Q.• What does the enzyme pepsin do?
• What are the initial and final products of digestion of proteins?

Proteins are the most structurally and functionally diverse class of macromolecule in the cell, making up about two-thirds of a cell's total dry mass. They are polymers of amino acids linked end to end by peptide bonds into polypeptide chains. Beyond their well-known role as enzymes, proteins serve many other functions: structural roles (e.g., keratin, collagen), transport (haemoglobin), defence (antibodies), and chemical signalling as hormones (e.g., insulin, growth hormone). A protein's specific three-dimensional shape - which determines its function - arises from four successive levels of folding: primary (the linear amino-acid sequence), secondary (local coiling into alpha-helices or folding into beta-pleated sheets via hydrogen bonding), tertiary (further folding of the whole chain into a compact globular domain), and, for multi-chain proteins, quaternary structure (assem …