Peptidase

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Introductions

Peptidase is an enzyme that can hydrolyze peptide chains, and can be divided into two main types, namely endopeptidase and exopeptidase: where endopeptidase acts on specific peptide bonds inside the polypeptide chain, such as pepsin, trypsin and chymotrypsin that exist in the animal digestive tract, and these enzymes are specific for peptide bonds composed of different amino acids; exopeptidase is an enzyme that acts on peptide bonds at both ends of the protein peptide chain, and those that act on amino-terminal peptide bonds are called aminopeptidases, and those that act on carboxyl-terminal peptide bonds are called carboxypeptidases. Those that act on the peptide bond at the carboxyl end are called carboxypeptidases. Food proteins in the digestive tract through the continuous action of various peptidases, and finally broken down into free amino acids.

Figure 1. Structure of peptidase.

Drug Concept

Peptidases are a general term for enzymes that break down short protein peptides, mostly found in microbial cells. when the cells autolysis during fermentation, they release peptidases to hydrolyze the peptides to form amino acids, which serve as flavor precursors. They are necessary for the survival of all organisms, and the genes encoding peptidases account for 2% of all protein codes. Peptidases can be broadly classified into six categories according to the catalytic type: serine, threonine, cysteine, aspartate, glutamate and metallo-ionic peptidases. They include: aminopeptidase, dipeptidase, dipeptidyl peptidase, peptidyl dipeptidase, serine-type carboxypeptidase, metallo-type carboxypeptidase, cysteine-type carboxypeptidase, serine endopeptidase, cysteine endopeptidase, aspartate endopeptidase, metallo-endopeptidase, threonine endopeptidase, and unknown types of endopeptidases.

Endopeptidase

Endopeptidases, also commonly known as proteases, mainly act on the peptide bonds within protein polypeptide chains to break down long protein chains into short peptide fragments. Proteases are divided into different clans based on structural homology, and subclasses are further divided into different families based on the degree of sequence homology. When proteases hydrolyze proteins, the site of action varies depending on the type of peptide bond. For example, trypsin requires a peptide bond with basic amino acids (arginine, lysine) on the carboxyl side of the cut site; pepsin requires aromatic amino acids at both ends of the cut site; Bacillus subtilis basic protease requires hydrophobic aromatic amino acids (tryptophan, tyrosine, phenylalanine) on the carboxyl side of the cut site. The specificity of the cut site required by this protease is called substrate specificity of the protease. The substrate specificity of proteases can be used to obtain peptides with specific structures in a targeted manner.

Figure 2. One of endopeptidases.

Exopeptidases

Exopeptidases are widely distributed in animals, plants, microorganisms, and soil, and can play important roles in both normal biological physiological processes and pathophysiological processes, mainly involving signal transduction, regulation of peptide hormones, protein binding and digestion. The most studied aminopeptidases are lactic acid bacteria aminopeptidases, and more than 100 kinds of aminopeptidases have been isolated and purified from lactic acid bacteria, but there are relatively few research reports on other microbial exopeptidases related to food fermentation.

Classification of Exopeptidases

Exopeptidases act only at the end of one side near the polypeptide chain; enzymes that act at the free N terminus release individual residues, dipeptides, or tripeptides (i.e., aminopeptidase, dipeptidyl peptidase, and tripeptidyl peptidase, respectively); enzymes that act at the free C terminus release individual residues (carboxypeptidase) or dipeptides (dipeptidyl carboxypeptidase). Other exopeptidases specifically hydrolyze dipeptides (dipeptidyl peptidases) or remove terminal residues that are replaced by heterogeneous peptide bonds, cyclized, or cross-linked (i.e., peptide bonds that are not alpha carboxyl to alpha amino groups).