The structure of the protein molecule has been studied for over 200 years. It is known for many proteins. Some of them are synthesized (for example, insulin, RNase). The main structural and functional unit of a protein molecule is amino acids. In addition to carboxyl and amino groups, proteins contain other functional groups that determine their properties. Such groups include those located in the side branches of the protein molecule: the carboxyl group of aspartic acid or glutamic acid, the amino group of lysine or oxylysine, the guanidine group of arginine, the imidazole group of histidine, the hydroxyl group of serine and threonine, the phenol group of tyrosine, the cishydryl disulfide group, the cysthide dishydride group the thioether group of methionine, the benzyl core of phenylalanine, the aliphatic chains of other amino acids.
There are four levels of structural organization of the protein molecule.
The primary structure of the protein. Amino acids in a protein molecule are linked together by peptide bonds, thus forming a primary structure. It depends on the qualitative composition of amino acids, their quantity and the sequence of connections 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 (DNF), resulting in the formation of dinitrophenyl protein (DNF protein). Subsequently, the DNF protein is hydrolyzed, the remainder of the protein molecule and the DNF amino acid are formed. The DNF amino acid is isolated from this mixture and hydrolyzed. The hydrolysis products are amino acid and dinitrobenzene. The remainder of the protein molecule reacts with new portions of DNP until the whole molecule breaks down into amino acids. Based on a quantitative study of amino acids, a diagram of the primary structure of an individual protein is drawn up. The primary structure of proteins is known (insulin, myoglobin, hemoglobin, glucagon and many others).
According to the Edman method, the protein is treated with phenylisothiocyanate. Sometimes proteolytic enzymes are used - trypsin, pepsin, chymotrypsin, peptidases, etc.
The secondary structure of the protein. American scientists using x-ray diffraction analysis found that protein polypeptide chains more often exist in the form of alpha-helices, and sometimes beta-structures.
Alpha spirals are compared with a spiral staircase, where amino acid residues function as degrees. In molecules of fibrillar proteins (silk fibroin), polypeptide chains are almost completely stretched (beta structure) and are placed in the form of balls stabilized by hydrogen bonds.
An alpha helix can spontaneously form in synthetic polypeptides (dederon, nylon), which have a molecular weight of 10 to 20 thousand. Yes. In certain areas of the protein molecule (insulin, hemoglobin, RNAase), the alpha-helical configuration of the peptide chain is disrupted, and another type of helical structure is formed.
Tertiary structure of the protein. Spiral-shaped sections of the polypeptide chain of the protein molecule are in different relationships, which determine 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 the hydrophilic groups are oriented towards the solvent, which leads to the formation of energetically favorable molecular confirmations. This process is accompanied by the formation of intramolecular bonds. The tertiary structure of the protein molecule has been decoded for RNAase, hemoglobin, chicken egg lysozyme.
Quaternary protein structure. This type of structure of the protein molecule occurs as a result of the association of several subunits into a single complex molecule. Each subunit has a primary, secondary and tertiary structure.