Biochemistry

Figure 2.3 - a-helix model and diagram Secondary protein structure. By the secondary structure of the polypeptide chain is understood the ordered spatial arrangement of its individual sections. In natural proteins, two main types of secondary structure are determined: the a-helix and fl­ structure (folded layers) (Fig. 2.3). The primary chemical bond supporting the secondary structure is the hydrogen bond. In the a-helix, dipoles of neighboring peptide bonds (each -С = О group has a hydrogen bond with the fourth chain along the chain - NH group) form an extensive system of intramolecular cooperative hydrogen bonds stabilizing the helical conformation of the polypeptide chain. The side chains of amino acids in the a-helix are directed outward from the central axis. The 0-structure is also stabilized by cooperative interpeptide hydrogen bonds within individual sections of the same polypeptide chain (in globular proteins) or between adjacent chains (in fibrillar proteins). In most natural proteins, highly structured regions of the chain alternate with regions without a pronounced secondary structure. For example, the spiralized portion of hemoglobin accounts for 75%, egg albumin - 45%, and pepsin - only 28%. Therefore, each protein is characterized by a certain degree of ordering (structuring) of its polypeptide chain. Spiral and folded structures are important to ensure the strength and stability of protein molecules, their functioning. Tertiary structure of the protein. The tertiary structure of the polypeptide chain determines the nature of the laying of spiral and linear sections in this volume (Fig. 2.4). Laying the chain in space occurs with great accuracy: a particular polypeptide chain under certain conditions is folded in only one single way, acquiring the form that is characteristic of the molecules of this native (natural) protein. Due to such a strict regularity, a protein can fully fulfill its functional purpose. 54