Catalysts accelerate the rate of a reaction by lowering the energy required to reach the transition state. Unlike reactants, catalysts are not consumed in a reaction. Though catalysts change reaction kinetics, they do not affect the equilibrium state of the reaction, and there is no increase in product formation.
Synthetic catalysts are used to accelerate a variety of industrial processes and are crucial to the chemical manufacturing industry. Catalysts are also found in nature in the form of proteins called enzymes. Enzymes are proteins that contain regions called active sites that can stabilize reaction transition states to speed reactions.
One model of enzyme mechanism is called the induced fit model, and it is depicted in the figure below. This model proposes that the binding of the reactant, or substrate, to the enzyme active site results in conformational changes to the enzyme that increase its binding affinity for the transition state complex, thus stabilizing this state and lowering the activation energy.
Enzymes can catalyze reactions through a variety of mechanisms. Some of these include:
- Bond strain: enzymes can destabilize the ground state through bond strain
- Proximity and orientation: changes upon substrate binding can bring reactive groups closer or orient them to react
- Proton donors and acceptors: the presence of acidic or basic groups can affect bond polarization and reaction speed
- Electrostatic catalysis: side chains or cofactors can stabilize the charge on the transition state
- Covalent catalysis: covalent bonding to side chains or cofactors can lower the energy of the transition state