As with all catalysts, all reactions catalyzed by enzymes must be "spontaneous" (containing a net negative Gibbs free energy). With the enzyme, they run in the same direction as they would without the enzyme, just more quickly. However, the uncatalyzed, "spontaneous" reaction might lead to different products than the catalyzed reaction. Furthermore, enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive" a thermodynamically unfavorable one. For example, the cleavage of the high-energy compound ATP is often used to drive other, energetically unfavorable chemical reactions.
Diagram of a catalytic reaction, showing the energy niveau at each stage of the reaction. The substrates usually need a large amount of energy to reach the transition state, which then reacts to form the end product. The enzyme stabilizes the transition state, reducing the energy of the transition state and thus the energy required to get over this barrier.
Enzymes catalyze the forward and backward reactions equally. They do not alter the equilibrium itself, but only the speed at which it is reached. Carbonic anhydrase catalyzes its reaction in either direction depending on the conditions. (in tissues - high CO2 concentration)
(in lungs - low CO2 concentration)
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