proteins

Proteins are large, complex molecules that perform most biological functions. They are polymers composed of twenty standard amino acids linked by peptide bonds, and their sequence is encoded by genes. The specific order of amino acids determines a protein’s chemical properties and its three‑dimensional structure, which in turn governs function.

Proteins fold into hierarchical structures. The primary structure is the linear sequence of amino acids. Local patterns form secondary structures such as alpha helices and beta sheets stabilized by hydrogen bonds. The overall three‑dimensional shape—the tertiary structure—results from interactions among side chains, and many proteins consist of multiple polypeptide subunits that assemble into a quaternary structure.

Proteins can be classified by shape and function. Globular proteins are generally soluble and carry out metabolic and regulatory roles, while fibrous proteins provide structural support. Some proteins contain non‑amino acid components called prosthetic groups, such as heme in hemoglobin or metal ions in metalloproteins.

Functions of proteins are diverse. Enzymes catalyze almost all cellular reactions. Structural proteins give cells and tissues shape and strength. Transport proteins move molecules across membranes or in the bloodstream. Storage, immune defense, signaling, and motility are also served by various proteins.

Protein synthesis occurs in cells via translation on ribosomes, using messenger RNA as a template. After synthesis, proteins may fold with the help of chaperones and may undergo post‑translational modifications. Denaturation—disruption of structure by heat, pH change, or chemicals—usually abolishes activity.

Dietary proteins supply essential amino acids the body cannot synthesize. Proteins also have broad applications in research, medicine, and industry.