Simple Proteinases and Conjugated Proteases

Proteins are divided into simple proteins and combined proteins. Similarly, according to chemical composition, enzymes can also be divided into two major categories: simple proteinases and conjugated proteases. General hydrolytic enzymes, such as urease, protease, amylase, lipase, ribonuclease, etc., are simple proteases. The activity of these enzymes depends only on their protein structure. The enzyme is composed only of amino acids and contains no other components. And transaminase, lactate dehydrogenase (LDH), carbonic anhydrase and other oxidoreductases are all bound proteases. In addition to protein components, these enzymes also contain non-protein small molecules that are stable to heat. The former is called apoenzyme, the latter is called cofactors. When enzyme protein and cofactor exist alone, there is no catalytic activity. Only when the two are combined into a complete molecule can they have enzyme activity. This complete enzyme molecule is called holoenzyme.

holoenzyme = enzyme protein + cofactor

Some of the cofactors of enzymes are metal ions and some are small molecule organic compounds. Sometimes both of them are required for enzyme activity. These small molecule organic compounds are usually called coenzymes or prosthetic groups. The metal is in the enzyme molecule, either as a constituent of the active site of the enzyme, or to help form the conformation necessary for enzyme activity. Enzyme proteins use polar groups on their side chains to bind cofactors through covalent, coordinate, or ionic bonds through reaction. Generally speaking, the small molecule organic substances that are loosely bound to the enzyme protein and easily detached from the enzyme protein and can be removed by dialysis are called coenzymes; and the small molecule substances that are tightly bound to the enzyme protein and are not easily removed by dialysis are called prosthetic groups. There is no essential difference between coenzymes and prosthetic groups, and there is no strict boundary between the two, except that they are strongly bound to the enzyme protein.

In the catalytic reaction of holoenzymes, enzyme proteins and cofactors play different roles. The enzyme protein determines the specificity and high efficiency of the enzyme reaction, and the cofactor directly acts as a carrier for electrons, atoms or certain chemical groups, participates in the reaction and promotes the entire catalytic process.

Usually an enzyme protein can only be combined with a coenzyme to form an enzyme, which acts as a substrate to carry out a chemical reaction in one direction. A coenzyme can be combined with several enzyme proteins to form several enzymes, catalyzing the same type of chemical reaction of several substrates. For example, the enzyme protein of lactate dehydrogenase can only be combined with NAD to form lactate dehydrogenase, which makes the substrate lactic acid dehydrogenate. But there are many kinds of enzyme proteins that can bind to NAD, such as lactate dehydrogenase, malate dehydrogenase (MDH) and glycerophosphate dehydrogenase (GDH) contain NAD, which can catalyze separately Lactic acid, malic acid and glycerol phosphate are dehydrogenated. It can also be seen that the enzyme protein determines the type of reaction substrate, that is, the specificity of the enzyme, and the coenzyme (base) determines the reaction type of the substrate.

Monomer Enzyme, Oligomerase and Multi-enzyme Complex System

According to the structural characteristics of proteins, enzymes can be divided into three categories:

1. Monomer enzyme

Enzymes with only one polypeptide chain are called monomeric enzymes, and they cannot dissociate into smaller units. Its molecular weight is 13 000 ~ 35 000. There are few such enzymes, and most of them are enzymes that promote the hydrolysis reaction of the substrate, that is, hydrolytic enzymes, such as lysozyme, protease, and ribonuclease.

2. Oligomerase

Enzymes composed of several or more subunits are called oligomeric enzymes. The subunits in the oligomerase can be the same or different. The subunits are connected by non-covalent bonds, which are easy to separate for acids, bases, high-concentration salts or other denaturants. The molecular weight of oligomerase ranges from 35,000 to several million. Such as phosphorylase a, lactate dehydrogenase, etc.

3. Multi-enzyme compound system

The complex formed by the chimerization of several enzymes with each other is called the multienzyme system. The multi-enzyme complex is conducive to the continuous progress of a series of reactions in the cell to improve the catalytic efficiency of the enzyme, and at the same time it is convenient for the body to regulate and control the enzyme. The molecular weights of multiple enzyme complexes are all above several million. Such as pyruvate dehydrogenase system and fatty acid synthetase complex are multi-enzyme systems.

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