3.1 Enzymes

What is a catalyst?

A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process.

👉 It lowers activation energy, allowing reactions to occur faster.

What is an enzyme?

An enzyme is a biological catalyst made of protein that speeds up metabolic reactions in living cells.

👉 Most enzymes are globular proteins with specific 3D shapes.

What is a substrate?

A substrate is the reactant molecule that an enzyme acts upon during a reaction.

What is a product?

A product is the molecule(s) formed after an enzyme-catalysed reaction.

What is an active site?

The active site is a specific region (pocket) on an enzyme where the substrate binds.

👉 Formed by R-groups of amino acids

👉 Has a complementary shape to the substrate

What is activation energy?

Activation energy is the minimum energy required to start a chemical reaction.

👉 Enzymes lower this energy barrier.

How do enzymes speed up reactions?

They provide a surface where substrates come together in the correct orientation, reducing activation energy.

👉 Makes reactions happen faster and more efficiently.

Why are enzymes necessary for life?

Without enzymes, metabolic reactions would occur too slowly to sustain life.

👉 Many reactions involve strong covalent bonds that require high energy to break.

Give examples of enzymes breaking down molecules (catabolic reactions).

• Amylase: starch → maltose

• Sucrase: sucrose → glucose + fructose

• Proteases: proteins → amino acids

• Lipase: lipids → fatty acids + glycerol

• Nucleases: nucleic acids → nucleotides

Give examples of enzymes building molecules (anabolic reactions).

• Starch synthetase: glucose → starch

• Peptide synthetase: amino acids → polypeptide
  👉 These involve condensation reactions (water removed)

What is enzyme specificity?

The ability of an enzyme to only bind to a specific substrate and catalyse a specific reaction.

What determines enzyme specificity?

The shape of the active site, which is complementary to the substrate.

Why won’t amylase break down proteins?

Because proteins do not fit the active site of amylase — wrong shape.

Are all enzymes completely specific?

No.

👉 Some are highly specific (one substrate)

👉 Others act on similar types of bonds (e.g. proteases)

What is the lock and key model?

The substrate fits exactly into the active site, like a key in a lock.

What is the induced fit model?

The enzyme changes shape slightly when the substrate binds to improve the fit.

👉 This is the more accurate model.

What is an enzyme-substrate complex?

A temporary complex formed when the substrate binds to the enzyme’s active site.

What happens to the substrate in the active site?

It is placed under strain, making bonds easier to break or form.

Why do biological reactions need activation energy?

Because they involve breaking or forming strong covalent bonds.

How do enzymes reduce activation energy?

• Bringing substrates together

• Orienting them correctly

• Straining bonds

Why are there many different enzymes in a cell?

Each enzyme is specific to a reaction, so many enzymes are needed to control all metabolic processes.

What type of protein are enzymes?

Globular proteins — compact, folded into specific 3D shapes.

Where is the active site formed from?

From amino acid R-groups brought together by tertiary structure.

What wording should you use to describe enzyme-substrate fit?

✅ “Complementary shape”

❌ NOT “same shape”