Biochemistry

Quaternary protein structure. The quaternary structure is characteristic of those proteins whose molecule consists of two or more polypeptide chains (subunits). And most often there are dimers, trimers and tetramers. The quaternary structure is stabilized by non-covalent interactions between amino acid residues located near the surface of each globule. Of the proteins of this type, hemoglobin is the most studied (Fig. 2.6). Figure 2.6 - Quaternary hemoglobin structure A protein molecule in the quaternary structure is a mutual spatial arrangement of globules, which are a single whole in a structural and functional sense. The quaternary structure plays an important role in the regulation of the biological activity of proteins, since it is very sensitive to external influences that cause a change in the relative position of the subunits and, as a consequence, a change in the activity of the protein. Thus, each protein has its own spatial structure — conformation. In its formation, the leading role belongs to the primary structure determined by the information stored in the DNA. Native structured protein molecules have a higher degree of ordering compared to the unfolded polypeptide thread. Spiralization of a linear peptide chain (formation of a secondary structure) reduces its size by about 4 times, and folding into a tertiary structure makes it tens of times more compact than the original polypeptide chain. 2.5 PHYSICAL AND CHEMICAL PROPERTIES OF PROTEINS The amino acid composition and spatial organization of each protein determine its physicochemical properties. The most important feature of proteins is their large molecular weight. It ranges from 6,000 to several million Da (Table 2.5). 56