Proteolytic enzymes are a general term for enzymes that catalyze the hydrolysis of polypeptides or proteins, referred to as proteases. It is widely distributed among animals, plants and bacteria. It is abundant in the digestive tract of animals and the lysosomes of various cells in the body. Proteases play an important role in the body's metabolism and biological regulation. The molecular weight is generally from 2 to 30,000. Protease can be divided into endopeptidase and exopeptidase according to the site of the hydrolysis substrate. The former hydrolyzes the peptide bond in the middle part of the protein, and the latter gradually degrades the amino acid residue from the amino or carboxyl end of the protein.
According to the nature of various protease active sites can be divided into four categories:
1. Serine protease
Its active center includes histidine and aspartate residues in addition to serine, such as various endopeptidases secreted by the pancreas and various proteases associated with blood coagulation, hemolytic fibers, and complement systems.
Figure 1. Protein structure of serine protease.
Its active center requires the involvement of histidine residues in addition to cysteine, such as certain cathepsins in plant-derived and cell lysosomal bodies.
3. Aspartic protease
Their active centers are composed of two aspartic acid residues, such as pepsin secreted by the gastric membrane, angiotensin-releasing enzyme in the kidney, and certain cathepsins in the cell lysosome.
Figure 2. Protein structure of aspartic protease.
In addition to metal ions, the active center needs to participate in other amino acid residues. For example, the active center of pancreatic carboxypeptidase A includes zinc ions (Zn2+) and glutamic acid and tyrosine residues. Most exopeptidases and certain bacterial proteases often fall into this category. In addition to the groups involved in the hydrolysis of peptide bonds, various proteases need to have certain sites that bind to the substrate. Because of these differences, the specificity of various proteases is determined.
Figure 3. Protein structure of carboxypeptidase A.
Since the discovery of the important role of proteases in the biological regulation process, they can be classified according to their physiological functions and their specificity.
1. Non-limiting hydrolyzing protease
It means that the specificity of the enzyme is very poor, and it can hydrolyze the very polypeptide bonds in the protein to produce various small peptides and even free amino acids. The physiological function of this kind of protease is mainly involved in the degradation of proteins in the body. For example, various proteases secreted by the gastrointestinal system digest and decompose the food protein ingested in vitro; various cathepsins in the cell lysosome can eliminate various metabolisms in the body. product. Red blood cells with an average lifespan of 120 days are degraded by cathepsin.
2. Restricted hydrolase
It refers to the specificity of the enzyme, which only acts on a specific protein substrate, hydrolyzes the specific peptide bond, and then produces various active polypeptides or proteins with different physiological functions. Such proteases play a biological regulation in vivo, and most of them belong to serine proteases, and their specificity is similar to that of trypsin, that is, a peptide bond composed only at the carboxyl terminus of arginine or lysine. Unlike other proteases, these proteases have low or no degrading activity on common proteins such as casein or hemoglobin. Even for their specifically hydrolyzed protein substrates, there are strict requirements on conformation, and once the substrate is denatured, the protease that cannot be degraded by it, but not the restricted hydrolysis, is the opposite. The greater the degree of denaturation of the protein substrate, the easier it is to hydrolyze. For example, when thrombin activates fibrinogen, only the sperm-glycopeptide bond near the N-terminus of the fibrinogen α and β subunits is hydrolyzed in the peptide bond of 150 arginine or lysine residues, releasing bleeding. Fibrin peptides A, B, thereby converting soluble fibrinogen into gel-like fibrin.
Many important physiological effects in the body are related to the biological regulation of proteases. As listed in the table, when the body is stimulated by external stimuli to mobilize the corresponding physiological reactions, the proteases in the body are mobilized to make certain polypeptides or proteins that are not physiologically active, and quickly become functional. Very strong corresponding products, thus achieving the purpose of the body's defense, survival and reproduction. Some mobilization processes are simpler and can be accomplished by a single catalytic reaction. For example, trypsinogen which is inactive in the gastrointestinal tract produces active trypsin when its N-terminus is hydrolyzed by enterokinase to remove a 6-peptide. Some processes are quite complex and involve more than 10 components in multiple catalytic reactions, such as blood coagulation and complement reactions.
Due to its rich source of proteases, it can be fermented by bacteria in addition to extraction from plants and plants. It has been widely used in medical, food, tanning, silk and other industries, and can also be used as an important biochemical reagent. For example, chymotrypsin is used in cataract extraction surgery to simplify surgery and improve success rate. Trypsin has obvious effects on removing necrotic tissue, anti-inflammatory and purulent, healing wounds; urokinase and streptokinase are used to treat venous thrombosis and vasculitis; Kallikrein is used to improve coronary heart disease, make microvascular relaxation, blood pressure drop; elastase has a certain effect on vascular sclerosis; bacterial protease is used for leather hair removal, silk gelatin. The chymosin is used to make cheese products; papaya or bromelain is used as a beer stabilizer to eliminate protein precipitation due to beer refrigeration.
1. Oda K.; et al. New families of carboxyl peptidases: serine-carboxyl peptidases and glutamic peptidases. Journal of Biochemistry. 2012, 151 (1): 13–25.