Protein

Polypeptide; that is, a long chain of amino acids held together by peptide bonds. The receptors in the cells that receive signals and identify them—for example, taste and smell—are proteins. Enzymes are a particular class of proteins that act as catalysts for chemical reactions under controlled circumstances. Proteins lose their functionality (denature) and undergo changes to their physical characteristics when they are heated, exposed to salt or acid when cooked, salted, or marinated, or subjected to the action of enzymes when fermented. Smaller peptides and free amino acids—for example, glutamic acid—are formed when proteins are broken down.