The mechanism by which an enzyme binds with the substrate to forward the reaction of producing products can be explained by Lock and key hypothesis and Induced fit mechanism.Lock and Key hypothesis: This hypothesis was putforth by Emil Fischer in 1894. Enzyme has a special site called as active site. the conformation of this active site corresponds to the shape of the substrate which fits into the active site. Enzyme is the lock and substrate is the key. A particular substrate can only fit into active site of the enzyme to form the enzyme substrate complex. (ES complex) This intermediate complex is later broken to form enzyme and the products. Enzyme undergoes no change but helps in forwarding the reaction. This is called as lock and key hypothesis.Induced-Fit mechanism: In this mechanism, the substrate wraps over the enzyme bringing about the change in the enzyme molecule. Here substrate determines the final shape of the enzyme. ES complex is formed after bnding. ES complex breaks to give rise to enzyme and products. Enzyme undergoes no change. Exactly to say the active site is modified during Enzyme-Substrate complex formation.