Enzymes as biological catalysts

Structure and functions in living organisms · Biological molecules and enzymes · note 3 of 5

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Enzymes as biological catalystsSpec 2.10

Metabolic reactions are the chemical reactions that happen in living cells. They are controlled by enzymes. Enzymes are proteins that act as biological catalysts. A catalyst speeds up a reaction without being used up. Each enzyme has a region called the active site, which has a particular shape.

The molecule an enzyme acts on is called the substrate. The substrate has a shape that is complementary to the active site, so it fits in, like a key in a lock. This is called the lock and key model.

The substrate collides with the active site of the enzyme.
The substrate fits into the active site, forming an enzyme-substrate complex.
The reaction takes place at the active site.
The product or products leave the active site and the enzyme is free to be used again.

Enzymes are specific. Only a substrate with a shape that fits the active site can bind to it, so each enzyme catalyses only one type of reaction.

Four-step lock and key model: the substrate collides with the active site, fits to form an enzyme-substrate complex, the reaction takes place, then the products leave and the enzyme is unchanged. — The lock and key model. The substrate is complementary to the active site, and the enzyme is not used up.

Exam tip:

The mark scheme word is complementary. Say the shape of the substrate is complementary to the shape of the active site, not that they are 'the same'.