Classification and chemical characteristics of complex proteins (proteids)

The group of proteids, i.e., conjugated proteins, includes: 1) nucleoproteins (ribonucleoproteins, deoxyribonucleoproteins); 2) chromoproteins; 3) glycoproteins; 4) phosphoproteins; 5) lipoproteins; 6) metalloproteins; 7) flavoproteins (Table 13; Shcherbakov V.G., Lobanov V.G., Prudnikova T.N. et al., 2003).

Table 13 – Prosthetic groups of conjugated proteins Prosthetic group Conjugated protein

 Metal atoms (ions) Metalloproteins Phosphate groups H3PO4 Phosphoproteins Chlorophyll Chromoproteins Mono- and oligosaccharides Glycoproteins Triacylglycerols and complex lipids Lipoproteins Flavin nucleotides Flavoproteins Ribonucleic acids Ribonucleoproteins (ribosomes) Deoxyribonucleic acids Deoxyribonucleoproteins (chromatin)

Metalloproteins contain cations of one or more metals. Most often these are iron, copper, zinc, molybdenum, and less often – manganese, nickel. The protein component is bound to the metal by a coordinate bond. This group primarily includes proteins that possess enzymatic properties. These are catalase, peroxidase, ascorbate oxidase, nitrate reductase, carbonic anhydrase, polyphenol oxidase, and cytochromes.

Deoxyribonucleoproteins are compounds whose molecule consists of a simple protein and deoxyribonucleic acid (DNA).

Ribonucleoproteins are compounds whose molecule consists of a simple protein and ribonucleic acids.

Phosphoproteins are proteids in which the protein is bound to phosphoric acid by an ester bond. Phosphoproteins include proteins that play an important role in the nutrition of young organisms. These are the main protein of milk, casein, which precipitates during curdling; vitellin and phosvitin of egg yolk; and ichthulin of fish roe.

Lipoproteins are conjugated proteins whose prosthetic group is formed by fat-like substances – lipids. In structure, they are small (150-200 nm) spherical particles, the outer shell of which is formed by proteins, and the inner part by lipids and their derivatives. Both polar and neutral lipids have been found in the composition of lipoproteins. These proteins are widely distributed in nature and are found in all representatives of living organisms. They are essential components of all cell membranes, where their non-protein part is represented mainly by polar lipids – phospholipids and glycolipids.

Glycoproteins are conjugated proteins whose prosthetic group is represented by carbohydrates and their derivatives. Based on the chemical structure of the carbohydrate components, two groups of glycoproteins are distinguished: 1) true glycoproteins – monosaccharides most often occur as their carbohydrate component; 2) proteoglycans – constructed from a very large number of repeating units having a disaccharide character (hyaluronic acid, heparin, chondroitin, keratan sulfates). The main difference between them lies in the fact that the carbon groupings of true glycoproteins usually contain up to 15–20 monosaccharide components that do not form repeating oligosaccharide fragments, whereas in proteoglycans they are constructed from a very large number of repeating units, mainly having a specific disaccharide character.

Structural features and functions of chromoproteins

Chromoproteins are conjugated proteins in which the non-protein part consists of colored compounds – heme, belonging to various classes of organic compounds:

• porphyrin structures;
• flavin adenine dinucleotide (FAD);
• flavin adenine mononucleotide (FMN).

A porphyrin ring with an ion of iron coordinated to it enters as a prosthetic part into the composition of a number of redox enzymes (catalase, peroxidase) and groups of electron carriers – cytochromes.