A prosthetic group is a non-peptide component of complex proteins that ensures the performance of their biological functions. Most often they talk about the prosthetic groups of enzymes. Prosthetic groups are firmly connected to the protein part by covalent bonds. They can be substances of inorganic (metal ions) and organic (carbohydrates, vitamins) nature.
Prosthetic groups of proteins
Complex proteins are classified according to the structure of the prosthetic group. The following classes of complex proteins are distinguished:
- Glycoproteins: true and proteoglycans. The prosthetic groups of the former are represented by monosaccharides, deoxysaccharides, sialic acids, oligosaccharides. True glycoproteins include all plasma globulins, immunoglobulins, interferons, fibrinogen, hormones corticotropin, gonadotropin. The prosthetic group of proteoglycans is represented by high molecular weight heteropolysaccharides - glycosaminoglycans. Examples of carbohydrates are hyaluronic acid, chondroitic acid, heparin. The carbohydrate part is connected to the protein covalent-glycosidic bond due to the hydroxyl group of threonine, serine or the amino group of lysine,glutamine, asparagine.
- Lipoproteins. The prosthetic group are lipids of various composition. The protein part can be combined with the lipid covalent bonds, then insoluble lipoproteins are formed, which perform mainly structural functions; and non-covalent bonds, then soluble lipids are formed, which perform mainly transport functions. Proteins (apoproteins) of soluble lipoproteins form a surface hydrophilic layer, lipids form a hydrophobic core, which contains transported substances of a lipid nature. Soluble lipoproteins include all lipoprotein complexes, which are rather conglomerates of proteins and lipids of variable composition.
- Phosphoproteins. The prosthetic group is phosphoric acid. Its residue is connected to the protein part by ester bonds due to the hydroxogroups of serine and threonine. Phosphoproteins include casein, vitellin, ovalbumin.
- Metalloproteins. These include more than a hundred enzymes. The prosthetic group is represented by an ion of one or more different metals. For example, transferrin and ferritin include iron ions, alcohol dehydrogenase - zinc, cytochrome oxidase - copper, proteinases - magnesium and potassium ions, ATPase - sodium, magnesium, calcium and potassium ions.
- Chromoproteins have a colored prosthetic group. In humans and higher animals, they are mainly represented by hemoproteins and flavoproteins. Heme is the non-protein part of hemoproteins. Heme is part of hemoglobin, myoglobin, cytochromes, catalases, peroxidases. Prosthetic group of flavoproteinsis FAD.
6. Nucleoproteins. The prosthetic group is nucleic acids - DNA or RNA. The protein part of nucleoproteins contains many positively charged amino acids - lysine and arginine, therefore it has basic properties. Nucleic acids themselves are acidic. The interaction between the protein and non-protein part is thus carried out by ion-ion interaction. Attaching the main protein part to a rather "loose" acidic DNA molecule allows you to get a compact structure - chromatin, which provides storage of hereditary information.
Prosthetic groups of enzymes
About 60% of known enzymes are simple substances. Their active center is formed only from amino acids. In this case, the enzyme-substrate bond is carried out by acid-base interaction. For a number of reactions to occur in the body, such a simple interaction is not enough. Then not only the substrate and the enzyme participate in the reaction, but also other non-protein compounds, which are called cofactors. There are two subclasses of cofactors: coenzymes and prosthetic groups. The former are connected to the protein part of the enzyme by weak non-covalent bonds, due to which they can act as carriers between individual enzymes. The prosthetic groups are firmly connected by covalent bonds with the apoenzyme and function as an intraenzymatic carrier. Examples of the prosthetic groups of some enzymes are presented intable.
| Table. Prosthetic groups, their sources of synthesis and corresponding enzymes | ||
| Prosthetic group | Source of synthesis | Examples of enzymes |
| FAD, FMN | Riboflavin | Aerobic and some anaerobic dehydrogenases |
| Pyridoxal Phosphate | Pyridoxine | Aminotransferases, decarboxylases |
| Thiamin pyrophosphate | Thiamin | Decarboxylases, transferases |
| Biotin | Biotin | Carboxylase |
| Gem | Glycine, succinate, ferritin | Cytochromes, hemoglobins, myoglobin, catalase, peroxidases |
Prosthetic lipid groups
In this case, the prosthetic group is the non-lipid part of complex lipids, such as phospholipids, glycolipids, sulfolipids.
