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 ( р I) 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.

Protein solution is called isoionic if it does not contain any other ions, except the ionized units of amino acids of protein molecule and the ions formed at their dissociation. Protein isoionic point is a value of рН of protein isoionic solution. Isoionic and isoelectric points coincide only in the case when both are equal to 7.

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

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.

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.

Prolamins and glutelins.

Prolamins are proteins of vegetative origin. They contain 20-25% of glutamic acid and 10-15% of proline. They are dissoluble in 60-80% ethanol water solution while all other simple proteins in these conditions usually form sediment.

Glutelins are simple proteins. They are contained in seeds of gramen, in green parts of plants. They are characterized by high amount of glutamic acid and lysine. They are dissoluble in dilute solutions of alkalis. Glutelins are reserve proteins.

CONJUGATIVE PROTEINS

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

1. Chromoproteins (from Gr. chroma - color) consist of simple protein and the pigmented non-protein component bound to it. We distinguish hemoproteins and flavoproteins. They participate in such processes as breath, transport of oxygen and carbon dioxide, redox reactions, light- and colour perception, etc.

The group of hemoproteins include hemoglobin, myoglobin, cytochromes, catalase, peroxidases. All of them contain ferriporphyrin, but vary in protein structure, and carry out various biological functions. Specific distinctions of hemoglobin are caused by globin.

Let's consider hemoglobin structure. It is a blood protein. Non-protein component of hemoglobin is the heme (fig. 5). It’s a pigment giving blood its red color. The basis of its structure is the protoporphyrin IX. In the heme centre the atom of iron is bounded to two atoms of nitrogen covalently and with two others by coordination bonds.

A heme is " wrapped up" by one polypeptide chain. In a molecule of hemoglobin of an adult person НbА there are four polypeptide chains which together form the protein part of a molecule - globin. Two α -chains contain 141 amino-acid residues, two β -chains – 146 (fig. 6).

                   Fig. 5. Heme                                             Fig. 6. Hemoglobin

In blood of an adult person there is also hemoglobin НbА₂ (2α, 2δ chains, 2, 5%) and НbA₃ (less than 1%, differs by structure of b-chains).

There is fetal hemoglobin HbF, consisting of 2 α - and 2 γ -chains. Hemoglobin F possesses the increased affinity to oxygen and allows rather small volume of blood of a fetus to carry out oxygen-bearing functions more effectively. Blood of the newborn contains up to 80% HbF, by the end of the 1st year of life it is almost entirely changed to НbА.

Diseases of hemoglobins (more than 200) are called hemoglobinoses.

1. Hemoglobinopathy, at the basis of which hereditary structural change of any chain of normal hemoglobin. In blood of a human about 150 various types of mutant hemoglobins are found.

Abnormal hemoglobins differ in physical and chemical properties (electrophoretic mobility, solubility, isoelectric point, affinity to oxygen).

Classical example of hemoglobinopathy is sickle-cell anemia. It widely spread in the countries of South America, Africa and South East Asia. Chemical defect is reduced to glutamic acid changing in 6th position from the N-end to valine in β -chains of a molecule of hemoglobin (HbS). It is the result of a mutation in DNA molecule. The HbS solubility and affinity to oxygen are reduced. Erythrocytes in the conditions of low partial pressure of oxygen take the form of a sickle. HbS after oxygen return in tissues turns in low solubility desoxiform and drops out into sediment in the form of spindle-shaped crystals. They distort a cell and lead to a hemolysis. The heterozygous form of anomaly proceeds asymptomatically or is accompanied by an easy hemolytic anemia. Homozygous individuals from the first months of life have the heavy form of sickle-cell anemia. Disease proceeds sharply, and children often die in an early age.

2. Thalassemia is a group of diseases with hereditary infringement of synthesis of one of globin chains. We distinguish α - and β -thalassemia. Hemoglobinopathy Н is one of the variants of a-thalassemia. It manifests in hemolytic anemia, the precipitation of hemoglobin H, enlarged spleen, severe osteal changes.

3. Iron-deficient anemia is infringement of synthesis of hemoglobin owing to deficiency of iron. Principal causes are blood loss and lack of nutrition rich with heme - meat and fish.

Hemoglobin derivatives

Oxyhemoglobin HbO₂. Molecular oxygen is joined to each heme of Hb by means of iron coordination bonds. Binding of each molecule of oxygen facilitates binding of the subsequent. This allosteric dependence has received the name Bohr effect. Oxyhemoglobin, getting to tissues, loses oxygen, becoming deoxyhemoglobin.

Carbhemoglobin HbCO₂ is hemoglobin bond with carbon dioxide. It is unstable and quickly dissociates in pulmonary capillars detaching СО₂.

Carboxyhemoglobin HbCO is a product of the addition of carbon monoxide to hemoglobin. Hemoglobin has high affinity to CO and is strong bound with it. Hemoglobin loses ability to bind oxygen, and here comes death from suffocation.

Methemoglobin MtHb is the hemoglobin form in which heme iron is in a trivalent state. It is not capable to carry oxygen. It is formed of free hemoglobin under the influence of various oxidants, and in an organism - after some poisonings.

Methemoglobinemia is an appearance of methemoglobin in blood. There are hereditary and acquired methemoglobinemia. Hereditary evolves as a result of presence of abnormal hemoglobins. Among the acquired can be toxic methemoglobinemia of exogenous parentage, arising under the influence of some chemicals (nitrates, nitrites, aniline, some medical products), and endogenous, owing to infringement of production and adsorptions of nitrates during enterocolitis. With significant methemoglobinemia there is an anoxemia (hypoxia).

Method of qualitative test of various derivatives of hemoglobin is research of their absorption spectrums.

Myoglobin is a globular protein which reserves in muscles molecular oxygen and transfers it to the oxidative systems of cells. It consists of one polypeptide chain. As well as in hemoglobin, the active centre of a molecule binding O₂, is the heme. Myoglobin defines the color of muscles.

Chromoproteins are also catalase, peroxidase, cytochromes enzymes.

Flavoproteins are the chromoproteins, which prosthetic groups are presented by derivatives of isoalloxasine - flavine mononucleotide (FMN) and flavine adenine dinucleotide (FAD). Flavoproteins are a part of some oxidoreductases - enzymes catalyzing redox reactions in a cell.

2. Lipoproteins consist of protein and the prosthetic group presented by any lipid (neutral fats, free fatty acids, phospholipids, derivatives of cholesterol). Lipoproteins are widely spread and also carry out various biological functions. Representatives of lipoproteins are protein of lungs tissue, lipovitellin of an egg yolk etc.

Lipoproteins are present in free state (mainly in blood plasma). Lipoproteins of blood serum contain hydrophobic lipid nuclei surrounded with polar lipids and a cover of proteins, called apoproteins. They provide transport of water insoluble lipids.

The lipids covalently bound with protein, serve as an anchor with the help of which proteins are attached to the membrane. These are structured lipoproteins (lipids of cells membranes, myelin sheath of nerve fibers).

3. Phosphoproteins are conjugative proteins into which composition as a non-protein component the phosphoric acid enters and is attached to a polypeptide chain by an ester bond through the residues of serine or threonine. The ionic bonding is also possible.

Phosphoproteins include caseinogens of milk, egg white ovalbumin, a number of enzymes such as RNA polymerase. A large number of phosphoproteins found in cells of the CNS.

Phosphoproteins are a valuable source of energy and plastic material in the process of embryogenesis and postnatal growth and development, participate in the regulation of nuclear activity of the cell, ion transport and oxidative processes in mitochondria.

4. Glycoproteins are conjugative proteins that contain, in addition to a simple protein or peptide, linear or branched geterooligosaccharide chains containing from 2 to 15 residues of hexose, pentose and the terminal carbohydrate (N-acetylgalactosamine or others). Carbohydrate component is connected to the protein by covalent bonds – N-glycoside and O-glycoside.

Glycoproteins are proteins of blood plasma (except albumin), certain enzymes, saliva mucin, proteins of cartilage and bone tissues. Glycoproteins are important structural component of cell membranes. They provide a cell adhesion, molecular and cellular recognition. Carbohydrate components in addition to the informative function increase the stability of the molecules, to which they belong, to various chemical or physical treatments, and protect them from the action of proteases.

Erythrocyte membrane glycoproteins determine the blood group in humans. The typical glycoproteins include interferons, immunoglobulins.

Interferons - inhibitors of reproduction of many types of viruses. They are formed in the cell in response to the invasion of viruses’ nucleic acids. Interferons are proteins thought to be essential not only as protective against viral infection, but also of tumors.

Immunoglobulins, or antibodies are protective proteins. They neutralize entering the body foreign substances of any chemical nature - antigens. There are three major classes of immunoglobulins: IgG, IgA, IgM; minor classes of immunoglobulins of human plasma are referred to as IgD and IgE. Immunoglobulins of different classes differ in molecular weight, the concentration in the blood, biological properties.

At rheumatic arthritis abnormal antibodies are synthesized with an unusually short sugar chains, which causes stimulation of the immune system against the organism.

Proteoglycans are complexes of proteins and glycosaminoglycans. Carbohydrate in these compounds is the main part of the molecule (95%). Typical glycosaminoglycan is hyaluronic acid. Its main function in the connective tissue is binding of water. Heparin involved in the regulation of blood coagulation.

5. Metalloproteins, in addition to protein, contain ions of a single metal or several metals.

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