## 11 Multifunctionality of the Newly Characterized Processive Endocellases
These novel GH5-tCel5A1 and GH5-p4818 Cel5_2A with an optimal temperature at 50-70 \({}^{\circ}\)C and their major endocellulae activities retained between 37 and 50 \({}^{\circ}\)C will be suitable as an exogenous feed enzyme additive in a wide range of livestock feeding, also including pelleted monogastric fish, poultry and swine diets. For non-herbivore monogastric animal species including swine, poultry and fish, a much higher thermostability is usually required for exogenous enzymes such as phytases and endocellulases because diets for these animals are commonly pelleted at high temperature (60-80 \({}^{\circ}\)C) [170]. Thus, a biochemical property of thermostability, as measured by residual activity after being heated at 80 \({}^{\circ}\)C for 10 min, will be essentially ideal for GH5-p4818 Cel5_2A endocellulae to be potentially used as an exogenous enzyme additive for non-herbivore monogastric animal species [170]. On the other hand, while saccharification of feedstock with cellulases following pretreatment has been tested and developed for potential lignocellulosic ethanol production at various temperatures such as 37, 45 and 50 \({}^{\circ}\)C [170, 76, 101], a higher thermophilic temperature ranging at 60-65 \({}^{\circ}\)C is considered to be advantageous for commercial lignocellulosic ethanol production [88, 172, 173]. Thus, thermostable cellulase such as the GH5-Ccel5A1, as reported by [PERSON] and [PERSON] [127], reflecting this required hyperthermophilic temperature, would be desirable for potential commercial lignocellulosic ethanol production. Two approaches are typically used to explore these desired thermophilic cellulases, including (i) the screening of novel and highly active cellulases from thermophilic (50-75 \({}^{\circ}\)C) and hyperthermophilic (80-120 \({}^{\circ}\)C) through targeted genomic sequencing the extremely thermophilic microorganisms such as _Thermotoga martima_ for the Cel5A1 and its variants [137, 174]; and (ii) improving of selected mesophilic cellulases through mutagenesis-based enzyme protein engineering [175, 176] and/or glycosylation modification-based enzyme property modification [177]. The first approach, represented by the Cel5A1, may be limited due to the fact that the selected thermostable cellulases that are expressed and screened out of thermophilic & hyperthermophilic microbes will most likely have optimal temperature within the thermophilic temperature zone, which will not be suitable as exogenous enzyme additives in livestock feeding sectors. For example, the _Clostridium Phytofermantas_ GH9 thermophilic processive endocillusae with an optimal temperature at 65 \({}^{\circ}\)C, as reported by [PERSON] et al. [78], is not ideal to serve as an exogenous enzyme additive in livestock feeding, since this enzyme will lose about 60-70% of its maximal activity once fed through the animals' digestive tract system at a temperature ranging from 37 \({}^{\circ}\)C in mammals to 41 \({}^{\circ}\)C in poultry. The hyperthermophilic _Calcidellulosiruptor bescii_ bi-functional processive endocellus CelA was effective in enzymatically converting pretreated and raw biomass materials at 75-85 \({}^{\circ}\)C, as demonstrated by [PERSON] et al. [77], however, this enzyme would not be very active in animals' digestive system as a good exogenous enzyme additive in livestock feeding. Thus, upon further optimization for improving its thermostability, reaching 75-85 \({}^{\circ}\)C through effective strategies of enzyme protein engineering and glycosylation modifications, this GH5-p4818 Cel5_2A processive endocellus, reported by [PERSON] et al. [45], has the potential to be used for various industrial applications at mesophilic, thermophilic and hyperthermophilic temperature conditions. Further research needs to be conducted to examine the efficacy of this hyperthermophilic processive endocellus GH5-tCel5A1 and its variants, as reported by [PERSON] and [PERSON] [127], as exogenous cellulases for food animal feeding.