## 9 Properties of Processive Endocellulases in Hydrolyzing Pre-Treated Cellulosic Substrates
While the _Fibrobacter succinogenes_ GH5 endocellase rcCel5H had non-detectable activity on the crystalline cellulose substrates Avicel and Sigmacel 100, the _Fibrobacter succinogenes_ GH9 endocellase rcCel9B (67.3 kDa) had very low activities on the crystalline cellulose substrates Avicel (0.05 umol D-glucose/mg protein.min) and Sigmacel 100 (0.07 umol D-glucose/mg protein.min) [154]. By comparison, the GH5-p4818 Cel5_2A activity on Avicel from this study was about two-fold higher than the GH9 endocellase rcCel9 Bactivity from the _Fibrobacter succinogenes_ in the study by [PERSON] et al. [154]. [PERSON] et al. [69] further cloned an atypical GH9 gene termed Cel9D (79.4 kDa) in expressing the exocellulose (E.C. 3.2.1.74) activity, and characterized its synergism with other four endocellulares of Cel8B (81.4 kDa), Cel9B (67.3 kDa), Cel45C (37.7 kDa) and Cel51A (119 kDa) that were all cloned from the ruminal _Fibrobacter succinogenes_. By comparison, the GH5-p4818 Cel5_2A activity (1 U = umol D-glucose/min) on Avicel from this study was about 431-fold higher than the atypical GH9 exocellulose Cel9D activity (0.0005 U/mg protein), 307-fold higher than the endocellulose Cel8B activity (0.0007 U/mg protein), 215 fold higher than the endocellulose Cel9B activity (0.0010 U/mg protein), 1079-fold higher than the endocellulose Cel45C activity (0.0002 U/mg protein) and 359 fold higher than the endocellulose Cel51A activity (0.0006 U/mg protein) from the ruminal _Fibrobacter succinogenes_ in the study by [PERSON] et al. [69]. Thus, as compiled in Table 1, in comparisons with the GH5-tCel5A1 and the GH5-p4818 Cel5_2A endoglucanase activities as reported by [PERSON] and [PERSON] [127] and [PERSON] et al. [45], cattle ruminal bacterial endocellulae and exocellulae activities reported so far in the literature are considerably much lower. Arguably, highly active processive endocellases may still exist in cow's ruminal microbes till all the catalogued 2365 cows' ruminal cellulase genes with large numbers of the GH5 and GH9 genes are fully expressed and characterized. [PERSON] et al. [159] reviewed that GH5 genes represent one of the largest glycoside hydrolase family genes within the 113 GH families. As summarized by [PERSON] et al. [159], while a few of the GH5 subfamilies did not encode for endocellase enzyme activities, a number of the GH5 genes subfamilies have encoded GH proteins that have lost their catalytic machinery, indicating evolution towards other novel functions. Interestingly, studies by [PERSON] et al. [157] and [PERSON] et al. [160] screened several novel GH5 endocellases with little or none detectable activities on crystalline cellulose substrates from the buffalo rumen, whereas [PERSON] et al. [54] screened a novel GH5 endocellase with high activity (0.20 U/mg protein) on Avicel from the yak rumen, and [PERSON] et al. [55] screened a novel GH5 endocellase with very high activity (2.5 U/mg protein) on crystalline cellulose Avicel from the buffalo rumen metagenome. Last but not least, the presence of lignin polymers that wrap around the surface of individual microfibrillar units encapsulates the microfibrillar contents of hemicellulose and celluloses from effective enzyme penetration and attack. The negative effect of lignin on fibre degradation has been well documented in animal nutrition [145,146,158,161]. Nevertheless, it can be concluded that the overall ruminal microbial cellulase activities, particularly processive endocellase and exocellase activities towards crystalline celluloses, are low and are limited to allow fast and efficient lignocellulosic degradation.