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.

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. 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 is characteristic to muscular proteins (actin and myosin).

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.

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 enzyme hydrolysis. It is known that in hydrolysis of protein 20 various amino acids are released.

Proteins are made of a-amino acids. All of them are L-isomers. 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.

Amino acids have an amphoteric character. 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 pI 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.

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, tryptophan, 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 phenylalanine), cysteine (from serine), histidine (with the assistance of glutamine).

The amount of various amino acids in proteins is unequal.