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 a- and w-carboxylic groups, for diaminodicarboxylic acids - as a half-sum of values рК a- and w-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.

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