3.1.4.2 Enzyme action

Enzymes

• Proteins

• Speed up chemical reactions by acting as biological catalysts

• They catalyse metabolic reactions

• Intracellular and extracellular action (within and outside the cells)

• Active site with a specific shape where substrates molecules bind to

How do enzymes speed up reactions?

• Lower the amount of activation energy that is needed, often making reactions happen at a lower temp. than they could without an enzyme

Catalyst

A substance that speeds up a chemical reaction without being used up in the reactions itself

Enzyme-substrate complex

Formed when a substrate fits into the enzyme’s active site

Why do the formation of an enzyme-substrate complex lower the activation energy?

• If 2 substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily

• If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily

Lock and key model

The active site and the substrate have a complementary shape

The induced fit model

• The substrate doesn’t only have to be the right shape to fit the active site, it has to make the active site change shape in the right way too

• Once the products are released, the active site returns to its original shape and can bind to the next substrate molecule

Enzyme properties

• Related to their tertiary structure

• The active site’s shape is determined by the enzyme’s tertiary structure

Tertiary structure pt2

• If the tertiary structure of a protein is altered, the shape of the active site changes

• This means that the substrate would no longer bind to the active site, so an enzyme-substrate complex would not be formed, therefore the reaction would not be catalysed as the enzyme would no longer be able to carry out its function

• The primary structure is determined by a gene and if there is a mutation in that gene, the tertiary structure may change