## 8 Discovered and Characterized Mono-Modular Processive Endoglucanase Properties  
The multi-functional endoglucanas GH5-Cel5A and variants were originally identified via targeted genome sequencing of the extremely thermophilic _Thermotoga maritima_ bacterium grown on glucan- and mannan-based polysaccharides [137]. [PERSON] and [PERSON] [127] further demonstrated truncation of GH5-Cel5A further enhanced its activity towards cellulosic substrates and GH5-Cel5A and its truncated variants such as GH5-tCel5A1 were monomodular processive endoglucanases. More recently, [PERSON] et al. [45] identified and characterized a mono-modular GH5-p4818 Cel5_2A processive endogluccanase from the porcine hindgut microbiome. Soluble substrates CMC, HEC and purified barley or oat \(\beta\)-glucans that are also associated with a viscous property are widely used to characterize an endo-\(\beta\)-1,4-glucanase activity [74; 138]. Since some exocellulases may also have considerable hydrolytic activities to these soluble substrates, simultaneous measurements of reduction in viscosity in one of the above soluble substrate solutions over a short period of incubation time are a traditional approach of distinguishing endocellulocytic activity from exocellulocytic activity [120; 132]. Substantial activities of theseetCel5A and GH5-p4818 Cel5_2A on CMC, HEC and purified barley \(\beta\)-glucan, as summarized in Table 1; and the time course of dramatic reduction in specific viscosity in the CMC solution were consistent with the data previously reported on three purified _Thermobifida fusca_ endoglucanases of Cel5A (E5), Cel6A (E2) and Cel9A (E4) [120], the _Constridium thermocellum_ noncellulosomal endoglucanase GH9B (Cell) [131], the _Constridium thermocellum_ cellulosomal GH9C (Cbha) [132], the _Saccharomyces degradans_ GH5 endoglucanase Cel5H [133], and the _Constridium Phytoferentans_ GH5 endoglucanase CpCel5B [77]. Meanwhile, hydrolytic activities of exocellulases were not effective in causing reduction in specific viscosity in the test CMC solutions as clearly demonstrated in some of the studies [132; 133; 120]. On the other hand, the non-detectable activities of the GH5-p4818 Cel5_2A on laminin (\(\beta\)-1,3-1,6-glucans) and curdlan (\(\beta\)-1,3-glucans) excluded the possibility that GH5-p4818 Cel5_2A could have the endo-\(\beta\)-1,3-glucanase (i.e., laminarinases, EC 3.2.1.39) and/or the endo-\(\beta\)-1,3-1,4-glucanase (i.e., lichenases, EC 3.2.1.73) activities [45; 139] (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))). Thus, the substrate specificity data in comparison with literature reports collectively suggest that these recently characterized tCel5A1 by [PERSON] and [PERSON] [127] and GH5-p4818 Cel5_2A by [PERSON] et al. [45] are the family-5 glycosyl hydrolase (GH5) \(\beta\)-1,4-endoglucanases. There are two types of processive cellulases: exocellulases and processive endocellulases [73]. Degree of processivity for exocellulases is usually determined through calculating the ratio of cellobiose to cellotriose based upon quantitative chromatographic analysis of hydrolytic products of a target exocellulase [73; 140; 141; 142]. The processive nature of endocellulases can be verified by calculating the processivity value, i.e., the ratio of the amount of total soluble reducing sugars to the amount of total insoluble reducing sugar ends, which is usually measured with pure crystalline cellulose substrates such as filter paper over a number of hours incubated with target endocellulases [73; 141; 142]. The following substrate hydrolytic data in comparison with literature reports support the notion that the GH5-p4818 Cel5_2A reported by [PERSON] et al. [45] is a novel processive endocellulase. Firstly, the pattern of dramatic reduction in specific viscosity in the CMC solution by [PERSON] et al.