Physical and chemical properties of proteins

Proteins are high-molecular compounds, their relative molecular weight varies from 6000 to 1000000 а.m.u. and more. For the definition of molecular weight of proteins we use sedimentation analysis methods, gel chromatography and gel-electrophoresis.

The most typical physical and chemical properties of proteins are high viscosity of solutions, insignificant diffusion, ability tj swell within wide limits, optical activity, mobility in electric field, low osmotic pressure and high oncotic pressure, ability to absorb UV rays at 280 nanometers (this property is used for quantitative definition of proteins). Protein molecules are not capable to inpour through semipermeable artificial membranes, and also biomembranes of vegetative and animal tissues.

In the isoelectric point (рI) the charge of the protein molecule is equal to zero, and proteins do not move in electric field. There are no repulsive forces between molecules, and protein molecules tend to form aggregates. In the isoelectric point proteins are least soluble and easily drop out into sediment. For the majority of animal tissue proteins рI = 5,5 - 7,0. Hb – 6.8, serum albumin – 4.9.

Classification of proteins

Depending on chemical composition proteins are divided into 2 groups.

Simple proteins are constructed of the amino acids residues and in hydrolysis give only free amino acids.

Conjugative proteins are two component proteins which consist of simple protein and non-protein component (prosthetic group).

Simple proteins

1. Albumins and globulins.

Albumins are simple proteins; are a part of animal and vegetative tissues; are contained in eggs, blood serum, milk, in seeds of plants. They are soluble in water, salting liquids, acids and alkalis. Approximately 75-80 % of osmotic pressure of proteins of blood serum are albumins; their one more function - transport of fat acids.

Globulins are group of animal and plant proteins widely spread in nature. They are globular proteins. They are soluble in weak solutions of neutral salts, weak acids and alkalis. The globulins presented by a-fraction are contained in blood in a complex with bilirubin and with lipoproteins of high density. The fraction of β-globulins includes a prothrombin which is the precursor of thrombin - the protein participating in blood coagulation. The fraction of g-globulins is the most heterogenous. The major g-globulins are immunoglobulins.

The ratio albumin/globulin has a diagnostic importance: in norm it is close to 2, and during inflammatory diseases it is decreases.

2. Protamines and histones.

Protamines are low molecular weight proteins in nuclei of spermatozoons of fishes and birds. They contain 60-85% arginine and possess the basic properties. They are well dissolve in water, acid and neutral medium and precipitate in alkalis. In nuclei of cells they associate with DNA.

Histones are small proteins of the basic character. Their composition has lysine and arginine which amount does not exceed 20-30%. There are five various types of histones: H1, H2A, H2B, H3 and H4. These proteins are mainly in nuclei of cells. They take part in the structural organization of chromatin.