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Enzyme Families



Enzyme is a protein or RNA produced by living cells that has a high specificity and a high catalytic efficiency for its substrate. The catalytic action of an enzyme depends on the integrity of the primary structure and the spatial structure of the enzyme molecule. Denaturation or subunit depolymerization of enzyme molecules can lead to loss of enzyme activity. Enzymes are biological macromolecules with molecular weights of at least 10,000 and large ones that can reach one million. Enzymes are an extremely important class of biocatalysts. Due to the action of enzymes, chemical reactions in living organisms can be performed efficiently and specifically under extremely mild conditions. With the deepening and development of the research on the structure and function of enzyme molecules and the kinetics of enzymatic reactions, the discipline of enzymology has gradually formed. The chemical nature of the enzyme is protein or RNA (Ribonucleic Acid), so it also has primary, secondary, tertiary, and even tertiary structures. According to their molecular composition, they can be divided into simple enzymes and binding enzymes. A protein that contains only proteins is called a simple enzyme; a binding enzyme consists of an enzyme protein and a cofactor. For example, most hydrolases consist solely of proteins; flavin mononucleases consist of enzyme proteins and cofactors. The enzyme protein in the binding enzyme is the protein part, and the cofactor is the non-protein part. Only when the two are combined to form a whole enzyme, it has catalytic activity.

Enzyme FamiliesFigure 1. Organisation of enzyme structure and lysozyme example. Binding sites in blue, catalytic site in red and peptidoglycan substrate in black.

Structure

Enzymes are usually much larger than their substrates. The monomer for 4-oxalylcrotonate tautomers ranges in size from only 62 amino acid residues to more than 2,500 residues in animal fatty acid synthase. Only a small part of their structure (about 2-4 amino acids) is directly involved in catalysis: the catalytic site. The catalytic site is adjacent to one or more binding sites of the residue-directed substrate. The catalytic site and the binding site together constitute the active site of the enzyme. Most of the remaining enzyme structures are used to maintain the precise orientation and dynamics of the active site. In some enzymes, no amino acid is directly involved in catalysis. In contrast, the enzyme contains sites that bind and target catalytic cofactors, and the structure of the enzyme may also contain allosteric sites. The binding of small molecules can cause conformational changes that increase or decrease. There are a few RNA-based biocatalysts called ribozymes, which in turn can work alone or in combination with proteins. The most common of these is the ribosome, which is a complex of proteins and catalytic RNA components.

Enzyme FamiliesFigure 2. Enzyme activity initially increases with temperature.

Physical and chemical properties

According to the chemical composition of enzymes, enzymes can be divided into simple enzymes and binding enzymes. A peptide chain consisting of only amino acid residues in a simple enzyme molecule. In addition to the protein consisting of the polypeptide chain, the binding enzyme molecule also has non-protein components, such as metal ions, iron porphyrins, or small molecular organic compounds containing B vitamins. The protein part of the binding enzyme is called the enzyme protein (apoenzyme), and the non-protein parts are collectively called cofactors. The two together form a holoenzyme; only the whole enzyme has catalytic activity. If the two enzymes are separated, the enzyme activity disappears. Non-protein parts such as iron porphyrin or compounds containing B vitamins are called prosthetic groups if they are covalently linked to the enzyme protein. They cannot be separated from the enzyme protein by dialysis or ultrafiltration. Coenzymes, which are linked by non-covalent bonds, can be separated by the methods described above. There are two main types of cofactors. One is metal ions, which are often auxiliary groups, and they are used to transfer electrons. The other are small molecule organic compounds, which are mainly used to transfer hydrogen atoms, electrons, or certain chemical groups.

Enzyme FamiliesFigure 3. The enzyme glucosidase converts the sugar maltose into two glucose sugars.

In this section, you will find a series of short reviews on various enzyme subfamilies based on their functional activity in enzymology or the properties targeted by many drugs.

Reference

  1. Porter KR.; et al. A study of tissue culture cells by electron microscopy: methods and preliminary observations. The Journal of Experimental Medicine. 1945, 81 (3): 233-46.

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