Humans and mammals contain at least 5,000 enzymes. They are either dissolved in the cytoplasm, or combined with various membrane structures, or located at specific positions in other structures within the cell, and are only activated when needed. These enzymes are collectively called intracellular enzymes; in addition, There are some enzymes that are synthesized within cells and then secreted outside the cells - extracellular enzymes.

According to reaction properties

According to the different properties of the reactions catalyzed by enzymes, enzymes are divided into seven categories:

Oxidoreductase is an enzyme that promotes oxidation-reduction reactions of substrates. It is a type of enzyme that catalyzes oxidation-reduction reactions and can be divided into two categories: oxidase and reductase.

Transferases are enzymes that catalyze the transfer or exchange of certain groups (such as acetyl, methyl, amino, phosphate, etc.) between substrates. For example, methyltransferase, aminotransferase, acetyltransferase, transsulfurase, kinase and polymerase, etc.

Hydrolases are enzymes that catalyze the hydrolysis of substrates. For example, amylase, protease, lipase, phosphatase, glycosidase, etc.

Lyases are enzymes that catalyze a reaction that removes a group from a substrate (non-hydrolytically) and leaves a double bond, or its reverse reaction. For example, dehydratase, decarboxylase, carbonic anhydrase, aldolase, citrate synthase, etc. Many lyases catalyze reverse reactions that allow the formation of new chemical bonds between two substrates and the elimination of a double bond in one substrate. Synthase falls into this category.

Isomerases are enzymes that catalyze the conversion between various isomers, geometric isomers or optical isomers. For example, isomerase, epimerase, racemase, etc.

Synthetic enzymes (ligase) catalyze the synthesis of two molecular substrates into one molecule of compound, and are coupled to enzymes that break the phosphate bond of ATP to release energy. For example, glutamine synthetase, DNA ligase, amino acid:tRNA ligase, and biotin-dependent carboxylase, etc.

Translocases are enzymes that catalyze the transport of ions or molecules across membranes or within membranes. Some of the enzymes involved in ATP hydrolysis reactions are classified as hydrolases (EC 3.6.3-), but hydrolysis reaction is not the main function of this type of enzyme. Therefore, the nomenclature committee recently decided to place this class of enzymes in the seventh major enzyme class.

According to the unified classification principles of enzymes published by the International Association of Biochemistry, based on the above seven major categories, each major category of enzymes is divided into several subcategories according to the characteristics of the groups or bonds that are acted on in the substrate; in order to better Precisely indicating the nature of the substrate or reactant, each subcategory is further divided into several groups (subsubcategories); each group directly contains several enzymes.

According to chemical composition

pure protein

Enzymes that are pure proteins and do not contain other substances except protein, such as urease, protease, amylase, lipase and ribonuclease, etc.

conjugated protein

Enzymes that are conjugated to proteins. In addition to proteins, they also combine with some heat-stable non-protein small molecule substances or metal ions. The former is called an apoenzyme, and the latter is called a cofactor. What happens after the apoenzyme and cofactor are combined The complex formed is called holoenzyme, that is, holoenzyme = apoenzyme + cofactor.

According to the form of existence

prozymogen

Some enzymes, such as various proteases in the digestive system, are synthesized and secreted in the form of inactive precursors, and then transported to specific parts of the body. When needed in the body, they are converted into active enzymes through the action of specific proteolytic enzymes and function. effect. The precursors of these enzymes without catalytic activity are called zymogens. Such as pepsinogen (pepsinogen), trypsinogen (trypsinogen) and chymotrypsinogen (chymotrypsinogen), etc. The process in which a certain substance acts on a zymogen to convert it into an active enzyme is called zymogen activation. The substance that converts inactive zymogen into active enzyme is called activin. Activin has certain specificity in the activation of zymogen.

For example, the chymotrypsinogen synthesized by pancreatic cells is a single peptide chain composed of 245 amino acid residues, with 5 pairs of disulfide bonds connected inside the molecule. The activation process of the chymotrypsinogen is that trypsin first hydrolyzes the peptide bond between the arginine residues at position 15 and the isoleucine residues at position 16, activating it into p-chymotrypsin with full catalytic activity. However, the enzyme molecule is not yet stable at this time. Through the catalysis of p-chymotrypsin itself, it removes two molecules of dipeptide to become α-chymotrypsin with catalytic activity and a stable structure.

Under normal circumstances, most coagulation factors in plasma exist in the form of inactive zymogens. Only when tissues or vascular endothelial cells are damaged can the inactive zymogens be converted into active enzymes, thereby triggering a series of cascade enzymatic reactions, leading to the conversion of soluble fibrinogen into stable fibrin polymers, which entrain platelets and form blood clots.

The essence of zymogen activation is to cut off the specific peptide bond in the zymogen molecule or remove some peptide segments, which is conducive to the formation of the enzyme active center. Zymogen activation has important physiological significance. On the one hand, it ensures that the cells that synthesize the enzyme are not digested and destroyed by proteases. On the other hand, they are activated under specific physiological conditions and specified locations and play their physiological roles. For example, after the tissue or vascular endothelium is damaged, coagulation factors are activated; pepsinogen secreted by gastric chief cells and chymotrypsinogen, trypsinogen, and elastaseogen secreted by pancreatic cells are activated into corresponding active enzymes in the stomach and small intestine respectively, which promotes the digestion of food protein. This is an obvious example. The activation of zymogens caused by the breakage of specific peptide bonds exists in large quantities in organisms and is an important way for organisms to regulate enzyme activity. If the activation process of zymogens is abnormal, a series of diseases will occur. The occurrence of hemorrhagic pancreatitis is because the zymogen is activated before entering the small intestine, and the activated protease hydrolyzes its own pancreatic cells, causing pancreatic bleeding and swelling.

Isozyme

The concept of isoenzyme: isoenzymes are a class of enzymes that catalyze the same chemical reaction but have different molecular structures, physicochemical properties, and immunogenicity. They exist in different tissues of the same species or individual, or even in different organelles of the same tissue or cell. There are no fewer than dozens of isoenzymes known today, such as hexokinase, lactate dehydrogenase, etc. Among them, lactate dehydrogenase (Lactic acid dehydrogenase) dehydrogenase, LDH) is well studied. In human and vertebrate tissues, there are five molecular forms that catalyze the same chemical reactions:

Each of the five isozymes is composed of four subunits. The subunits of LDH are divided into skeletal muscle type (M type) and cardiac muscle type (H type). The two types of subunits have different amino acid compositions. They are tetramers composed of two subunits in different proportions. There are five forms of LDH. . Namely H4(LDHl), H3M1(LDH2), H2M2 (LDH3), H1M3 (LDH4) and M4 (LDH5).

The amino acid compositions of M and H subunits are different, which is determined by different genes. The proportions of M and H subunits in the five types of LDH are different, which determines the differences in their physical and chemical properties. Usually, the five types of LDH can be separated by the electro-ice method. LDH1 swims towards the positive electrode quickly, while LDH5 swims slowly. The other ones are in between, including LDH2, LDH3 and LDH4. The amounts of various LDHs contained in different tissues are different. LDH1 and LDH2 are more abundant in myocardium, while LDH5 and LDH4 are dominant in skeletal muscle and liver. The differences in LDH isozyme profiles in different tissues are related to the physiological process of tissue utilization of lactic acid. LDH1 and LDH2 have high affinity for lactic acid, which dehydrogenates and oxidizes lactic acid into pyruvate, which is beneficial for the myocardium to obtain energy from lactic acid oxidation. LDH5 and LDH4 have a high affinity for pyruvate and can reduce pyruvate to lactate, which is consistent with the physiological process of muscle obtaining energy in anaerobic glycolysis. When tissue disease occurs, these isoenzymes are released into the blood. Due to differences in the distribution of isoenzymes in tissues, the serum isoenzyme spectrum changes. Therefore, serum isozyme spectrum analysis is commonly used clinically to diagnose diseases.

allosteric enzyme

Allosteric enzymes are often multi-subunit oligomeric enzymes with a quaternary structure. In addition to the catalytic active center in the enzyme molecule, it is also called a catalytic site. site; there is also an allosteric site. The latter is where allosteric agents bind effector), when it binds to an allosteric agent, the molecular conformation of the enzyme will change slightly, affecting the affinity of the catalytic site for the substrate and the catalytic efficiency. If the binding of an allosteric agent increases the affinity between the enzyme and the substrate or the catalytic efficiency, it is called an allosteric activator. activator), and conversely, those that reduce the affinity of the enzyme substrate or catalytic efficiency are called allosteric inhibitors.

The regulation of enzyme activity by allosteric agents is called allosteric regulation) function. The catalytic site and allosteric site of an allosteric enzyme can be located in different parts of a subunit, but more often they are located in different subunits. In the latter case, the subunit with the catalytic site is called the catalytic subunit, and the subunit with the allosteric site is called the regulatory subunit. Most allosteric enzymes are at the beginning of metabolic pathways, and the allosteric agents of allosteric enzymes are often some physiological small molecules and substrates for the enzyme, or intermediate products or end products of the metabolic pathway. Therefore, the catalytic activity of allosteric enzymes is regulated by the concentration of substrates, metabolic intermediates or final products in the cell. The end product inhibits allosteric enzymes in the pathway is called feedback inhibition. inhibition). This means that once the end product in the cell increases, it acts as an allosteric inhibitor to inhibit the enzyme at the beginning of the metabolic pathway, and timely adjusts the speed of the metabolic pathway to meet the needs of the cell's physiological functions. Allosteric enzymes play an important role in regulating the metabolism of cellular substances. Therefore, allosteric enzymes are also called regulatory enzymes. enzyme).

Modifying enzyme

Some enzymes in the body need to modify the molecular structure of the enzyme under the action of other enzymes before they can become catalytically active. Such enzymes are called modification enzymes (modification enzymes). enzyme). Among them, covalent modification is the most common. For example, the functional group -OH of serine and threonine residues of enzyme proteins can be phosphorylated. At this time, modification changes of covalent bonds are generated, so it is called covalent modification. modification). The change in enzyme activity caused by this modification is called covalent modification of the enzyme. regulation). Common covalent modifications in the body are enzyme phosphorylation and dephosphorylation, in addition to enzyme acetylation and deacetylation, uridylation and deuridylation, methylation and demethylation. Because covalent modification reacts quickly and has a cascade amplification effect, it is also an important way to regulate substance metabolism in the body. For example, glycogen phosphorylase that catalyzes the glycogen decomposition reaction has two forms: active and inactive. The active one is called phosphorylase a, and the inactive one is called phosphorylase b. The interconversion of these two forms is Through the process of phosphorylation and dephosphorylation of enzyme molecules.

Multi-enzyme complexes and multi-enzyme systems

Some enzymes in the body aggregate with each other to form a physical combination. This combination is called a multienzyme complex. complex). If the multi-enzyme complex is disassembled, the catalytic activity of each enzyme disappears. There are many enzymes involved in making up a multi-enzyme complex. For example, the pyruvate dehydrogenase multi-enzyme complex that catalyzes the oxidative decarboxylation reaction of pyruvate is composed of three enzymes, while the multi-enzyme complex that catalyzes the β-oxidation of fatty acids in mitochondria Composed of four enzymes. The product of the enzyme-catalyzed reaction of the multi-enzyme complex becomes the substrate for the second enzyme, and this continues until the final product is generated.

Due to the physical combination of the multi-enzyme complex, the spatial conformation is conducive to the rapid progress of this assembly line operation, and is an effective measure for organisms to improve enzyme catalytic efficiency.

Many enzymes often participate in various pathways of substance metabolism in the body to complete the reaction process in sequence. These enzymes are different from multi-enzyme complexes and are not structurally related to each other. Therefore, it is called multienzyme system (multienzyme system). For example, the 11 enzymes involved in glycolysis are all present in the cytosol, forming a multi-enzyme system.

multifunctional enzyme

In the 21st century, it was discovered that some enzyme molecules have multiple catalytic activities. For example, Escherichia coli DNA polymerase I is a polypeptide chain with a molecular mass of 109kDa, which can catalyze the synthesis of DNA chains, 3'-5' exonuclease and 5'-3 'Exonuclease activity, two peptide fragments were obtained by mild hydrolysis with proteolytic enzyme, one containing 5'-3' exonuclease activity, and the other containing the activities of the other two enzymes, indicating that E. coli DNA polymerase The molecule contains multiple active centers. Mammalian fatty acid synthase is composed of two polypeptide chains, each of which contains the catalytic activity of seven enzymes required for fatty acid synthesis. Enzymes with multiple catalytic active sites in the enzyme molecule are called multifunctional enzymes. enzyme) or tandem enzyme (tandem enzyme). Multifunctional enzymes are more advantageous than multi-enzyme complexes in terms of molecular structure, because the relevant chemical reactions are performed on one enzyme molecule, which is more efficient than multi-enzyme complexes. This is also the result of biological evolution.