Chapter 4 - Cellobiohydrolases

Abstract

Cellobiohydrolases (CBHs) or exoglucanases are the enzymes produced by microorganisms to hydrolyze cellulose. CBHs belong to the glycoside hydrolases (GH) families 5, 6, 7, 9, 48, and 74. CBHs hydrolyze 1,4-β-d-glycosidic bonds present toward the end of cellulose chain, releasing cellobiose and glucose. There are two types of CBHs: CBHI, which cleaves cellulose chain progressively from the reducing end and CBHII, which cleaves the cellulose chain progressively from the nonreducing end. The catalytic domain of CBH possesses a tunnel-shaped structure that includes two surface loops covering the part of the active-site, which ensures processive mode of action as the enzyme progresses along the cellulose chain. CBHs are the major component of many fungal derived cellulolytic cocktails. Many fungal CBHs are associated with family 1 carbohydrate-binding module (CBM1), which may assist the catalytic core in the processive cleavage. The cellobiose produced by CBHs is eventually converted to glucose by β-glucosidases. The glucose then can be fermented by microbes to fuels, chemicals, plastics, fibers, detergents, pharmaceuticals, and many other industrial products. Therefore, CBHs have been the target for improving their biochemical properties by protein engineering, which is the focus of this chapter.