Enzymes and Coenzymes:
The cellular reactions with the help of enzymes proceed at rates much faster than those achievable using synthetic catalysts. This is possible due to the following reasons:
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The concentration of the substrate increases at the catalytic site of the enzyme;
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The enzyme holds all the molecules of the substrate in correct orientation for the reaction to take place;
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Some of the binding energy contributes to the catalysis so that the activation energy of the reaction is lowered due to the enzyme;
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Substrate molecules pass through a series of intermediate forms for which the enzyme has greater affinity and which themselves have free energy levels suitable for the reaction to take place;
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Many enzymes temporarily make covalent bonds with the substrate so that the substrate reacts with a second molecule on the enzyme surface that breaks the covalent bond of the enzyme with the substrate, thus releasing the free enzyme;
Each of these two steps has a relatively low activation energy thus increasing the reaction speed
Although an enzyme may speed up the reaction, it can not make it more (or less) energetically favorable. In other words, enzymes can not alter the difference in free energies of the substrates and the products. For instance, in a reaction A + B → AB. At the equilibrium point, the enzyme equally speeds up forward and backward reactions depend only on the concentrations of the substrates and the products rather than on the enzyme.
