Enzymes are highly substrate specific i.e. they catalyze the reaction of a particular substrate only. They facilitate occurrence of a reaction and transition of reactant into product. A chemical reaction $S \rightarrow P$ (where $S$ is the substrate and $P$ the product or products) will take place when a certain number of $S$ molecules at any given instant possess enough energy to attain an activated condition called the 'transition state'. The enzyme combines transiently with the substrate to produce a transient state having a lower energy of activation than that of substrate alone. This results in acceleration of the reaction. Once the products are formed, the enzyme (or catalyst) is free or regenerated to combine with another molecule of the substrate and repeat the process.