A protein functioning as a biological catalyst, classified according to chemical reaction catalysed and substance acted on; eg hydrolases, carboxylases, proteinases.
An adaptive (inducible) enzyme is one which is formed by an organism in appreciable amounts only in response to the presence of a specific agent, which may be a chemical compound or a physical stimulus such as heat or light.
A constitutive enzyme is one which is synthesized by the cell in the absence of any specific stimulus.