Protein

A protein (in Greek πρωτεϊνη = first thread) is a complex, high-molecular-weight organic compound that consists of amino acids joined by peptide bonds. Proteins are essential to the structure and function of all living cells and viruses.

Diversity

Proteins are generally large molecules, having molecular masses of up to 3,000,000 (the muscle protein titin has a single amino acid chain 27,000 subunits long). Such long chains of amino acids are almost universally referred to as proteins, but shorter strings of amino acids are referred to as "polypeptides," "peptides" or rarely, "oligopeptides". The dividing line is undefined, though "polypeptide" usually refers to an amino acid chain lacking tertiary structure which may be more likely to act as a hormone (like insulin), rather than as an enzyme (which depends on its defined tertiary structure for functionality).

Related Topics:
Molecular mass - Titin - Peptide - Hormone - Insulin

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Proteins are generally classified as soluble, filamentous or membrane-associated (see integral membrane protein). Nearly all the biological catalysts known as enzymes are soluble proteins (with a recent notable execption being the discovery of ribozymes, RNA molecules with the catalytic properties of enzymes.) Antibodies, the basis of the adaptive immune system, are another example of soluble proteins. Membrane-associated proteins include exchangers and ion channels, which move their substrates from place to place but do not change them; receptors, which do not modify their substrates but may simply shift shape upon binding them. Filamentous proteins make up the cytoskeleton of cells and much of the structure of animals: examples include tubulin, actin, collagen and keratin, all of which are important components of skin, hair, and cartilage. Another special class of proteins consists of motor proteins such as myosin, kinesin, and dynein. These proteins are "molecular motors," generating physical force which can move organelles, cells, and entire muscles.

Related Topics:
Integral membrane protein - Catalyst - Enzyme - Ribozyme - RNA - Antibodies - Adaptive immune system - Exchanger - Ion channel - Substrates - Receptors - Cytoskeleton - Tubulin - Actin - Collagen - Keratin - Cartilage - Myosin - Muscle

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Click to see a list of proteins.

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