Biochemical and genetic features of the plant cell wall degrading enzyme complex (cellulosome) from Clostridium cellulovorans
Roy H. Doi and Yutaka Tamaru
Section of Molecular & Cellular Biology, University of California, Davis, California 95616, USA
The biological degradation of cellulose, a major component of the earthis biomass, plays an important role in the carbon cycle on earth. There are generally two types of microorganisms that degrade cellulose: (a) aerobic microorganisms that produce a set of individual enzymes that hydrolyze cellulose to cellobiose and then to glucose and (b) primarily anaerobic microorganisms that produce an enzyme complex called the cellulosome that degrade cellulose, hemicellulose, and other plant cell wall components to smaller compounds that can be used for energy.
We have been studying the properties of the Clostridium cellulovorans cellulosome, which is an extracellular enzyme complex. The cellulosome has a mass of about 1 x 106 daltons and is composed of 10 subunits including a non-enzymatic scaffolding protein called CbpA and nine enzymatic subunits. Scanning electron micrographs have shown large protuberances on the cell surface which are believed to be comprised of large numbers of cellulosomes. Depending on the source of carbon during growth, the protuberances vary in size, with the largest occurring when cellulose is the carbon source.
The scaffolding protein CbpA, a major subunit of the cellulosome, contains a single cellulose binding domain (CBD), that binds the cellulosome to the substrate, nine hydrophobic domains (HBDs or cohesins) that bind the enzymatic subunits to CbpA, and four hydrophilic domains (SLH or surface layer homologous domains) that are now believed to bind the cellulosome to the cell surface.
Several genes for cellulosomal enzymes have been cloned and sequenced and the enzyme products have been characterized. The genes code for several types of endo-glucanase, an exoglucanase, a xylanase, a mannanase and a pectate lyase. Thus, the cellulosome is able to degrade cellulose, xylan, mannan, and pectin, components of the plant cell wall. The cellulosome is able to convert Arabidopsis and tobacco cells to protoplasts very efficiently.
