Chapter 3.03 - Multifunctional Enzyme Systems for Plant Cell Wall Degradation

Multifunctional enzymes refer to proteins that consist of two or more catalytic modules. Many microorganisms use multifunctional enzymes to efficiently break down the recalcitrant polymeric networks that constitute plant cell walls. Future applications of multifunctional enzymes may represent a potential solution to the problem of high enzyme cost for processing lignocellulosic biomass into fermentable sugars. Currently, commercial enzyme mixtures used in simultaneous saccharification fermentation process for biofuel production are derived primarily from free enzyme systems produced by fungi. In this context, we have analyzed the modular structures of 16. 937 genes corresponding to 34 glycoside hydrolase families putatively related to the degradation of lignocellulose in the Carbohydrate Active enZyme (CAZy) database. Among these genes, 64 gene sequences have been identified to putatively encode multifunctional enzymes, and up to five catalytic modules have been found in a single polypeptide. Based on their deduced polypeptide sequences, they can be classified into four types, that is, cellulase-cellulase, cellulase-hemicellulase, hemicellulase-hemicellulase, and hemicellulase-carbohydrate esterase. The compositional modules and architectural structures of these enzymes are analyzed here, and their putative activities on breaking down cell walls are discussed. We further discuss the predicted intramolecular synergistic mechanisms between the catalytic modules, including substrate channeling, which is a mechanism often proposed for carbohydrate-binding modules residing in multifunctional enzymes. Furthermore, the potential applications of native and engineered multifunctional enzymes for biomass conversion technology are also reviewed.