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Updated Mar 20, 2026

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14 pages

EDUQAS A Level Biology: Enzymes Topic Revision Notes

Molly Gowar

@mollygowar

Ever wondered how your body manages thousands of chemical reactions... Show more

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The Protein Nature of Enzymes

Your body is basically a massive chemical factory running non-stop reactions called metabolism. These reactions happen in organised sequences called metabolic pathways - think of them like assembly lines where each step feeds into the next.

There are two main types of reactions: anabolic reactions that build things up (like making proteins) and catabolic reactions that break things down (like digestion). What's brilliant is that the product from one reaction becomes the starting material for the next - it's perfectly coordinated.

Enzymes are globular proteins that work as biological catalysts. They're made by living cells and only speed up reactions that would happen anyway - they just make them fast enough for life to actually work. Without enzymes, your cellular reactions would be so slow you'd basically stop functioning.

Key Point: Enzymes aren't used up or changed during reactions - they can be reused thousands of times, which is why your body only needs small amounts of them.

Structure of Enzymes

Enzymes are proteins with a specific tertiary structure that folds into a globular shape. The hydrophilic R groups sit on the outside, making enzymes water-soluble - pretty handy since your cells are mostly water!

Each enzyme has its own unique sequence of amino acids, and the bonds between them (hydrogen bonds, disulphide bonds, and ionic bonds) hold the whole molecule in its precise 3D shape. This shape is absolutely critical because it creates the active site - a small area with a specific 3D structure.

Enzymes work in three different locations: extracellular enzymes get secreted outside cells (like digestive enzymes), intracellular enzymes in solution work inside cells (like those in glucose breakdown), and membrane-bound enzymes attach to membranes (like those making ATP in mitochondria).

Remember: The enzyme's shape determines its function - change the shape and you change what it can do.

Active Sites

The active site is where all the action happens - it's the specific 3D area where the substrate (the molecule being worked on) temporarily binds to form an enzyme-substrate complex. Once the reaction finishes, products are released and the enzyme is ready to go again.

The lock and key model explains this beautifully: the active site has a complementary shape to its specific substrate, like a lock that only fits one key. This specificity means each enzyme only catalyses one type of reaction - no accidents or mix-ups.

When the substrate fits into the active site, temporary bonds form between them. After the reaction, products are released, leaving the enzyme completely unchanged and ready for the next substrate molecule.

Top Tip: Think of enzymes like reusable tools - they do their job perfectly every time and never wear out from normal use.

The Induced Fit Model

The induced fit model gives us a more accurate picture than the simple lock and key idea. Here, the enzyme's active site actually changes shape slightly to accommodate the substrate - like a flexible glove moulding around your hand.

As the enzyme changes shape around the substrate, it puts strain on the substrate's chemical bonds. This lowers the activation energy needed to break those bonds, making the reaction much easier to happen.

Lysozyme is a perfect example - it's the antibacterial enzyme in your saliva and tears. Its active site forms a groove that bacterial cell wall sugars fit into, then the groove closes around them, changing the enzyme's shape to catalyse the reaction.

Key Insight: The shape change isn't just about fitting together - it actually makes the chemical reaction easier by weakening the bonds that need to break.

Enzymes and Activation Energy

For any chemical reaction to happen, molecules need enough kinetic energy to get close enough to react. The minimum energy required is called the activation energy - think of it as the energy barrier that must be overcome.

In non-living systems, you'd add heat to give molecules more kinetic energy. But in living organisms, temperatures above 40°C cause proteins to denature (permanently lose their shape), which would be fatal.

Enzymes work by lowering the activation energy required for reactions. When a substrate enters the active site, the enzyme's shape change makes it much easier for the reaction to occur at normal body temperature.

Why This Matters: Without enzymes lowering activation energy, your body would need to be dangerously hot for essential reactions to happen fast enough.

The Course of an Enzyme-Controlled Reaction

When you plot an enzyme-catalysed reaction over time, you get a characteristic curve that tells the whole story. Initially, there's loads of substrate and empty active sites, so collisions happen frequently and products form rapidly.

As the reaction progresses, enzyme concentration becomes the limiting factor when there's still plenty of substrate around. More enzymes would mean more active sites and faster reactions.

Later, as substrate gets used up, substrate concentration becomes limiting instead. Eventually, all substrate is converted to products and the line plateaus - no more substrate means no more reaction.

Rate Calculation: Rate = Change in Mass ÷ Time, and % Change = (Change in Mass ÷ Initial Mass) × 100

Effect of Temperature on Enzyme Action

Temperature has a dramatic effect on enzyme activity. Higher temperatures increase the kinetic energy of both enzyme and substrate molecules, making successful collisions more frequent and speeding up reactions.

However, above 40°C, the increased molecular vibration starts breaking the hydrogen bonds that maintain the enzyme's tertiary structure. This changes the active site shape so the substrate no longer fits - the enzyme becomes denatured.

Denaturation is permanent - once those bonds are broken, the enzyme can't regain its original shape. At low temperatures, enzymes become inactivated (temporarily inactive) but aren't damaged, so warming them up restores their activity.

Temperature Sweet Spot: Most human enzymes work best around 37°C - your normal body temperature isn't a coincidence!

Effect of pH on Enzyme Action

Most enzymes have an optimum pH where they work best - usually around pH 7, though some (like pepsin in your stomach) prefer more extreme conditions.

pH affects the charges on amino acid side-chains in the active site. At low pH, excess H⁺ ions neutralise negative charges. At high pH, excess OH⁻ ions neutralise positive charges. Both extremes disrupt the ionic and hydrogen bonds maintaining the active site's shape.

Small pH changes around the optimum cause small, reversible changes in enzyme structure. But extreme pH values cause denaturation - the active site permanently loses its shape and can't form enzyme-substrate complexes.

pH Memory Trick: Think of Goldilocks - enzymes need pH to be "just right," not too acidic or too basic.

Effect of Enzyme Concentration

Since enzymes can be reused once products leave the active site, you only need low enzyme concentrations to catalyse loads of reactions. The turnover number tells you how many substrate molecules one enzyme can convert per second.

Catalase is incredibly fast with a turnover number of 40 million molecules per second - it breaks down toxic hydrogen peroxide in your cells. Most enzymes aren't quite this speedy, but they're still remarkably efficient.

As enzyme concentration increases, more active sites become available, so the rate of reaction increases proportionally. When enzyme concentration is low, it becomes the limiting factor controlling how fast reactions can happen.

Efficiency Fact: Enzymes are so efficient that your body can run thousands of different reactions simultaneously with relatively small amounts of each enzyme.

Effect of Substrate Concentration

Substrate concentration also controls reaction rates, but in a different way. With low substrate concentration, enzyme molecules have few substrate molecules to collide with, so active sites aren't working to full capacity.

As you add more substrate, more active sites get filled and the reaction rate increases. Substrate concentration is the limiting factor until you reach a critical point where all active sites are occupied.

Once the enzyme becomes saturated (all active sites full), adding more substrate won't increase the reaction rate - the line plateaus. Now substrate concentration is no longer limiting because there simply aren't enough free active sites to use the extra substrate.

Saturation Point: Think of it like a car park - once it's full, more cars arriving won't increase the number of parked cars until some spaces become free.

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Updated Mar 20, 2026

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14 pages

EDUQAS A Level Biology: Enzymes Topic Revision Notes

Molly Gowar

@mollygowar

As the reaction progresses, enzyme concentration becomes the limiting factor when there's still plenty of substrate around. More enzymes would mean more active sites and faster reactions.

Rate Calculation: Rate = Change in Mass ÷ Time, and % Change = (Change in Mass ÷ Initial Mass) × 100

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Effect of Temperature on Enzyme Action

Temperature Sweet Spot: Most human enzymes work best around 37°C - your normal body temperature isn't a coincidence!

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Effect of pH on Enzyme Action

Most enzymes have an optimum pH where they work best - usually around pH 7, though some (like pepsin in your stomach) prefer more extreme conditions.

pH Memory Trick: Think of Goldilocks - enzymes need pH to be "just right," not too acidic or too basic.

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Effect of Enzyme Concentration

Efficiency Fact: Enzymes are so efficient that your body can run thousands of different reactions simultaneously with relatively small amounts of each enzyme.

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Effect of Substrate Concentration

Saturation Point: Think of it like a car park - once it's full, more cars arriving won't increase the number of parked cars until some spaces become free.