Though thousands of diverse GHs have been discovered and hundreds characterized, their enzymatic activities are still too low to meet our demands. Scientists are employing rational, random, and semi-random methods to enhance GH catalytic efficiency and optimize substrate specificity. Site-directed mutagenesis is a common strategy of protein engineering in which mutations are purposefully introduced into the polypeptide. It has been successfully applied to increase the pH stability and range of the catalytic activity of Cel6A from T. reesei (Wohlfahrt et al. 2003), and to improve the thermostability of cellulase C and Xyn II from C. thermocellum and T. reesei, respectively (Nemeth et al. 2002; Fenel et al. 2004). To engineer novel enzymatic properties, researchers have also empolyed directed evolution, which consists of random mutagenesis followed by screens or selections to identify enzymatic features of interest. Error prone PCR-based mutagenesis produced modified Cel12A from T. reesei, and Cel6F from Orpinomyces PC-2, both of which showed changes in their pH optima (Wang et al. 2005; Hughes et al. 2006). Another powerful approach used for creating diversity is called in vitro recombination or DNA shuffling, which is based on random or directed recombination among different DNA fragments or conserved motifs (Coco et al. 2001). In this way, Heinzelman et al. (2009) identified highly thermostable class II cellobiohydrolase chimeras (Heinzelman et al. 2009). Similarly, a fusion of the Thermotoga maritima Cel5A with the CBM1 from T. reesei and CBM6 from C. stercorarium xylanase A generated a CBM-engineered Cel5A with 14- to 18-fold higher hydrolytic activity towards Avicel (Mahadevan et al.

2008) . In another example, a chimeric bifunctional enzyme endo5A-GS — Xyl11D, in which a glycine-serine (GS) linker separated the two enzymes, demonstrated 1.6-fold and 2.3-fold higher activity than the parent enzymes, respectively (Adlakha et al. 2011).

Researchers have also been inspired by the apparent modularity of cellulosome architecture to create artificial mini-cellulosomes in vitro and in heterologous bacterial systems. In a recent study, synthetic cellulosomes significantly enhanced cellulose hydrolysis, especially in the case of highly recalcitrant cellulose at low enzyme loading rates (You et al. 2012). Toward enabling industrial usage of the powerful cellulosome biochemical machinery, heterologous surface assembly of cellulosomes has been realized in yeast and Bacillus subtilis (Wen et al. 2010; Goyal et al. 2011; Fan et al.

2012) . The engineered yeast are capable of simultaneous saccharification and fermentation of crystalline cellulose to ethanol (Fan et al. 2012). The cellulosome engineering processes would be further enhanced by additional structural and biochemical characterization of component GHs and other enzymes.