# Explain how enzymes act as biological catalysts by reference to their structure and interaction with substrates.

> IB DP Biology Standard Level (2023 syllabus) — B1.2 Proteins · Explain · 4 marks

## Mark scheme (4 marks)

1. Enzymes are proteins with a specific three-dimensional shape determined by their amino acid sequence.
2. The active site is a region of the enzyme with a shape complementary to the specific substrate(s).
3. The enzyme–substrate complex forms when the substrate binds to the active site, lowering the activation energy of the reaction.
4. The enzyme is released unchanged after the reaction, allowing it to catalyse further reactions.

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