Why are enzymes great catalysts?
The three reasons an enzyme decreases activation energy
- The formation of transient covalent bonds that active a substrate and lower activation energy. These bonds are found between groups on the substrate and the active site. They decrease the entropy of a system and help correct positions of certain side chains.
- The formation of weak non-covalent bonds, which results in the release of a small amount of energy, the sum of which is called the binding energy. This is the main source of energy responsible for the decrease of activation energy.
A decrease of activation energy by 5.7 kJ/mol usually results in a 10-fold increase in the rate of reaction. Most enzymes will decrease the AE by 60-100 kJ/mol.
The energy released by one weak non-covalent bond is ~4-30 kJ/mol, which means you only a need a few WNC bonds for the reaction to proceed.
3. Enzymes are “exquisitely specific.” For example, β-Galactosidase: changing the orientation of one hydroxyl group renders it ineffective.
Mode of Substrate binding
Emil Fischer proposed the “lock-key” model in 1890. This was shown to violate the Free Energy of the reaction. A locked ES would have lower free energy than the product, thus there’s no incentive to proceed.
Daniel Koshland corrected this in 1958 with his Induced Fit model. The Enzyme is complementary to the transition state instead of the substrate.
In a cell, water molecules surround a substrate. This leads to the formation of weak interactions between the two. Desolvation is the process by which weak interactions between the water and substrate are replaced by weak interactions between the substrate and enzyme.
The displacement of ordered water molecules increases entropy and thus makes the formation of an enzyme-substrate complex more thermodynamically favorable.