Enzyme action

Biology · Enzymes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The chemical reactions in living cells would be far too slow at body temperature without help. Enzymes are proteins that speed these reactions up. They are biological catalysts: they make the reaction faster but are not changed by it, so one enzyme molecule can be used again and again.

Each enzyme has a region called the active site. The substance the enzyme works on is the substrate. The substrate fits into the active site because the two have complementary shapes, like a key in a lock. Together they form an enzyme-substrate complex. The reaction happens, the products leave, and the active site is free again.

Rules to remember

Catalyst: a substance that increases the rate of a chemical reaction and is not changed by the reaction.
Enzymes are proteins that act as biological catalysts in all metabolic reactions.
Sequence: enzyme + substrate → enzyme-substrate complex → enzyme + products.
Lock and key: the enzyme is the lock, the substrate is the key.
Enzymes are specific: the active site has a shape complementary to one substrate only.
Examples: amylase breaks down starch to maltose; protease breaks down protein to amino acids; lipase breaks down lipids to fatty acids and glycerol.

Common mistake

Writing that the active site and the substrate have the same shape. They have complementary shapes, so that one fits into the other. Also, enzymes are not used up in the reaction.

Worked example

Lipase is mixed with a starch solution and kept at 37 °C for 30 minutes. A sample then still turns iodine solution blue-black. Explain why.

Blue-black with iodine means starch is still present, so it was not broken down.
The active site of lipase has a shape that is complementary to lipid molecules.
Starch has a different shape, so it cannot fit into the active site.
No enzyme-substrate complex forms, so there is no reaction.

Answer: Lipase is specific to lipids; starch does not fit its active site, so the starch stays unchanged.

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