a crystalline substrate. We consider two random walkers, representing the catalytic domain (CD) and the carbohydrate binding module (CBM), whose rates for stepping are biased by the coupling through the linker and the energy required to lift the cellulose polymer from the crystalline surface.  Our results show that the linker length and stiffness play a critical role in the cooperative action of the CD and CBM domains and that, for a given linker length, the steady state hydrolysis shows a maximum at some intermediate linker stiffness.  The maximum hydrolysis rate corresponds to a transition of the linker from a compressed to an extended conformation, where the system exhibits maximum fluctuation as measured by the variance of the separation distance between the two domains and the dispersion around the mean hydrolysis speed.  In the range of experimentally known values of the parameters of our model, improving the intrinsic hydrolytic activity of the CD leads to proportional increase in the overall hydrolysis rate.