Enzymes

• Enzymes are proteins with a specific 3D shape
• Active site: where the substrate binds
• Lock and key model: substrate fits into active site like a key into a lock
• Enzyme-substrate complex forms → reaction occurs → products released
• Enzyme is reused (not used up)

Key points:

• Enzymes are specific — each enzyme catalyses only one reaction
• Enzymes lower the activation energy of reactions

Temperature

• As temperature increases → rate increases (more kinetic energy → more collisions)
• Optimum temperature: ~37°C (for human enzymes) → maximum rate
• Above optimum → enzyme denatures (active site changes shape; substrate can't fit)

pH

• Each enzyme has an optimum pH
• Too acidic or too alkaline → enzyme denatures
• Pepsin: optimum pH 2 (stomach)
• Amylase: optimum pH 7 (mouth)

Substrate Concentration

• More substrate → faster rate (more collisions with active sites)
• Until all active sites occupied → rate plateaus (enzyme saturation)

Bile: produced in liver, stored in gall bladder, emulsifies fats, neutralises acid.

1. Set up test tubes with starch solution + amylase at different pH (buffer solutions)
2. At regular intervals, test samples with iodine on a spotting tile
3. Iodine: blue-black with starch, brown when starch has been digested
4. Record time for starch to be completely digested (iodine stays brown)
5. Shorter time = faster reaction rate

When an enzyme is denatured:

• The active site changes shape permanently
• The substrate can no longer fit
• The reaction can no longer be catalysed
• Caused by: extreme temperatures or pH

This is NOT the same as the enzyme being "killed" — enzymes are not alive!

• Enzymes: biological catalysts; proteins with specific active sites
• Lock and key model: substrate fits active site
• Affected by temperature, pH, substrate concentration
• Denaturation: active site changes shape; enzyme stops working
• Digestive enzymes: amylase, protease, lipase