Question 1

What is the difference between lock-and-key and induced fit models?

Accepted Answer

The lock-and-key model suggests the active site is a perfect shape for the substrate, like a key fitting a lock. The induced fit model is more accurate: the active site is flexible and changes shape slightly when the substrate binds, creating a tighter fit and stressing bonds to lower activation energy. OCR A-Level expects you to describe both but emphasise induced fit as the modern view.

Question 2

How does temperature affect enzyme activity?

Accepted Answer

As temperature increases, molecules move faster, increasing collisions between enzyme and substrate, so reaction rate rises up to an optimum (usually around 37°C for human enzymes). Beyond this, heat breaks hydrogen bonds and other interactions, denaturing the enzyme's active site irreversibly, causing a rapid drop in rate. The graph shows a bell-shaped curve.

Question 3

What is the difference between competitive and non-competitive inhibition?

Accepted Answer

Competitive inhibitors have a similar shape to the substrate and bind reversibly to the active site, blocking the substrate. Increasing substrate concentration can overcome this. Non-competitive inhibitors bind to an allosteric site, changing the active site shape so substrate cannot bind effectively. This cannot be overcome by adding more substrate, and Vmax decreases.

Question 4

Why do enzymes have an optimum pH?

Accepted Answer

Enzymes have an optimum pH because their active site shape depends on ionic bonds and hydrogen bonds formed by amino acid R-groups. At extreme pH, these bonds break, altering the active site's 3D structure and reducing activity. Each enzyme has a different optimum (e.g., pepsin works best at pH 2, trypsin at pH 8) based on its environment.

Question 5

What is the role of cofactors and coenzymes?

Accepted Answer

Cofactors are inorganic ions (e.g., Mg2+, Zn2+) that help stabilise the enzyme-substrate complex or participate in catalysis. Coenzymes are organic molecules (e.g., NAD, FAD) that carry chemical groups between enzymes. Both are essential for some enzymes to function; without them, the enzyme may be inactive.

Question 6

How do you calculate the rate of an enzyme-controlled reaction from a graph?

Accepted Answer

To calculate the initial rate, draw a tangent to the curve at time zero (or the steepest part) and calculate its gradient (change in product concentration divided by change in time). This gives the rate in units like mmol/s. Ensure you use the correct units from the axes and show your working for full marks.

Enzymes

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

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How to Revise Enzymes — OCR A-Level Biology

Examiner Tips for Enzymes

Common Mistakes in Enzymes

Key Marking Points