The prosthetic group is a non-peptide component of complex proteins, which ensures the fulfillment of their biological functions. Most often they talk about prosthetic groups of enzymes. Prosthetic groups are firmly connected to the protein part by covalent bonds. They can be inorganic substances (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 blood plasma globulins, immunoglobulins, interferons, fibrinogen, corticotropin hormones, 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 coupled to a protein covalent glycosidic bond due to the hydroxyl group of threonine, serine or the amino group of lysine, glutamine, asparagine.
- Lipoproteins. The prosthetic group is lipids of various compositions. The protein part can combine with lipid covalent bonds, then insoluble lipoproteins are formed, performing mainly structural functions; and non-covalent bonds, then soluble lipids are formed, performing mainly transport functions. Proteins (apoproteins) of soluble lipoproteins form the surface hydrophilic layer, lipids - the hydrophobic core, which contains transported substances of lipid nature. Soluble lipoproteins include all lipoprotein complexes, which are rather conglomerates of proteins and lipids of variable composition.
- Phosphoproteins. The prosthetic group is orthophosphoric acid. Its residue combines with the protein part with ester bonds due to the hydroxy groups 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, the composition of transferrin and ferritin includes iron ions, alcohol dehydrogenase - zinc, cytochrome oxidase - copper, proteinase - magnesium and potassium ions, ATP-ase - 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. The non-protein part of hemoproteins is heme. Gem is part of hemoglobin, myoglobin, cytochromes, catalases, peroxidases. The prosthetic group of flavoproteins is FAD.
6. Nucleoproteins. A prosthetic group is a nucleic acid - DNA or RNA. The protein part of nucleoproteins contains many positively charged amino acids - lysine and arginine, therefore, it has the basic properties. Nucleic acids themselves have acidic properties. The interaction between the protein and non-protein part is, thus, due to 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 of amino acids. In this case, the enzyme-substrate bond is due to the acid-base interaction. For a series of reactions in the body, such a simple interaction is not enough. Then the reaction involves not only the substrate and the enzyme, but also other compounds of a non-protein nature, 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 serve as carriers between the individual enzymes. Prosthetic groups are firmly connected by covalent bonds with the apoenzyme and act as an intra-enzyme carrier. Examples of prosthetic groups of some enzymes are presented in the table.
| Table. Prosthetic groups, their sources of synthesis and corresponding enzymes |
| Prosthetic group | Synthesis source | Enzyme Examples |
| FAD, FMN | Riboflavin | Aerobic and some anaerobic dehydrogenases |
| Pyridoxalphosphate | Pyridoxine | Aminotransferases, decarboxylases |
| Thiamine pyrophosphate | Thiamine | Decarboxylases, transferases |
| Biotin | Biotin | Carboxylases |
| Gem | Glycine, succinate, ferritin | Cytochromes, hemoglobins, myoglobin, catalase, peroxidase |
Prosthetic lipid groups
In this case, the prosthetic group is the non-lipid part of complex lipids, for example, phospholipids, glycolipids, sulfolipids.