BIOG 1105-1106 | Cornell Introductory Biology, Individualized Instruction

How exactly does the temporary unstable complex formed by an enzyme and its substrate reduce the amount of activation energy needed for the reaction? One possible explanation is that enzymes, by binding substrate molecules on their active sites, greatly increase the effective concentrations of the substrates in the active sites, and thereby increase the probability that the reaction will take place. Second, it is thought that when substrate molecules are bound to the enzyme, the molecules are oriented in such a way that their chemical groups are optimally positioned to react (see figure). And third, it is thought that the weak bonds that form between the enzyme and substrate during the formation of the enzyme-substrate complex place a strain on the substrate by disturbing the distribution of electrons within the substrate molecules. The susceptible bonds in the substrate molecules may be weakened as a result, becoming more easily broken, and therefore less energy is necessary for the reaction to occur. In other words, enzymes may not only orient substrate molecules so that their reactive groups are properly aligned, but they may also enhance the reactivity of those groups..

Orientation of substrate for greatest activity. When substrate molecules collide (left), their orientation often fails to bring together their reactive groups (red). When the substrate molecules are bound to an enzyme, they are oriented for maximal interaction of their reactive groups.