CELLULASES FOR BIOMASS CONVERSION
7. OUTLOOK FOR CELLULASE RESEARCH
chitin, -1,3-glucans and -(1,3)-(1,4) mixed linkage glucans, xylan, mannan, galactan, and starch. Some CBMs display “lectin-like” specificity and bind to a variety of cell-surface glycans (Borastonet al., 2004; Sorimachiet al., 1996, 1997;
Williamsonet al., 1997; Sigurskjoldet al., 1994). Based on structural and functional similarities, CBMs are been grouped into three types:
6.3.1. Type A Surface
Binding CBMs This class of CBMs binds to insoluble, highly crystalline cellulose and/or chitin. The aromatic amino acid residues play key role in the binding sites.
The planar architecture of the binding sites is thought to be complementary to the flat surfaces presented by cellulose or chitin crystals (Bayer et al., 1999).
The substrate binding site comprises the “hydrophobic” face of cellulose (Bayer et al., 1999). Upon binding to the substrate, the cellulosome is thought to undergo a supramolecular rearrangement so that the components redistribute to interact with the different target substrate. For this purpose, the various cellulosomal enzymes include different types of CBMs from different families that exhibit appropriate specificities that complement the action of the parent enzyme (Bayeret al., 2004).
6.3.2. Type B Polysaccharide-Chain-Binding CBMs
This class of CBMs binds to individual glycan chains. As with type A CBMs, aromatic residues play a pivotal role in ligand binding, and the orientation of these amino acids are key determinants of specificity. The binding sites often described as grooves or clefts, and comprise several sub-sites able to accommodate the individual sugar units of the polymeric ligand (Simpsonet al., 2000). In sharp contrast with the Type A CBMs, direct hydrogen bonds also play a key role in the defining the affinity and ligand specificity of Type B glycan chain binders (Notenboom et al., 2001; Xieet al., 2001).
6.3.3. Type C Small-Sugar-Binding CBMs
This class of CBMs has the lectin-like property of binding optimally to mono-, di-, or tri-saccharides and thus lacks the extended binding-site grooves of type B CBMs. The distinction between Type B CBMs and Type C CBMs can be subtle (Borastonet al., 2003).
6.3.4. Type D CBMs
This class of CBMs is always found in close spatial proximity with the catalytic domains of their respective proteins. Examples include the cellulase family 9 enzymes fromT. fusca(Sakonet al., 1997).
Technologies yields cellulase costs to the bioethanol process of $0.32 and $0.18 per gallon ethanol produced, respectively. These costs must be reduced to less than $0.05 per gallon ethanol by 2020 and this requires further increases in specific activity or production efficiency or some combination thereof (Wooley and Ruth, 1999).
It is most likely that the needed further improvements in cellulase performance will come via continued research aimed at understanding the basic principles by which these enzymes function on microcrystalline cellulose surfaces. Specifically, the mode of action of the “processive” enzymes, such as T. reesei CBH I and CBH II, must be more deeply understood before further improvement in activity via enzyme engineering tools can be realized.
ACKNOWLEDGEMENT
This work was funded by the U.S. Department of Energy’s Office of the Biomass Program.
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