Designer Cellulosomes: a Broad Platform for Synthetic Biotechnology

The plant cell wall comprises a collection of natural polymers, which include numerous complex carbohydrates, e.g., cellulose, xylans, mannans, arabinans, etc., and the aromatic polymer lignin, of which cellulose is the most abundant. Cellulose is composed entirely of the simple sugar glucose linked in β(1→4) bonds to form the repeating unit, the disaccharide cellobiose, which is arranged into long linear chains. This arrangement affords near-perfect hydrogen bonding within and between neighboring chains, forming a crystalline-like material, whereby its glucose residues are “locked” in place, virtually inaccessible to the organisms in nature that would otherwise exploit the glucose as an excellent food source.

Despite its recalcitrance, an ample corps of microorganisms (bacteria and fungi) can cope with decaying cellulosic matter, by virtue of the cellulolytic enzymes, the cellulases, that they produce. Aerobic fungi and bacteria tend to produce large amounts of cellulases and hemicellulases that together act synergistically in decomposition of the target polysaccharides to their component soluble sugars, as opposed to selected anaerobic bacteria that produce a multi-enzyme complex called the cellulosome. The cellulosome contains numerous cellulases, hemicellulases and related enzymes, attached to the bacterial cell surface, thus enabling efficient degradation of cellulosic substrates.

Recent work in our lab has centered on dismantling the cellulosome into its component parts and reassembling them into “designer cellulosomes” of precise enzymatic content and configuration. Rational bioengineering of cellulase and cellulosomal components for production of tailor-made multi-functional enzymes and “designer cellulosomes” is now being developed for improved cellulose degradation. The designer cellulosome approach shows promise for understanding the rationale behind its catalytic efficiency, and knowledge gained from these studies may provide the basis for creating improved designer cellulosomes for conversion of plant-derived biomass into liquid biofuels.

In our original proposal of the designer cellulosome concept more than two decades ago, we envisioned a much broader set of applications, outside of the biofuels arena. Integration of alternative enzymes and/or noncatalytic macromolecules render the designer cellulosome concept a general platform for self-assembly of biologically active nanomaterials.