Enzymes (enzymes) are specific proteins that play the role of biological catalysts; are produced by cells of living organisms.
Enzymes differ from conventional catalysts by their greater specificity (see below), and also by the ability to accelerate the course of chemical reactions in conditions of normal vital activity of the organism.
Enzymes are present in all living cells - animals, plant, bacterial. Most of the enzymes are in tissues in negligible concentrations, but there are cases when the protein that constitutes a significant part of the cell plasma, for example, myosin in muscle tissue, has enzymatic activity. Molecular weight of enzymes varies widely: from several thousand to several million, with the same type of enzymes, but isolated from different sources, can have different molecular weight, differ in the sequence of amino acid composition.
Enzymes that have the same catalytic effect, but differ in their physico-chemical properties, are called isoenzymes (isoenzymes). Enzymes can be simple or complex proteins. The latter, in addition to the protein (apoenzyme), have in their composition and non-protein component - the residue of an organic molecule or an inorganic ion. The non-protein component easily separated from the apoenzyme is called coenzyme. Strongly bound to the enzyme, the non-protein part is called a prosthetic group. Many prosthetic groups and coenzymes are derivatives of vitamins, pigments, etc. Enzymes have a strict specificity with respect to the substrate (ie, selectively interacts with one or another chemical substance and compounds). For example, lactase (found in intestinal juice) cleaves only disaccharide-lactose and lactose derivatives (lactobionic acid, lactoureids, etc.) with the formation of a mixture of glucose and galactase; maltase splits maltose into two glucose molecules, and amylase acts only on starch, glycogen and other polysaccharides .The
As a result of the sequential action of these, as well as other enzymes, the carbohydrates of food products are converted into monosaccharides and absorbed by the intestinal wall. The specificity of enzymes is determined by the fact that they interact with a certain chemical grouping of the substrate. For example, pepsin (see) acts on proteins, splits bonds within the polypeptide chain of the protein molecule, and the protein molecule splits into polypeptides, which then under the action of other enzymes - trypsin (see), chymotrypsin (see) and peptidase can to cleave up to amino acids. The specificity of enzymes plays an important biological role; thanks to it, a sequence of chemical reactions is achieved in the body. Inorganic ions activate a number of enzymes; some enzymes (metalloenzymes) are generally inactive, if there is no one, a specific ion for the given enzyme. The enzyme sites responsible for the localization and activation of the substrate in the enzymatic process are called active enzyme centers. The formation of the active site involves specific amino acid residues of the protein molecule, sulfhydryl groups and prosthetic groups, if any. So, in the composition of enzymes bearing the group name of flavoproteins, as a prosthetic group includes the flavin derivative (usually this is flavinadenidine dinucleotide - FAD). Easily oxidized and recovered, flavin prosthetic groups act as biological hydrogen carriers, for example, in the dehydrogenation of amino acids with the participation of oxygen or in the dehydrogenation with the participation of cytochromes in the mitochondria of the initial components of the respiratory chain (such as succinate, choline , sarcosine, etc.). Similar functions are performed by other respiratory pigments (hemoglobin and myoglobin - in higher animals and humans, as well as hemaritrin, erythrocricrin, hemocyanin and others - in lower animals). All these enzymes combine the presence in the active center of the atoms of the metal (iron or copper).