In the digestive systems of animals, enzyme always occupies a principal role in assimilating biomacromolecular nutrients. Large molecules can be broken down by enzymes into small fragments which can then be absorbed by human body easily. Many nutritional ingredients are in the form of large molecules such as sugar, proteins, and fat, which cannot be up taken easily by human body. Hence, these ingredients need to be broken down by enzymes into smaller pieces before the absorption by intestines, and this process is called catabolism. Following absorption, these small molecules will be used as building blocks to refresh the body through tissue repairing, regeneration, and growth, and this process is called anabolism. Different food substances can be digested by different enzymes.

There are mainly four enzymes required to be employed in catabolism and anabolism, such as amylases and proteases that break down starch and proteins, respectively. In ruminants utilizing herbivorous diets, microorganisms in the gut produce cellulase to decompose the cellulose cell walls of plant fiber. Lipases as a subclass of the esterases perform essential roles in the digestion of dietary lipids.

Cellulase

Cellulase is chiefly produced by fungi, bacteria, and protozoans to the decompose cellulose and some related polysaccharides into monosaccharides, shorter polysaccharides or oligosaccharides. by a procedure of cellulolysis. Cellulase could also represent any naturally occurring mixture or complex of various enzymes that serially or synergistically break down cellulosic materials. Cellulose breakdown has considerable economic importance, since it endows a major constituent of plants promising values of consumption and application in chemical reactions. The cleavage of the 1,4-β-D-glycosidic linkages in cellulose, hemicellulose, lichenin, and cereal β-D-glucans is involved in this hydrolysis that is relatively difficult compared to the degradation of other polysaccharides. Most mammals have only very limited ability to digest dietary fibres by themselves due to the short of cellulases. Symbiotic bacteria in many herbivorous animals and hindgut fermenters could produce cellulases, which could also be obtained from a few types of animals, such as termites and snails.

Cellulase has been applied for commercial food processing in coffee by hydrolyzing cellulose during drying of beans. Additionally, cellulases could be also noticed in textile industry, laundry detergents, paper industry for various purposes. They are even implicated in pharmaceutical applications as a treatment for phytobezoars, a form of cellulose bezoar found in the human stomach. Cellulase takes part in the fermentation of biomass into biofuels, which though is at relative experimental stage presently, and is also efficient in disrupting polymicrobial bacterial biofilms by breaking the β(1-4) glycosidic linkages within the matrix exopolysaccharides of the extracellular polymeric substance.

Protease

Proteases widespreadly occurring in all organisms, from prokaryotes to eukaryotes to viruses, perform protein catabolism by hydrolyzing peptide bonds. Different kinds of proteases can conduct the same reaction via completely distinct catalytic mechanisms, and are involved in an abundance of physiological reactions ranging from simple digestion of food proteins to highly regulated cascades. Proteolysis is such a promiscuous procedure that a broad range of substrates could be hydrolyzed. This is the case for digestive enzyme trypsin that is capable of cleaving an array of proteins ingested into smaller peptide fragments. However, promiscuous proteases have specificity for residue by typically binding to a specific amino acid on the substrate. Some proteases with strict selectivity only break down substrates with a certain sequence, which are essential in blood clotting and viral polyprotein processing in order to achieve precise cleavage events. The activity of proteases can be a destructive change by abolishing the function of protein or digesting it into corresponding principal components, an activation of function, or a signal in a signalling pathway. It is noteworthy that proteases, being themselves proteins, could be sliced by other proteases, sometimes of the same variety, which functions as a technique of regulating protease activity.

Lipase

Lipases catalyze the hydrolysis of fats or lipids, and occupy indispensable roles in digestion, transportation and processing of dietary lipids in most living organisms. Most lipases have a specific action site on the glycerol backbone of a lipid substrate. For example, human pancreatic lipase, the main enzyme breaking down dietary fats in the human digestive system, could transform triglyceride substrates in ingested oils into monoglycerides and two fatty acids. During an infection, lipases are expressed and secreted by pathogenic organisms. Particularly, a large number of different lipases that possibly reveal broad-lipolytic activity are found in Candida albicans and make possible contribution to the persistence and virulence of C. albicans in human tissue.

Lipases have long served for human practices as ancient as yogurt and cheese fermentation, and are also exploited as cheap and versatile catalysts to degrade lipids in more modern applications such as baking, laundry detergents and even as biocatalysts in innovative energy strategies to convert vegetable oil into fuel. Highly active lipase can be as a replacement for traditional catalyst in processing biodiesel with high energy intensity, environmental benefit and safety. In particular, gastric lipase secreted by the gastric chief cells can partially compensate for the decrease in pancreatic lipase production associated with pancreatic dysfunction, supplying additional means for the body to digest lipids.