This sequence is called the primary structure of the protein. These polypeptides may undergo coiling or pleating, the nature and extent of which is described as the secondary structure . The three-dimensional shape of the coiled or pleated polypeptides is called the tertiary structure . Quaternary structure specifies the structural relationship of the component polypeptides. Proteins may be broadly classified into globular proteins and fibrous proteins.
Globular proteins have compact rounded molecules and are usually water-soluble.
Of prime importance are the enzymes , proteins that catalyse biochemical reactions. Fibrous proteins are generally insoluble in water and consist of long coiled strands or flat sheets, which confer strength and elasticity. In this category are keratin and collagen . Actin and myosin are the principal fibrous proteins of muscle, the interaction of which brings about muscle contraction. Blood clotting involves the fibrous protein called fibrin.