1.2. Functions of proteins

Catalytic function. The majority of enzymes known as biological catalists, are proteins. Now, some thousand enzymes are described.

Transport function. Respiratory function of blood, in particular oxygen transfer, is carried out by molecules of hemoglobin protein of erythrocytes. Albumins of blood serum take part in transport of lipids.

Protective function. In response to bacteria, toxins, viruses or alien proteins entering in organism protective proteins-antibodies are synthesized (immune defenses). A number of proteins of blood plasma are capable to coagulation that protects from wounds blood loss (physical protection).

Hormonal function. A series of hormones is presented by proteins or polypeptides, for example, pancreas hormone - insulin.

Structural function. In complex with lipids proteins participate in the formation of biomembranes of cells. Cytoskeleton structural proteins provide shape of cells and many organoids. Structural proteins are collagen in connective tissue, keratin in hair, nails, skin, elastin in a vascular wall, etc.

Nutrient (reserve) function. Proteins of egg (ovalbumins) are a source of nutrition for the fetus. The basic protein of milk (casein) also carries out nutrient function.

Receptor function. Protein receptors can be integrated into the cell membrane or in the cytoplasm. The receptor accepts a signal which is often a chemical substance.

Contractile (motor) function. Contractile function is inherent to muscular proteins (actin and myosin), to proteins of cytoskeleton and that provides discrepancy of chromosomes in the course of mitosis.

Other important functions of proteins - ability to sustain oncotic pressure in cells and blood, the buffer properties sustaining physiological рН value of internal environment, etc.

1.3. Amino-acid composition of proteins

To establish the amino-acid composition of proteins we use a combination of acid (НС1), alkaline (Ba (OH)₂) and, less often, enzyme hydrolysis or one of them. It is known that in hydrolysis of pure protein, free from admixings, 20 various amino acids are released.

The amino acids which are part of proteins, are a-amino acids. All of them belong to an L-number, and the size and optical rotation sign depend on the nature of radicals of amino acids and the value of рН solution. In human proteins D-amino-acids are not found, however they are found in cellular wall of bacteria, as part of some antibiotics (actinomycins).

Amino acids classified on the base of the chemical nature of radical R which does not participate in peptide bond formation. Modern rational classification of amino acids is based on polarity of radicals.

There are: 1) non-polar (hydrophobic); 2) polar (hydrophilic); 3) aromatic (mostly non-polar), 4) negatively charged and 5) positively charged amino acids.

In some proteins derivatives of amino acids are found. In the protein of connective tissue collagen there are hydroxyproline and hydroxylysine. Diiodotyrosine is the basis of structure of thyroid gland hormones.

Oxylysine Oxyproline Diiodotyrosine

Amino acids possess one universal property. They have an amphoteric character (from Gr. amphoteros - bilateral). In рН interval from 4,0 to 9,0 almost all amino acids exist mainly in the form of zwitterions (bipolar ions). The value of IEP of amino acid is calculated by the formula:

For monoaminodicarboxylic acids рI is calculated as a half-sum of values рК - and -carboxylic groups, for diaminodicarboxylic acids - as a half-sum of values рК - and -amino groups.

There are nonessential amino acids which can be synthesised in a human body, and essential which are not formed in the organism and should be received from food.

Essential amino acids or indispensable amino acids are an amino acids that cannot be synthesized de novo by the organism, and therefore must be supplied in the diet. They are: valine, leucine, isoleucine, lysine, methionine, threonine, indole amino-propionic acid, phenyl alanine.

Non-essential amino acids are those that are produced within the human body. These include: glycine, lactamic acid, asparagine, aspartate, glutamine, glutamate, proline, serine.

Conditionally essential amino acids can be synthesised in the organism from other amino acids: arginine (from citrulline), thyrosin (from phenyl alanine), cysteine (from serine), histidine (with the assistance of glutamine).

The amount of various amino acids in proteins is unequal. For discovering in biological objects and quantitative definition of amino acids we use the reaction with ninhydrin.