A catalyst works simply by lowering the energy barrier of a reaction, (Figure 11.4). By doing so, the catalyst increases the fraction of molecules that have enough energy to attain the transition state, thus making the reaction go faster in both directions.

The position of the equilibrium (the amount of product versus reactant) is unchanged by a catalyst. Even though k1 and k-1 many be greatly changed from their values in the absence of a catalyst, each one changes by the same factor and the equilibrium constant, K, is unchanged, because K = k1/k-1.

Catalysts lower the energy barrier in two ways:

1. The catalyst binds a substrate in an intermediate conformation that resembles the transition state, but has a lower energy. This may lead to multiple intermediate states that bypass the transition state (Figure 11.5). An intermediate state is a metastable state of a molecule.

2. In a non-catalyzed reaction the entropy may be highly negative due to the highly specific orientation required in order for a reaction to occur. (Note that a more negative entropy contributes to a more positive free energy of transition.) Catalysts can lower the negative entropy by binding reacting molecules only in the proper mutual orientataion, thus increasing their reactivity (Figure 11.6).

1. Characteristics of Enzymes

2. Enzyme Outline