Chapter 5 – Processive Cellulases

Cellulases are currently the third largest industrial enzyme class in sales volume and are likely to become the largest when and if the cellulosic ethanol industry matures. Cellulases also are key enzymes in recycling plant cell walls in nature. They are very diverse enzymes, because there are at least 10 different protein folds for cellulases, and in addition there is extensive sequence variation in each of the 12 cellulase families. This diversity probably result from two factors; one is the difficulty in hydrolyzing crystalline cellulose, which is reported to have a half-life greater than 107 years at pH 7, and the other is the extreme heterogeneity of plant cell walls, which are the main substrates of cellulolytic organisms. Thus, cellulolytic organisms may evolve a set of cellulases that are optimal for the types of cell walls that they use. There is evidence for positive selection for some cellulase genes, which is consistent with cellulases being evolved for specific substrates.