The Cellulosomes: Multienzyme Machines for Degradation of Plant Cell Wall Polysaccharides

Abstract

The discrete multicomponent, multienzyme cellulosome complex of anaerobic cellulolytic bacteria provides enhanced synergistic activity among the different resident enzymes to efficiently hydrolyze intractable cellulosic and hemicellulosic substrates of the plant cell wall. A pivotal noncatalytic subunit called scaffoldin secures the various enzymatic subunits into the complex via the cohesin-dockerin interaction. The specificity characteristics and tenacious binding between the scaffoldin-based cohesin modules and the enzyme-borne dockerin domains dictate the supramolecular architecture of the cellulosome. The diversity in cellulosome architecture among the known cellulosome-producing bacteria is manifest in the arrangement of their genes in either multiple-scaffoldin or enzyme-linked clusters on the genome. The recently described three-dimensional crystal structure of the cohesin-dockerin heterodimer sheds light on the critical amino acids that contribute to this high-affinity protein-protein interaction. In addition, new information regarding the regulation of cellulosome-related genes, budding genetic tools, and emerging genomics of cellulosome-producing bacteria promises new insight into the assembly and consequences of the multienzyme complex.

The Enigmatic Universe of the Herbivore Gut

“…Cellulosomes were first identified in the bacterial family Clostridiaceae [9]. These large multienzyme complexes link diverse enzymes that function synergistically for high-efficiency plant biomass degradation ( Figure 1) enzymes within the complex.…”

“…In bacteria, the cellulase and hemicellulase enzymes and proteins containing carbohydrate-binding modules (CBMs) are complexed to a scaffold protein (scaffoldin) via interactions between dockerin and cohesin molecules. The cellulosome complex is often tethered to the bacterial cell surface [9]. In anaerobic fungi, cellulases, hemicellulases, and CBMs can be complexed to dockerin proteins.…”

Preparation and Characterization of Bacterial Protein Complexes for Structural Analysis

“…First discovered in the thermophilic anaerobe Clostridium thermocellum, cellulosomes have since been identified in a variety of other anaerobic bacteria, ruminal bacteria, and anaerobic fungi (Bayer et al, 2004). In general, the cellulosome can be divided into three modular protein components: (1) cell-surface proteins, (2) scaffold proteins, and (3) enzymes (Fig.…”

“…7A). Cellulosome assembly is mediated by conserved calcium-dependent protein-protein interaction modules called cohesins (Coh) and dockerins (Doc) (Bayer et al, 2004). At least three types of Coh-Doc pairings exist, although they do not exhibit any cross specificity for other types (Ding (Fujino et al, 1993) (Fig.…”

“…7A). To date, three type II Coh-containing cell-surface proteins and more than 70 type I Doc-containing cellulases and hemicellulases have been identified in C. thermocellum, which offers a large degree of subunit plasticity (Bayer et al, 2004). We have characterized the mechanism of cellulosome cell-surface attachment in C. thermocellum, both biochemically and structurally, using several methods including ITC, DSC, SAXS, and X-ray crystallography.…”