## 7 Microbial Processive Cellulase Properties of Hydrolyzing Natural Crystalline Substrates  
The chemically modified soluble cellulose substrates, such as CMC and HEC, are widely used for biochemically characterizing various endocellulaases and exocellularases. However, cellulase hydrolytic activities measured with CMC and HEC are not necessarily related to and cannot be simply extrapolated to their potential activities on pure crystalline or amorphous cellulose substrates, pretreated lignocellulosic biomass and natural raw lignocellulosic biomass materials [4] (the Braunschweig Brenda Enzyme Database at [[http://www.brenda-enzyme.org](http://www.brenda-enzyme.org)]([http://www.brenda-enzyme.org](http://www.brenda-enzyme.org))). It is a standard research practice to further characterize and compare the hydrolytic potential of cellulases on native lignocellulosic biomass by using model insoluble cellulose substrates. Major model insoluble cellulasic substrates include nearly pure celluloses such as cotton fibre, Whatman No. 1 filter paper, bacterial cellulose, bacterial microcrystalline cellulose (BMCC), microcrystalline cellulose (Avicel) and Sigmacell (20, 50 and 100) as well as impure cellulose-containing substrates such as dyed celluloses, \(\alpha\)-cellulose and various pretreated lignocellulosic substrates (e.g., Solska-Floc(r)) [4; 124]. There are some recognized intrinsic differences in the physiochemical properties among the major model insoluble cellulasic substrates including crystallinity index, cellulose size as marked by their DP and intraparticulate pore size [4; 19]. Crystallinity index (CrI) differs among the model insoluble native cellulasic substrates and variability of the CrI values largely reflects different intramolecular and intermolecular hydrogen bonding among polymer cellulose glucan molecules within each of the elementary fibrillar bundles as well as different techniques used for measuring the CrI values [4; 96; 106]. There are inconclusive views regarding the relationship between hydrolysis rates and CrI values in the major model insoluble cellulasic substrates. [PERSON] and [PERSON] [96] and [PERSON] et al. [125] showed that CrI values negatively predicted hydrolysis rates of the major model insoluble cellulasic substrates, whereas [PERSON] et al. [126] and [PERSON] et al. [4] concluded that substrate recalcitrance and hydrolysis rates were not strongly associated with their CrI values but reflected their overall accessibility of reactive sites to cellulases. Intrinsic differences among cellulases involved in the studies such as mechanisms of enzyme actions might have been responsible for the differences in the role of CrI values in the afore-discussed major model insoluble cellulasic substrate hydrolysis studies. Since lignin and celluloses are essentially removed in the major model insoluble cellulose substrates, intraparticulate porosity (radius of 1-10 nm) reflects free space or the capillary structure between each of the elementary fibrillar bundles in the major insoluble cellulasic substrates [19; 96; 107]. Avicel (PH101, PH102 and PH105) products are processed from wood pulp with removal of the amorphous fraction and are thus associated with intermediate CrI values (0.5-0.6) but relatively small DP values (DP of 150-500) [4]. The main Avicel (PH101, PH102 and PH105) products are further characterized with an intraparticulate pore peak radius between 2 and 3 nm [94]. Thus, model insoluble cellulasicsubstrates such as filter paper (Whatman #1) and Avice are widely used to characterize and compare hydrolysis rates of endocellulars and exocellulars for their potential commercial applications while Avicel is a more suitable substrate for assessing exocellalase activity due to its much smaller chain length. It is important to note that the GH5-tCel5A1 processive endoglucanase reported by [PERSON] and [PERSON] [127] and the GH5-p4818 Cel5_2A processive endoglucanase by [PERSON] et al.