protein
n. a molecule that consists of a long-chain polymer of amino acids. Proteins are involved in virtually every function performed by a cell; they are the principal building blocks of living organisms and, in the form of enzymes, the basic tools for construction, repair, and maintenance. Proteins play an essential role in human nutrition, including the provision of all of the essential amino acids that humans cannot produce themselves. Protein deficiency leads to a variety of symptoms and conditions. Excess protein can cause overreaction of the immune system and liver dysfunction and is implicated in obesity. See also peptide.