Mechanisms of Enzyme Activity
High-Yield Summary
- Enzymes speed reactions via a favorable microenvironment, transition-state stabilization, and positioning reactive groups close together.
- Substrate binds the enzyme's active site, forming the enzyme-substrate complex — where catalysis actually happens.
- Lock and Key: rigid, pre-shaped active site — explains specificity but not flexibility. Induced Fit: active site conformationally molds around substrate — explains both; the modern, accurate model.
- Cofactors = inorganic helpers (metal ions: Mg²⁺, Zn²⁺, Fe²⁺). Coenzymes = organic helpers, usually vitamin-derived (NAD⁺, FAD, coenzyme A).
- Apoenzyme = enzyme without its cofactor (inactive). Holoenzyme = enzyme + cofactor (active).
Key Terms
- Substrate
- The molecule an enzyme acts on.
- Active site
- Specialized pocket on the enzyme that binds the substrate.
- Enzyme-substrate complex
- Temporary bound association essential for catalysis.
- Conformational change
- Shape change the enzyme undergoes upon substrate binding (Induced Fit).
Lock and Key vs. Induced Fit
| Lock and Key | Induced Fit |
|---|---|
| Active site fixed, pre-shaped to substrate | Active site flexible, changes shape on binding |
| Explains specificity | Explains specificity AND flexibility |
| Simplified/historical model | More accurate, modern model |
Cofactors vs. Coenzymes
| Cofactors | Coenzymes |
|---|---|
| Inorganic (usually metal ions) | Organic (usually vitamin-derived) |
| Mg²⁺, Zn²⁺, Fe²⁺ | NAD⁺, FAD, coenzyme A |
| Stabilize charge, assist electron transfer | Carry functional groups/electrons between reactions |
| Enzyme without it: apoenzyme; with it: holoenzyme | Enzyme without it: apoenzyme; with it: holoenzyme |
Common MCAT Trap
- Cofactor = inorganic; coenzyme = organic — easy to swap under time pressure since both use the same apo-/holo- terminology.
- "Apoenzyme" always means inactive/missing its helper molecule, regardless of whether that helper is a cofactor or coenzyme.
Quick Recall
Which binding model explains enzyme flexibility that Lock and Key can't?
An enzyme is inactive without magnesium — is that a cofactor or coenzyme requirement?
What's the difference between an apoenzyme and a holoenzyme?