Enzymes as Biological Catalysts
High-Yield Summary
- Enzymes are protein catalysts: they speed up reactions without being consumed, by lowering activation energy — the sole mechanism behind their effect.
- Enzymes affect KINETICS ONLY — they never change the equilibrium constant (Keq) or the overall Gibbs free energy (ΔG) of a reaction.
- On a reaction-progress diagram, a catalyzed reaction has the same start/end points as uncatalyzed — only the transition-state peak is lower.
- Six enzyme classes by reaction type (mnemonic LIL HOT): Ligases, Isomerases, Lyases, Hydrolases, Oxidoreductases, Transferases.
- Enzymes show substrate specificity and remain sensitive to environmental conditions like pH and temperature.
Key Terms
- Catalyst
- Substance that increases reaction rate without being consumed.
- Transition state
- High-energy, unstable intermediate reactants must pass through to become products.
- Activation energy
- Energy required to reach the transition state.
- Equilibrium constant (Keq)
- Ratio of products to reactants at equilibrium — unaffected by enzymes.
- Endergonic vs. exergonic
- Endergonic requires energy input (+ΔG); exergonic releases energy (−ΔG). Either type still faces an activation-energy barrier.
Enzymes DO / DON'T
- DO: lower activation energy, increase reaction rate, get reused (not consumed), show substrate specificity, stay sensitive to pH/temperature.
- DON'T: alter Keq, change overall ΔG. Any choice claiming an enzyme "shifts equilibrium" or "makes a reaction more thermodynamically favorable" is wrong.
The Six Enzyme Classes (LIL HOT)
- Oxidoreductases
- Oxidation-reduction/electron transfer — dehydrogenase, reductase, oxidase.
- Transferases
- Move a functional group between molecules — kinase (transfers phosphate).
- Hydrolases
- Break a bond using water (hydrolysis).
- Lyases
- Cleave bonds without water or redox chemistry, often forming a double bond/ring.
- Isomerases
- Rearrange bonds within a molecule to form an isomer — e.g., phosphohexoisomerase in glycolysis.
- Ligases
- Join two large molecules, typically using ATP — e.g., DNA ligase.
Common MCAT Trap
- "Enzymes make a reaction more favorable/shift equilibrium" — always false. Enzymes are a kinetics tool only; they change how fast equilibrium is reached, never where it sits.
- A catalyzed vs. uncatalyzed reaction-progress diagram has the SAME start and end free-energy points — only the peak (activation energy) differs.
Quick Recall
What is the single mechanism behind every enzyme effect on a reaction?
Do enzymes change Keq or ΔG?
What does LIL HOT stand for?