The cellulose crystal which is the substrate for enzymatic attack in Cellulase 4D. (A) Three-dimensional representation of a cellulose rod where individual glucose molecules are represented by colored spheres. Orange spheres are the accessible edge glucose moieties in the cellulose chain; pink and green spheres signify the reducing and non-reducing ends, respectively, of the cellulose chain. The white spheres are inaccessible subsurface, flatsurface residues. The circled section indicates the site of endoglucanase hydrolysis of the cellulose chain at two positions five glucose units apart, leaving a reducing and non-reducing end to the chain. The released cellopentaose was solubilized and not indicated in the figure. (B) A view down the length of a smaller fragment of cellulose generated from its crystal structure, showing chains that are more exposed and hence more accessible to cellulases than those that are bordered by four adjacent chains.