在活细胞里催化化学反应的蛋白质。在必要的物理化学反应里酶是能够加速形成和打断化学连接的蛋白分子。酶可以是细胞内的，催化细胞内的反应如：新陈代谢、调节、信号传导或合成过程；或细胞外的，在细胞间或血液里循环，并在那里催化对于多细胞生物的功能具有重要作用的反应。酶的催化中心是由一组氨基酸组成，其三维结构形成活性位点，允许它高效地结合或修饰底物。酶的作用一般分三步：结合底物；改变底物结构使其便于修饰；化学反应的完成修饰了底物并使酶恢复到原来的状态。其它促成催化的因素来自结合水分子，金属离子或其它因子。许多酶是由几个基组成，有些具有不同区域形成的结合和催化功能。

A protein that catalyzes chemical reactions in living cells. Enzymes are protein molecules whose function it is to speed the making and breaking of chemical bonds required for essential physiochemical reactions. Enzymes are either intracellular, catalyzing reactions within the cell such as those involved in the metabolic, regulatory, signaling, or synthesis pathways; or extracellular, circulating in the spaces between cells or in the bloodstream, where they catalyze reactions critical to the functioning of a multicellular organism. The catalytic center of an enzyme is composed of a set of amino acids whose three-dimensional arrangement forms the “active-site” that allows them to efficiently bind and modify the substrate. Enzymes typically function in a three-step process: binding of the substrate (the molecule to which a chemical bond will either be added or cleaved); straining of the substrate into a conformation where it is susceptible to modification, (called the enzyme-substrate complex, in which the enzyme and substrate are joined by a transient covalent attachment); completion of the chemical reaction modifies the substrate and returns the enzyme to its initial state. Additional contributions to catalysis can come from bound water molecules, metal ions, or other factors. Many enzymes are composed of several subunits (domains), and some have binding (recognition) and catalytic (effector) functions encoded by different domains.