TheClostridium cellulovorans cellulosome: An enzyme complex with plant cell wall degrading activity

Abstract

Cellulose comprises a major portion of biomass on the earth, and the turnover of this material contributes to the CO2 cycle. Cellulases, which play a major role in the turnover of cellulosic materials, have been found either as free enzymes that work synergistically, or as an enzyme complex called the cellulosome. This review summarizes some of the general properties of cellulosomes, and more specifically, the properties of the Clostridium cellulovorans cellulosome. The C cellulovorans cellulosome is an extracellular enzyme complex with a molecular weight of about 1 x 10(6), and is comprised of at least ten subunits. The major subunit is the scaffolding protein CbpA, with a molecular weight of 189,000. This nonenzymatic subunit contains a cellulose binding domain (CBD) that binds the cellulosome to the substrate, nine conserved cohesins or enzyme binding domains, and four conserved surface layer homologous (SLH) domains. It is postulated that the SLH domains help to bind the cellulosome to the cell surface. The cellulosomal enzymes include cellulases (family 5 and 9 endoglucanases and a family 48 exoglucanase), a mannanase, a xylanase, and a pectate lyase. The cellulosome is capable of converting Arabidopsis and tobacco plant cells to protoplasts. One of the endoglucanases, EngE, contains three tandemly repeated SLHs at its N-terminus, and therefore appears capable of binding to the scaffolding protein CbpA as well as to the cell surface. Cellulosomes can attack crystalline cellulose, but the free cellulosomal enzymes can attack only soluble and amorphous celluloses. Nine genes for the cellulosome are found in a gene cluster cbpA-exgS-engH-engK-hbpA-engL-manA-engM-engN. Other cellulosomal genes such as engB, engE, and engY are not linked to the major gene cluster or to each other. By determining the structure and function of the cellulosome, we hope to increase the efficiency of the cellulosome by genetic engineering techniques.

Secretory pathway of cellulase: a mini-review

“…This is similar to the secretion of hemicellulase, for example aerobic fungi Trichoderma reesei and Aspergillus niger secrete 8 and 12 hemicellulases in high concentrations, respectively [85]. Fungi, such as T. reesei and A. niger , produce large amounts of extracellular cellulolytic enzymes, whereas some strains, including bacteria mainly from the class Clostridia, such as Clostridium cellulovorans [86-88] and Clostridium thermocellum [89,90], a few anaerobic fungi [91,92], and Eisenia fetida [93], mostly produce cellulolytic enzymes in a multienzyme complex called cellulosome, which is associated with the degrading cell wall [94,95]. …”

Profile of native cellulosomal proteins of Clostridium cellulovorans adapted to various carbon sources

“…We sequenced the entire genome of Clostridium cellulovorans (Tamaru et al [2010a]) and identified all genes, including those that encode proteins of known and unknown functions, related to cellulosome composition (Tamaru et al [2010a]). Genome analysis of C. cellulovorans indicated the presence of 57 cellulosomal enzymes-encoding genes including 4 scaffold proteins-encoding genes and 53 cellulosomal enzymes-encoding genes with dockerin domains (Doi and Tamaru [2001]). The major scaffold protein, CbpA, is comprised of 9 cohesin domains that bind to various cellulosomal enzymes via cohesin-dockerin interactions.…”

Properties of Selected Hemicellulases of a Multi-Enzymatic System fromPenicillium funiculosum

“…3,4) The efficiency of the synergistic or combined action of hemicellulases has been reported to be higher than that of their individual actions. 5) Some cellulolytic bacteria and filamentous fungi produce a set of cellulases and hemicellulases which synergistically hydrolyze crystalline celluloses and hemicelluloses to smaller oligosaccharides and subsequently to monomeric sugars upon the cleavage action of exo-enzymes.…”