All enzymes are proteins, required in small quantities, reusable and denatured at high temperature. Enzyme reaction is reversible and specific. Two hypotheses about enzyme actions are lock-and-key hypothesis and induced-fit hypothesis.
Lock-and-key hypothesis
In lock-and-key hypothesis, enzyme acts as the lock and substrate acts as the key. Substrate binds to enzyme at active site, forming enzyme-substrate complex which lowers the activation energy. Products are formed and enzyme remains unchanged and reusable.
Induced-fit hypothesis
In induced-fit hypothesis, substrate binds to enzyme at active site, enzyme adjusts its shape to fit substrate, forming enzyme-substrate complex which lowers the activation energy. Products are formed and enzyme remains unchanged and reusable.