## 9 Properties of Processive Endocellulases in Hydrolyzing Pre-Treated Cellulosic Substrates
[150] isolated and characterized high GH5 endocellase celA activity at 1.4 umol D-glucose/mg protein.min by using Avicel from the ruminal anaerobic fungus _Piromyces triazinflata_. Indeed, these reported ruminal anaerobic fungal endocellase activities were several folds higher than our GH5-p4818 Cel5_2A endoglucanase activity in hydrolyzing Avicel. However, anaerobic fungi only represent a small population (5-10%) of the total microbes in rumen [151]. Secondly, these ruminal anaerobic fungal endocellases are not processive in nature, thus cannot lead to efficient degradation of crystalline cellulose into soluble cellodextrin in vivo. [PERSON] et al. (2003) were the first to recognize and propose that the lack of expression of GH6 and GH7 exo-acting cellulases was likely the primary enzymatic factor limiting efficient degradation of crystalline cellulose in rumen [30]. Intriguingly, more recent work on microbial metagenomic sequencing and the GH gene cataloguing of cow's ruminal microbiome by [PERSON] et al. (2011) [41] confirmed the original view by [PERSON] et al. [30]. No GH6 and only one GH7 cellulase gene were identified out of a total of 2365 cow's ruminal microbial cellulase genes catalogued in the work conducted by [PERSON] et al. [41]. Also characterized with the cellulase systems in the aerobic fungal _Trichoderma reesei and Humicola insolens_, the type-I and type-II exo-acting cellulases, i.e., cellobiohydrolases, are largely classified to have GH6 and GH7 catalytic domains, and belong to GH6 and GH7 families [49]. Thus, the metagenomic analysis-based gene cataloguing by [PERSON] et al. [41] would support the notion that there are essentially very little or no exo-acting microbial cellulase genes and their activities in the bovine rumen. On the other hand, a large number of GH5 and GH9 cellulase genes were identified and catalogued from cow's ruminal microbiome by [PERSON] et al. [41]. It is known that some of the well-characterized processive endocellases belong to the GH5 and GH9 cellulase families. A large number of studies have been conducted to isolate and characterize ruminal microbial cellulases by using the classic chromatography-based biochemical protein purification [152, 153], gene cloning and expression [69, 154], organism genome sequencing based cloning and expression [82, 151, 155], metagenomic library based functional screening [53, 156], and gene-centric metagenomic sequencing, cataloguing and expression [41] during the past three decades. [PERSON] et al. [156] and [PERSON] et al. [157] screened and characterized some ruminal GH5 endocellases with non-detectable activities on crystalline cellulose substrates. [PERSON] et al. [53] characterized a number of novel cellulases with activities on several tested crystalline cellulose substrates; however, they did not classify catalytic domain GH families on these enzymes, and their enzymes were not purified and assayed by using the international unit for direct comparison with other studies. _Fibrobacter succinogenes_ and _Eubacterium cellulosolvens_ are some of the main fibrobl bacteria in rumen [158]. [PERSON] et al. [83] cloned and characterized a large bifunctional _Eubacterium cellulosolvens_ GH5 endocellase celA (127 kDa) showing activities on soluble substrate CMC and hemicellulose xylan but did not report activity on crystalline cellulose substrates. [PERSON] and [PERSON] [155] cloned and characterized a _Fibrobacter succinogenes_ endocellase with activities on several tested crystalline cellulose substrates; however, they did not classify a catalytic domain GH family on this enzyme, and their enzyme was not purified and assayed by using the international unit for direct comparison with other studies. [PERSON] et al. [154] cloned and characterized two main bifunctional _Fibrobacter succinogenes_ GH5 and GH9 endocellases of rcCel5H and rcCel9B with activities on soluble substrates CMC and HEC, amorphous substrate Ball-milled cellulose and hemicellulose xylan.