noun, plural: proteins
A molecule composed of polymers of amino acids joined together by peptide bonds. It can be distinguished from fats and carbohydrates by containing nitrogen. Other components include carbon, hydrogen, oxygen, sulphur, and sometimes phosphorus.
A protein is a linear polymer built from about 20 different amino acids. The type and the sequence of amino acids in a protein are specified by the DNA in the cell that produces them. This sequence of amino acids is essential since it determines the overall structure and function of a protein.
A protein may be classified based on its form and main functions: it can be a globular protein like most enzymes, fibrous protein which are for structural role; and membrane proteins that serve as receptors or channels for polar or charged molecule to pass through the cell membrane.