Describe how enzymes take part in a chemical reaction.

Enzymes are molecules that speed up the rate of a reaction without being used up. They provide an alternate reaction pathway with lower activation energy and remain unchanged at the end of the reaction. Enzymes react with reactant molecules, which are known as substrates. Enzymes have particular places to fit these substrate molecules into and such sites are known as active sites. As shown in the attached illustration, substrate molecules bind to the active sites on an enzyme. There are two commonly accepted hypotheses of the working of enzymes: the lock-and-key hypothesis and induced fit hypothesis. In the former, the active sites are good fits for the substrate molecules, while the latter suggests that the enzyme molecules get modified to some extent to get a good fit. Either way, an intermediate substrate-enzyme complex is formed, which has a lower activation energy and from there, products are formed and detached from the active site and the enzyme is left unchanged. 

This is the way an enzyme-catalyzed reaction propagates. 

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