Cysteine protease

Related Reading

Cysteine proteases include cathepsins B, H, L, etc., which are generally found in lysosomes and are mainly involved in phagocytosis and the removal and digestion of excess substances in cells. Cysteine proteases, also known as thiol proteases, are enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad. Cysteine proteases are commonly encountered in fruits including the papaya, pineapple, fig and kiwifruit. The proportion of protease tends to be higher when the fruit is unripe. In fact, dozens of latices of different plant families are known to contain cysteine proteases. Cysteine proteases are used as an ingredient in meat tenderizers.

Figure 1. Protein structure of cysteine proteases.

Introductions

In recent years, the role of Cathepsin B (CB) has received increasing attention. The optimal pH of CB isolated from normal tissues is acidic and is inactive under neutral or alkaline conditions, but CB in tumor tissues is more active at neutral or alkaline conditions, which seems to be more suitable for the metabolism of malignant tumors disorder. CB can degrade type I collagen, LN, PG, and activate interstitial collagenase and type IV collagen, and degrade type I, II, III and IV collagen fibers in the matrix. Therefore, it is involved in tumor invasion and metastasis. Scientist detected the concentration of cysteine protease and its inhibitors in the cancerous tissue and adjacent tissues of laryngeal cancer, and found that the concentration of cysteine protease and its inhibitors in cancer tissues was significantly higher than that in adjacent tissues The concentration indicates that they may be involved in the whole process of tumor invasion.

Catalytic mechanism

The first step of the reaction mechanism in which cysteine proteases catalyze the hydrolysis of peptide bonds is that the sulfhydryl group at the active site of the enzyme is deprotonated by adjacent amino acids with basic side chains (usually histidine residues). The next step is the nucleophilic attack of the deprotonated cysteine anion sulfur on the carbonyl carbon of the substrate. In this step, the amine terminus is used to release fragments of the substrate, the histidine residue in the protease is reduced to its deprotonated form, and a new carboxy terminus of the substrate is connected to the cysteine thiol. Thioester intermediates. Therefore, they are sometimes called thiol proteases. The thioester bond is then hydrolyzed to generate carboxylic acid moieties on the remaining substrate fragments while regenerating the free enzyme.

Regulation

Proteases are usually synthesized into large precursor proteins called zymogens, such as serine protease precursor trypsinogen and chymotrypsin, and aspartic protease precursor pepsinogen. Protease is activated by removing inhibitory segments or proteins. Once the protease is delivered to a specific intracellular compartment (for example, lysosome) or the extracellular environment (for example, stomach), activation occurs. This system prevents protease-producing cells from being destroyed by it. A protease inhibitor is usually a protein with a domain that enters or blocks the active site of the protease to prevent entry of the substrate. In competitive inhibition, the inhibitor binds to the active site, thereby preventing the interaction of the enzyme with the substrate. In non-competitive inhibition, the inhibitor binds to the allosteric site, thereby changing the active site and making the substrate inaccessible.

Inhibitors

Cysteine protease inhibitor can be divided into three types: Type I is an intracellular single-chain protein containing 100 amino acid residues, without disulfide bonds and sugar chains; Type II is an exocrine type containing about 115 amino acid residues and two disulfides Bond; Type III is a multi-domain protein, mammalian kininogen belongs to this category. In addition, there are untyped cysteine protease inhibitors.

Reference

1. Grudkowska M.; et al. Multifunctional role of plant cysteine proteinases. Biol Chem. Acta Biochim. Pol. 2004, 51 (3): 609–24.