Any of numerous organic compounds, complex amino acid s that are involved in nearly every aspect of the physiology and biochemistry of living organisms.
Twenty different amino acids are common to proteins, linked in chains of hundreds to thousands of units. An active protein molecule has three important levels of structure: primary (the amino acid sequence), determined by the gene s; secondary (the geometric shape, often a helix), determined by the angles of the covalent bond s between and within amino acids; and tertiary (the looped and folded overall shape), determined largely by attraction between oppositely charged groups (and repulsion between like charged groups) on amino-acid side chains and especially by hydrogen bonding . The tertiary structure, which can be globular or sheetlike with ridges, crevices, or pockets, often holds the key to a protein's biological activity. Proteins can serve, e.g., as structural material (as in connective tissue and hair ; see collagen ; keratin ), as enzyme s and hormone s, as transporters of essential substances such as oxygen (see hemoglobin ), as antibodies , or as regulators of gene expression. Some proteins are simple (amino acids only), some conjugated (see conjugation ) to other groups, often vitamin s or metal atoms needed in tiny amounts in the diet (see coenzyme ; cofactor ). Rhodopsin and hemoglobin are conjugated proteins. Proteins may be covalently linked to other atoms or molecules, as to {{link=sugar">sugar s (glycoproteins), phosphate groups (phosphoproteins), or sulfur (sulfoproteins). Proteins are an essential human nutrient, obtained from both plant and animal foods. Their greatest commercial use is in food products; they are also employed in adhesives, plastics, and fibres.