Cellulosomes from Mesophilic Bacteria

Abstract

Plant cell wall-degrading enzymes have become increasingly important, since the development of efficient biomass degra- dation methods and the conversion of sugars to valuable prod- ucts such as butanol and amino acids and utilizable forms of energy such as ethanol and methane could lead to less depen- dence on imported petroleum as a fuel and chemical source. Plant biomass is an abundant renewable resource. Since cellu- lose and hemicellulose comprise about 40 to 50% of plant cell walls and are considered to be the largest components of the earth's biomass, efficient conversion of this material by engi- neered enzymes and/or microorganisms would be highly desir- able. The rate-limiting step in biomass degradation is the con- version of the cellulose and hemicellulose polymers to sugars. Basic research is essential to elucidate and improve the properties of enzymes that can degrade plant biomass, such as cellulose. Generally two types of cellulase systems occur. One type consists of independent extracellular cellulases that act synergistically to degrade cellulose, while the second type is represented by an enzyme complex called the "cellulosome," which consists of a nonenzymatic scaffolding protein associated with various enzymatic subunits that act in concert to degrade cellulose and hemicellulose. This minireview will focus on de- scribing the properties of cellulosomes from mesophilic bacte- ria and recent findings related to this interesting enzyme com- plex. The cellulosome from Clostridium thermocellum has been discussed in a number of recent reviews (2, 4, 5, 29). Reviews on cellulosomes (1, 3, 14, 15, 20, 57) and noncellulosomal cellulases (37, 48, 65) should be consulted for previous and complementary discussions of this subject. CELLULOSOMES What are cellulosomes? Cellulosomes are large extracellular enzyme complexes that are capable of degrading cellulose, hemicelluloses, and pectin and are produced by anaerobic bac- teria such as Clostridium, Acetivibrio, Bacteroides, and Rumino- coccus (Table 1). These microorganisms are found in various environmental niches, including soil, wood chip piles, sewage, and rumens. Cellulosomes actually function to degrade plant cell wall material and not only crystalline cellulose as its initial nomenclature suggested. Cellulosomes may be the largest ex- tracellular enzyme complexes found in nature, since polycellu- losomes have been reported to be as large as 100 MDa, al- though the individual cellulosomes range in mass from about 650,000 Da to 2.5 MDa. Cellulosomes were first observed by Bayer and Lamed as large protuberances on the surface of C. thermocellum (2, 31), and the complex was found to consist of a nonenzymatic scaf- folding protein to which were attached a number of enzymatic subunits (3). These observations on the complex structure were supported by the findings of Mayer et al. (38), who found by electron microscopy that cellulosomes had an ordered arrange- ment of subunits. There has been a relatively rapid expansion in identification of cellulosomes from various anaerobic cellu- lolytic bacteria (Table 1), and it is likely that cellulosomes will be identified in a large number of anaerobic bacteria. Cellulo- somes are now thought to degrade not only crystalline cellu- lose, but also hemicelluloses, chitin, and even pectin, depend- ing on the source of the cellulosomes. Interestingly, in one case, cellulosome-like structures were observed with Clostrid- ium acetobutylicum, but they did not have any activity in de- grading crystalline forms of cellulose (55). Recent studies have been devoted to characterizing and comparing the properties of the scaffolding protein and the cellulosomal enzymes from various bacterial sources. They have provided a glimpse into the diversity of cellulosomes and a partial rationale for their existence.