B5 ENZYMES: THE BIOLOGICAL CATALYSTS

Hello future scientists! Welcome to the exciting world of Enzymes. This short chapter is incredibly important because enzymes are essential for *everything* living things do—from digesting your lunch to breathing. If you can understand how these tiny protein machines work, you'll unlock a huge part of Biology!

In these notes, we will look at what enzymes are, how they perform their magical tasks, and why factors like heat and acidity can easily stop them in their tracks.

1. What are Enzymes? (B5.1 Core 1)

Think about mixing sugar and water. That dissolves quickly. Now think about digesting a meal. That takes time! Living things need chemical reactions to happen fast enough to sustain life. That's where enzymes come in.

1.1 Definition and Role

An enzyme is a protein that acts as a biological catalyst.

  • Catalyst: A substance that speeds up a chemical reaction without being used up itself.
  • Biological: Meaning they work inside living organisms.

Enzymes are involved in all metabolic reactions. Metabolism is the sum of all chemical reactions that occur inside a living cell or organism. These reactions fall into two categories:

  1. Building Up: Synthesising large molecules (e.g., making proteins from amino acids).
  2. Breaking Down: Digesting large molecules (e.g., breaking down starch into glucose).

Quick Takeaway: Enzymes are reusable protein tools that speed up life-sustaining chemical reactions.

2. The Mechanism of Enzyme Action (The Lock and Key Model) (B5.1 Supplement 3 & 4)

Enzymes are unique because they are highly specific. An enzyme designed to break down starch cannot break down protein. This specificity is explained by the analogy known as the Lock and Key Model.

2.1 Key Components
  • Substrate: The molecule(s) the enzyme acts on (the 'key').
  • Active Site: A specific region on the enzyme molecule where the substrate binds (the 'lock').
  • Product: The new substance(s) formed after the reaction (the 'key parts' once the key is broken or fixed).
2.2 Step-by-Step Enzyme Action

The job of the enzyme is to hold the substrate in the correct position so the reaction can happen quickly.

  1. The substrate approaches the enzyme.
  2. The substrate fits perfectly into the active site because their shapes are complementary (Supplement 4). This forms the enzyme-substrate complex (E-S Complex) (Supplement 3).
  3. The enzyme holds the substrate(s) and facilitates the chemical reaction (either breaking bonds or forming new bonds).
  4. The newly formed products leave the active site.
  5. The enzyme is unchanged and immediately ready to take on another substrate molecule.

Analogy Check: Imagine trying to open a lock (enzyme active site). Only one specific key (substrate) will fit and work. If the wrong key tries to fit, nothing happens—this is specificity.

Quick Takeaway: The complementary shape and fit between the active site and the substrate dictates enzyme specificity, allowing the reaction to proceed fast.

3. Factors Affecting Enzyme Activity (B5.1 Core 2, Supplement 5 & 6)

Since enzymes are delicate proteins, their activity is highly sensitive to changes in the environment, especially temperature and pH. Enzymes work best under specific conditions, called their optimum conditions.

3.1 The Effect of Temperature

In IGCSE Biology, you must explain temperature effects using terms related to particle movement and shape.

A. Low Temperatures

At very low temperatures (e.g., 0°C), enzyme activity is slow.

Why?

  • The enzyme and substrate molecules have very little kinetic energy (movement energy) (Supplement 5).
  • This means there is a low frequency of effective collisions, so they meet and bind infrequently.
B. Optimum Temperature

This is the temperature where the enzyme is most active. For most human enzymes, this is around 37°C.

  • Molecules have maximum kinetic energy without damaging the enzyme structure.
  • The frequency of effective collisions is at its peak.
C. High Temperatures (Above Optimum)

If the temperature rises too high (e.g., above 45°C), the rate of reaction drops dramatically because the enzyme becomes denatured.

Why Denaturation Occurs (Supplement 5):

  1. High heat causes the enzyme protein structure to vibrate violently.
  2. The weak bonds holding the precise 3D shape of the enzyme, especially the active site, break.
  3. The active site changes shape, meaning the substrate can no longer fit (loss of complementary shape and fit).
  4. This change is permanent. Denatured enzymes cannot return to their original shape.

Think of denaturing an enzyme like frying an egg. The protein (egg white) changes shape permanently due to heat. You can't un-fry an egg!

3.2 The Effect of pH

pH measures acidity or alkalinity. Each enzyme has a narrow, specific pH range in which it works best—its optimum pH.

A. Optimum pH

Different enzymes have different optimum pH values. For example, the enzyme found in your mouth (salivary amylase) prefers neutral pH 7, while the protease enzyme in your stomach prefers very acidic pH 2.

B. Deviation from Optimum pH

If the pH moves too far above or below the optimum, enzyme activity decreases rapidly.

Why? (Supplement 6):

  • Extreme changes in pH interfere with the chemical bonds and electrical charges within the enzyme molecule.
  • This causes the enzyme's 3D structure to change, distorting the active site.
  • The substrate can no longer fit into the misshapen active site (loss of shape and fit), leading to denaturation.

Common Mistake Alert: Students often confuse "slow activity" (at low temperature) with "denaturation" (at high temperature/extreme pH). Slow activity is temporary; denaturation is permanent and destroys the enzyme’s function.

4. Digestive Enzymes: Real-World Applications (Linking to B7.3)

Enzymes are essential for digestion, breaking down large insoluble food molecules into small soluble molecules that can be absorbed into the blood.

Enzyme Type Breaks Down... Into... Where it Acts (Optimum pH)
Amylase Starch (complex carbohydrate) Simple reducing sugars (e.g., maltose) Mouth (neutral) and Small Intestine (alkaline)
Proteases Proteins Amino acids Stomach (acidic, pH 2) and Small Intestine (alkaline)
Lipase Fats and Oils (lipids) Fatty acids and glycerol Small Intestine (alkaline)

Did you know? The stomach is extremely acidic (pH ~2) due to hydrochloric acid. This acid has two main roles (B7.3, point 7): 1) Killing harmful microorganisms, and 2) Providing the optimum acidic pH required for the protease enzymes (like pepsin) to work efficiently!

Extra Tip for Lipase (B7.3, point 9): Lipase works best in the small intestine, which is alkaline. However, fats are large lumps. Bile (produced by the liver) emulsifies the fats—breaking them into smaller droplets—which increases their surface area dramatically. This larger surface area means lipase can work much faster!

Quick Review Box

  • Enzymes are proteins acting as biological catalysts.
  • The Active Site determines specificity (complementary shape).
  • High temperature/extreme pH causes denaturation (permanent loss of shape).
  • Low temperature causes slow activity (low kinetic energy/collisions).