Science - Combined (0653) | Enzymes

🔬 Chapter B5: Enzymes – The Body’s Biological Catalysts

Welcome to the fascinating world of enzymes! These molecules are the most important workers in your body, responsible for speeding up virtually every chemical reaction that keeps you alive, from breathing to digesting food.
Understanding enzymes is crucial because they explain how biological processes happen quickly and efficiently. Don't worry if the vocabulary seems tricky at first—we'll break it down using simple analogies!

Key Takeaway from the Introduction

Enzymes are essential for life because they allow complex chemical reactions (metabolism) to happen fast enough to support living organisms.

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1. What Exactly Are Enzymes? (Core 1)

Enzymes are special types of proteins that act as biological catalysts.

What does 'Biological Catalyst' mean?

A catalyst is a substance that speeds up a chemical reaction without being used up itself.
When we say an enzyme is a biological catalyst, it means it does this job inside living organisms (like your cells).

• Function: Enzymes are involved in all metabolic reactions.
• Example: Breaking down the large food molecules you eat (digestion) or releasing energy from glucose (respiration).
• Identity: All enzymes are chemically classified as proteins.

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2. How Enzymes Work: The Lock and Key Model (Supplement 3 & 4)

Enzymes don't just speed up *any* reaction; they are highly specific. Think of it like a very specialised locksmith!

The Analogy: The Lock and Key

The classic way to understand enzyme action is the Lock and Key Model.

• The Enzyme is the Lock.
• The molecule the enzyme acts upon (the reactant) is the Substrate (the key).

Step-by-Step Enzyme Action (The Process)

The reaction follows a specific path:

1. The Substrate finds the enzyme.
2. The substrate fits precisely into a specific region on the enzyme called the Active Site.
3. When the substrate is held in the active site, a temporary structure is formed: the Enzyme-Substrate Complex.
4. The enzyme catalyses the reaction (e.g., breaking the bond in the substrate).
5. The substrate is converted into new molecules called Products.
6. The products leave the active site, and the enzyme is now free to bind to a new substrate molecule. (Remember, catalysts are not used up!)

Enzyme Specificity (Supplement 4)

Enzymes are highly specific. This means one enzyme can usually only catalyse one specific reaction.

• This specificity is explained by the complementary shape and fit between the active site of the enzyme and the shape of the substrate.
• If the substrate molecule is the wrong shape, it cannot fit into the active site, and no reaction will occur.

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

Enzymes are very delicate! If the conditions around them are not optimal, they stop working efficiently. We must investigate and explain how temperature and pH affect them.

A. The Effect of Temperature (Core 2, Supplement 5)

1. At Low Temperatures

When the temperature is low (e.g., close to 0°C), the enzyme and substrate molecules have very low kinetic energy.
They move slowly, leading to fewer successful effective collisions between the substrate and the active site. The reaction rate is therefore very slow.

2. At Optimum Temperature

As temperature increases, the kinetic energy of both enzyme and substrate molecules increases. They move faster, leading to a much higher frequency of effective collisions.
This results in the maximum rate of reaction.
(Did you know? For most human enzymes, the optimum temperature is around 37°C.)

3. Above Optimum Temperature (Denaturation)

This is critical. If the temperature gets too high (e.g., above 45°C):

• The intense heat causes the bonds holding the complex 3D structure of the enzyme protein to vibrate violently and break.
• This causes the enzyme to lose its specific shape. This process is called denaturation.
• Crucially, the active site changes shape.
• Because the active site is no longer the complementary shape to the substrate, the substrate cannot fit (the key no longer fits the lock), and the enzyme stops working.
• Denaturation is usually irreversible. Once the protein shape is lost, it cannot easily be restored, meaning the enzyme is permanently damaged.

B. The Effect of pH (Core 2, Supplement 6)

The pH scale measures how acidic or alkaline a solution is. Enzymes also require an optimal pH level to maintain their structure.

1. At Optimum pH

Each enzyme has an optimum pH where its activity is highest. At this pH, the enzyme’s 3D structure (including the active site) is perfectly maintained.

• Example: The enzyme amylase (which breaks down starch) works best at a neutral or slightly alkaline pH (around pH 7 to 8).

2. At Extreme pH (Acidic or Alkaline)

If the pH is too acidic (low) or too alkaline (high) relative to the optimum:

• The change in hydrogen ion concentration interferes with the electrical charges within the protein structure, causing bonds to break.
• Like with temperature, this results in denaturation and the active site changes shape.
• The enzyme can no longer bind to the substrate, and enzyme activity stops.

Interesting Connection: Digestive Enzymes

Different enzymes in the body are adapted to work at different pH levels depending on where they are found:

• Protease (e.g., Pepsin) in the stomach: Optimum pH is very low/acidic (around pH 2) because the stomach contains hydrochloric acid. This acid also serves the function of killing harmful microorganisms in food (B7.3, Supplement 7).
• Amylase in the small intestine: Optimum pH is alkaline (around pH 8.5) because bile neutralises the acid coming from the stomach.

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4. Specific Examples of Enzyme Functions (Review from B7.3)

When studying enzymes, it's essential to know the function of the main digestive enzymes:

Functions of Digestive Enzymes (B7.3 Supplement 5)

Enzyme Type	Substrate (What it breaks down)	Product (What it breaks into)
Amylase	Starch (a large carbohydrate)	Simple reducing sugars (e.g., maltose)
Proteases	Proteins (large molecules)	Small amino acids
Lipases	Fats and oils (lipids)	Fatty acids and glycerol

Location of Secretion and Action (B7.3 Supplement 6)

• Amylase: Secreted by salivary glands and pancreas. Acts in the mouth and small intestine.
• Proteases: Secreted by the stomach (pepsin) and pancreas. Acts in the stomach and small intestine.
• Lipase: Secreted by the pancreas. Acts in the small intestine.

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Study Summary: Enzymes

What you must know:

1. Definition: Enzymes are proteins acting as biological catalysts, speeding up metabolic reactions.
2. Action: Explained by the Lock and Key Model, where the substrate fits into the complementary-shaped active site to form an enzyme-substrate complex, yielding products.
3. Temperature: Works optimally at body temperature. Too cold slows reaction (low kinetic energy). Too hot causes denaturation (active site loses shape), which is irreversible.
4. pH: Each enzyme has an optimum pH. Extreme pH levels cause denaturation by altering the protein structure.