Levels of structural organization of the protein molecule or structure of the protein

The structure of the protein molecule is studied in more than 200years. It is known for many proteins. Some of them are synthesized (for example, insulin, RNase). The main structural and functional unit of the protein molecule are amino acids. In addition to carboxyl and amino groups, proteins also contain other functional groups, which determine their properties. Such groups include the protein molecule located in the side branches of the protein molecule: the carboxyl group of aspartic acid or glutamic acid, the amino group of lysine or oxylizine, the guanidine group of arginine, the imidazole group of histidine, the hydroxyl group of serine and threonine, the tyrosine phenolic group, the cysteine ​​sulfhydryl group, the cystine disulfide group, thioester group of methionine, benzene nucleus of phenylalanine, aliphatic chains of other amino acids.

There are four levels of structural organization of the protein molecule.

Primary structure of the protein. Amino acids in the protein molecule are joined together by peptide bonds, forming a primary structure. It depends on the qualitative composition of amino acids, their number and the sequence of the connection between each other. The primary structure of the protein is most often determined by Senger. The test protein is treated with a solution of ditrofluorobenzene (DNP), resulting in the formation of dinitrophenyl-protein (DNP-protein). Later the DNP-protein is hydrolyzed, the residue of the protein molecule and the DNP-amino acid are formed. The DNP-amino acid is isolated from this mixture and is hydrolyzed. The products of hydrolysis are amino acid and dinitrobenzene. The rest of the protein molecule reacts with new portions of DNP until the entire molecule breaks down into amino acids. On the basis of a quantitative study of amino acids, a scheme of the primary structure of an individual protein is made. The primary structure of proteins is insulin, myoglobin, hemoglobin, glucagon and many others).

By the Edman method, the protein is treated with phenyl isothiocyanate. Sometimes proteolytic enzymes - trypsin, pepsin, chymotrypsin, peptidase, etc. are used.

Secondary structure of the protein. American scientists, using X-ray analysis, found that protein polypeptide chains often exist in the form of alpha helices, and sometimes beta structures.

Alpha-spirals are compared with a spiral staircase,where the function of the degrees is performed by the amino acid residues. In molecules of fibrillar proteins (silk fibroin), the polypeptide chains are almost completely stretched (beta structure) and placed in the form of spheres stabilized by hydrogen bonds.

The alpha helix can spontaneously form insynthetic polypeptides (dederon, nylon), which have a molecular mass of 10 to 20 thousand Da. At certain sites in the protein molecule (insulin, hemoglobin, RNAase), the alpha-helical configuration of the peptide chain is disrupted, and spiral structures of a different type are formed.

Tertiary structure of the protein. The spiral sections of the polypeptide chain of the protein molecule are in different relationships, which predetermine the tertiary (three-dimensional) structure, form the volume and shape of the protein molecule. It is believed that the tertiary structure arises automatically due to the interaction of amino acid radicals with solvent molecules. In this case, hydrophobic radicals are "drawn" into the protein molecule, forming their dry zones, and hydrophilic groups are oriented toward the solvent, which causes the formation of energetically favorable confirmation of the molecule. This process is accompanied by the formation of intramolecular bonds. The tertiary structure of the protein molecule is deciphered for RNAse, hemoglobin, lysozyme chicken egg.

Quaternary structure of the protein. This kind of structure of the protein molecule arises as a result of the association of several subunits into a single complex molecule. Each subunit has a primary, secondary and tertiary structure.