Biochemistry

2.6 CLASSIFICATION AND CHARACTERISTICS OF PROTEINS Depending on the chemical composition, proteins are divided into two groups: simple and complex. Simple proteins are made up of amino acids only. In addition to amino acids, complex proteins include a non-protein component called the prosthetic group. In turn, within each of these groups, proteins are divided into subgroups. Simple proteins are conventionally classified according to their solubility in various substances, and complex proteins are divided on the basis of the chemical nature of the non-protein part of the molecule. Simple proteins Simple proteins include albumin, globulins, prolamins, glutelins, protamines, histones, and proteinoids. Albumins. The proteins of this subgroup have a small molecular weight (15000-70000 Da); they are classified as acidic proteins due to the high content of glutamic acid. Albumin is highly hydrated; it dissolves well in water; precipitated from aqueous solutions upon saturation with neutral salts, for example ammonium sulfate. Albumins have a high adsorption capacity. Thus, plasma albumin due to nonspecific adsorption of various substances perform a physiologically important transport function. Albumin is widespread in nature. In human blood plasma, chicken eggs they make up to 50% of all proteins. Milk and dairy products are rich in albumin. Globulins. These proteins are larger than albumin; their molecular weight exceeds 100,000 Da. Globulins dissolve in weak solutions of various salts (insoluble in water). At 50% saturation of the solution, ammonium sulfate precipitates. Globulins are slightly acidic and neutral proteins. They make up the bulk of seed proteins, especially legumes and oilseeds. A lot of globulins in blood and other body fluids. This subgroup includes: blood protein - fibrinogen, as well as pea seed protein - legumin, beans - phaseolin, hemp - edestin. Albumins and globulins are very diverse groups of proteins that perform various functions in living organisms. Prolamins. Proteins readily soluble in 70 percent ethanol. Prolamins are insoluble in water and salt solutions. They contain a lot of proline and glutamic acid. Prolamins are found in cereals, where they play the role of reserve substances. Each of them has a specific name according to the source from which they were isolated: gliadin - protein of wheat and rye, hordein - barley, zein - corn. Glutelins. These are plant proteins, insoluble in water, salt solutions, ethanol. They dissolve well in weak alkalis (0.2-2%). Glutelins contain more arginine and less proline than prolamins. The complex of alkali-soluble proteins of wheat seeds is called glutenin, rice - orizenin. 63