Mode of Enzyme Action: MDCAT Biology notes
Mode of enzyme action MDCAT notes: activation energy, active site, binding and catalytic sites, lock and key vs induced fit models, turnover number.
Activation energy
Every reaction needs an initial input of energy to start. This is called the activation energy. It lets reactants reach an unstable transition state. Enzymes lower the activation energy, so many more molecules can react at body temperature. They do this by changing conditions within the active site, which stabilizes the transition state. They also bend and strain bonds, distorting the substrate bound in the active site.
Enzymes do not supply energy, raise temperature, change the equilibrium, or turn endergonic reactions into exergonic ones. They only make equilibrium arrive faster.
The active site
- The active site is a small pocket or groove on the enzyme surface, made of only a few amino acids. Catalytic activity is restricted to this region.
- Its shape is complementary to the substrate. This shape decides the enzyme's specificity.
- It has two parts. The binding site recognizes and holds the substrate, forming the enzyme–substrate complex. The catalytic site converts the substrate into product.
- The substrate is held by weak bonds: hydrogen bonds, ionic interactions and hydrophobic interactions. Covalent bonds are not formed in normal binding.
- The active site depends on the enzyme's 3-D (tertiary) shape. If this shape is lost, activity is lost.
The substrate is the molecule that binds the active site and is changed into product. The overall reaction is:
$$\mathrm{E + S \rightleftharpoons ES \rightarrow E + P}$$
Models of enzyme action
| Feature | Lock and key model | Induced fit model |
|---|---|---|
| Proposed by | Emil Fischer (1894) | Daniel Koshland (1959) |
| Active site | Rigid and specific, like a lock | Flexible; it changes shape as the substrate binds |
| Substrate range | One enzyme is restricted to one substrate and reaction type | The enzyme can act on a range of related substrates and catalyse related reactions |
| Explains | High specificity | Specificity plus flexibility |
In the induced fit model, the substrate binding causes the active site to mould itself around the substrate, much like a glove fitting a hand. The active site gets its final shape only after the substrate arrives.
Turnover number
The turnover number is the number of substrate molecules one enzyme molecule (one active site) converts into product per unit time. It measures how efficient an enzyme is. Catalase, for example, has one of the highest turnover numbers known.
Worked example
Question: An enzyme digests several closely related peptides, and its active site changes shape slightly with each one. Which model fits?
Answer: The induced fit model. A flexible active site that accepts related substrates is the key idea. The lock and key model predicts a rigid site and a single substrate.
Common MDCAT traps
- Enzymes lower activation energy. They never raise it or replace it.
- Lock and key means rigid; induced fit means flexible. Fischer and Koshland are often swapped.
- The active site is made of amino acids, not nucleotides or sugars.
- The substrate is recognized at the binding site, not at the allosteric site.
- No covalent bonds form between substrate and active site.
Quick revision
- Activation energy is the energy needed to start a reaction.
- The active site is complementary to the substrate.
- Koshland proposed induced fit in 1959.
- Emil Fischer proposed the lock and key model.
- Turnover number is substrate converted per enzyme per unit time.