The basis of the structure of proteins and proteides is a polypeptide chain, and a protein molecule can consist of one, two or more chains. Nevertheless, the physical, biological and chemical properties of biopolymers are determined not only by the general chemical structure, which may be “meaningless”, but also by the presence of other levels of organization of the protein molecule.
The primary structure of the protein is determined by the quantitative and qualitative amino acid composition. Peptide bonds are the basis of the primary structure. This hypothesis was first expressed in 1888 by A. Ya. Danilevsky, and later on his assumptions were confirmed by the synthesis of peptides, which was carried out by E. Fisher. The structure of the protein molecule was studied in detail by A. Ya. Danilevsky and E. Fisher. According to this theory, protein molecules are composed of a large number of amino acid residues that are connected by peptide bonds. A protein molecule may have one or more polypeptide chains.
In the study of the primary structure of proteins, chemical agents and proteolytic enzymes are used. So, using the Edman method it is very convenient to identify terminal amino acids.
The secondary structure of the protein shows the spatial configuration of the protein molecule. The following types of secondary structure are distinguished: alpha-helical, beta-helical, collagenic helix. Scientists have found that the alpha helix is most characteristic of the structure of peptides.
The secondary structure of the protein is stabilized by hydrogen bonds. The latter arise between the hydrogen atoms connected to the electronegative nitrogen atom of one peptide bond and the carbonyl oxygen atom of the fourth amino acid from it, and they are directed along the spiral. Energy calculations show that the right alpha helix, which is present in native proteins, is more effective in the polymerization of these amino acids.
Secondary Protein Structure: Beta-Folded Structure
The polypeptide chains in the beta folds are fully extended. Beta folds are formed by the interaction of two peptide bonds. The indicated structure is characteristic of fibrillar proteins (keratin, fibroin, etc.). In particular, beta-keratin is characterized by a parallel arrangement of polypeptide chains, which are further stabilized by interchain disulfide bonds. In silk fibroin, adjacent polypeptide chains are antiparallel.
Secondary Protein Structure: Collagen Helix
The formation consists of three helical chains of tropocollagen, which has the shape of a rod. Spiral chains twist and form a supercoil. The spiral is stabilized by hydrogen bonds arising between the hydrogen of the peptide amino groups of the amino acid residues of one chain and the oxygen of the carbonyl group of the amino acid residues of the other chain. The presented structure gives collagen high strength and elasticity.
Tertiary protein structure
Most proteins in their native state have a very compact structure, which is determined by the shape, size and polarity of amino acid radicals, as well as the sequence of amino acids.
Hydrophobic and ionogenic interactions, hydrogen bonds, etc. have a significant impact on the formation of the native conformation of a protein or its tertiary structure. Under the influence of these forces, a thermodynamically expedient conformation of the protein molecule and its stabilization are achieved.
Quaternary structure
This type of molecular structure arises as a result of the association of several subunits into a single complex molecule. Each subunit contains primary, secondary and tertiary structures.