The interaction of an enzyme with its substrate is highly specific, which means that the enzyme can only bind certain substrates. This specificity is determined by the shape and chemical characteristics of the active site— the region of the enzyme that binds to the substrate. Induced fit is a widely accepted model to explain enzyme specificity and catalysis. Unlike the lock-and-key model, which hypothesizes that the substrate fits into the enzyme's active site, the induced-fit model proposes that the enzyme undergoes a change in its three-dimensional shape upon substrate binding. This conformational change alters the relative positions of specific amino acids of the enzyme to increase their interactions with the substrate. The enzyme-substrate interaction lowers the activation energy for the reaction, increasing the reaction rate. After the product of the reaction is formed and released from the active site, the enzyme reverts to its original shape and can catalyze additional rounds of the reaction.