QUARTERNARY STRUCTURE OF PROTEINS: Some proteins contain two or more separate polypeptide chains, or subunits, which may be identical or different. The arrangement of these protein subunits in three-dimensional complexes constitutes a quaternary structure. In considering these higher levels of structure, it is useful to classify proteins into two major groups: fibrous proteins, having polypeptide chains arranged in long strands or sheets, and globular proteins, having polypeptide chains folded into a spherical or globular shape. The two groups are structurally distinct: fibrous proteins usually consist largely of a single type of secondary structure; globular proteins often contain several types of secondary structure.
Fibrous Proteins: α-Keratin, collagen, and silk fibroin are examples of fibrous proteins. Fibrous proteins give strength and/or flexibility to the structures in which they occur. All fibrous proteins have a high concentration of hydrophobic amino acid residues both in the interior of the protein and on its surface and so, are insoluble in water.
Globular Proteins: In a globular protein, different segments of a polypeptide chain (or multiple polypeptide chains) fold back on each other to form a spherical shape. Globular proteins include enzymes, transport proteins, motor proteins, regulatory proteins, immunoglobulins, and proteins with many other functions. They are soluble in water. Ex: Insulin and albumin.