## 3 Limiting Steps of the Cellulose Hydrolytic Pathway  
The word cellulases is a generic term referring to a group of cellulose hydrolases classified under different glycosyl hydrolase families (GH), including endo-acting endocellulases, exo-acting exocellulaases and \(\beta\)-glucosidase that independently and synergistically degrade polymer cellulose substrates into soluble cellodext and glucose [15]. The soluble cellodext typically have a degree of polymerization (DP) of DP of 2-6 and include cellobiose (DP = 2), cellottories (DP = 3), cellotetraose (DP = 4), cellopentaose (DP = 5) and cellohexose (DP = 6) [19]. Classic non-processive endocellularases (EC 3.2.1.1), also termed 1,4-\(\beta\)-D-glucan-4-glucuronhydrolases, carry out hydrolysis by inserting a water molecule in the 1,4-\(\beta\)-glucosidase bond randomly at internal amorphous sites in the polymer cellulose, generating cello-oligosaccharides of various lengths with new chain ends [5; 15]. The first group of exocellulaases (EC 3.2.1.74), specifically 1,4-\(\beta\)-D-glucan glucanohydrolases or cellodext in [15], are also known as glucan-1,4-\(\beta\)-glucosidase according to the Braunschweig Brenda Enzyme Database ([[http://www.brenda-enzyme.org](http://www.brenda-enzyme.org)]([http://www.brenda-enzyme.org](http://www.brenda-enzyme.org)), accessed on 31 May 2021) or 1,4-\(\beta\)-D-glucan cellobiohydrolases II (CBHII) [68]. These exocellulaases primarily cut the \(\beta\)-1,4-glycosidic linkage in cello-oligosaccharides as well as amorphous and crystalline polymer celluloses from non-reducing chain ends with low specific activities towards cellobiose, thus, processively releasing cellobiose as the main product with very little glucose release [69; 70]. The second group of exocellulaases (EC 3.2.1.91), namely, 1,4-\(\beta\)-D-glucan cellobiohydrolases [15] or cellobiohydrolases I (CBHI) [67], are also termed cellulose 1,4-\(\beta\)-cellbiosidases according to the Braunschweig Brenda Enzyme Database ([[http://www.brenda-enzyme.org](http://www.brenda-enzyme.org)]([http://www.brenda-enzyme.org](http://www.brenda-enzyme.org))). These exocellulaases work on the \(\beta\)-1,4-glucoside linkage in low-DP cellodext from reducing chain ends with activities towards soluble cellodextrins and cellobiose, hence primarily releasing cellobiose and glucose in a processive manner [70]. The _Trichoderma_ fungal exocellulaases are so far the best elucidated processive cellulases in terms of biological mechanisms [71; 72]. Thus, exocellulaases are also the classic group of processive cellulases. More recently, processive endocellularases have also been discovered and characterized to contain a GH5 or a GH9 catalytic domain (Table 1), and these are very rare endocellulars that can continue to hydrolyze their initially broken-apart cello-oligosaccharides into soluble cellodext (DP of 2-6) and glucose with one such type of cellulases playing both endo-acting and exo-acting hydrolytic roles [72; 73]. \(\beta\)-Glucosidases (EC 3.2.1.21) are primarily responsible for the terminal hydrolysis of cellobiose and, to some extent, may also hydrolyze some soluble cellodext into D-glucose [5; 15]. [PERSON] et al. (2012) further suggested that the classical \(\beta\)-glucosidasees hydrolyze soluble cellodext (DP of 2-6) at similar rates whereas the rate of hydrolysis of exo-\(\beta\)-glucosidasees would increase as the DP increases from 2 to 6 [74], further distinguishing between the classic \(\beta\)-glucosidase and the exo-\(\beta\)-glucosidase activities. Furthermore, it is believed that endocellase and exocellase activities in hydrolyzing natural and pretreated cellulose substrates are the rate-limiting steps of the polymer cellulose biochemical degradation process [4; 5; 10; 15]. This notion can be further substantiated by the following facts. For comparison, one standard enzyme activity unit (U) is defined to be 1 umol D-glucose or reducing sugar released per min.