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Enzymes

Biological catalysts & factors affecting rate

What Are Enzymes?

Proteins that speed up chemical reactions (biological catalysts)

Enzymes are NOT used up in reactions — they're reusable

Every enzyme catalyses ONE specific reaction (substrate specificity)

Without enzymes, reactions would be too slow for life

Examples: amylase (breaks starch), protease (breaks protein), lipase (breaks fat), catalase (breaks H₂O₂)

Lock & Key Model

Enzyme = lock; substrate = key

Active site: Specific 3D pocket in enzyme where substrate binds

Substrate fits into active site (complementary shape) → enzyme-substrate complex forms → reaction happens → product released → enzyme unchanged

Only correct substrate fits active site (explains specificity)

Temperature & Enzyme Activity

Below 37°C: Reaction rate increases with temperature. More particle collisions.

~37°C (human body): Optimum temperature for most human enzymes. Maximum reaction rate.

Above 37°C: Enzyme denatures. H-bonds break. 3D active site shape lost. Substrate no longer fits. Reaction rate falls to ZERO.

⚠ Denaturation is PERMANENT — even if cooled, enzyme doesn't refold correctly

pH & Enzyme Activity

Each enzyme has an optimum pH:
Pepsin (stomach) = pH 2 (very acidic)
Amylase (saliva/pancreas) = pH 7 (neutral)
Trypsin (small intestine) = pH 8 (slightly alkaline)

Wrong pH → H-bonds break → active site shape lost → denaturation

Enzymes are adapted to pH of their environment

Substrate Concentration & Enzyme Rate

Low substrate: Reaction rate increases with more substrate (more enzymes have substrate to work on)

High substrate (saturation): Rate plateaus. All enzyme active sites occupied. Adding more substrate = no speed increase. Enzyme is saturated.

To increase rate when saturated: add more enzyme OR increase temperature OR change pH

Enzyme Cofactors & Coenzymes

Cofactors: Inorganic (metal ions like Mg²⁺, Zn²⁺, Fe²⁺). Help enzyme work. Example: Mg²⁺ in photosynthesis enzymes

Coenzymes: Organic (vitamins). Help enzyme work. Example: NAD (carries electrons in respiration), coenzyme A (carries acetyl groups)

Without cofactors/coenzymes, enzyme may not work properly

Inhibitors

Competitive inhibitors: Molecules similar to substrate. Compete for active site. Can be overcome by adding more substrate.

Non-competitive inhibitors: Bind to enzyme (not active site). Change enzyme shape. Cannot be overcome by adding more substrate.

Example: Cyanide is a non-competitive inhibitor of cytochrome c oxidase (stops respiration)

Exam Tips

✓ Enzymes denature (permanently) above optimum temperature

✓ Substrate saturation = all active sites occupied

✓ Know optimum pH for pepsin (2), amylase (7), trypsin (8)

✓ Cofactors = metal ions; coenzymes = vitamins

✓ Competitive inhibitors blocked by more substrate; non-competitive are not

Key Takeaways

✅ Enzymes: biological catalysts, not used up

✅ Lock & key: substrate fits active site

✅ Optimum temp: 37°C (humans); denatures above

✅ pH varies by enzyme (pepsin ≠ amylase)

✅ Substrate saturation = rate plateaus

Enzymes make life possible! ⚡✓