Enzyme Action — Active Sites, Specificity and Catalysis

Enzymes are biological catalysts — proteins that speed up specific biochemical reactions without being used up themselves. They make life as we know it possible: without enzymes, the chemical reactions of metabolism would be far too slow to sustain a living cell. This lesson covers the OCR A-Level Biology A specification point 2.1.4 (a) and (b) — the role of enzymes in catalysing intracellular and extracellular reactions, and the mechanism of enzyme action, including the lock-and-key and induced-fit models.

1. What Is an Enzyme?

Key Definition — Enzyme: A biological catalyst, usually a globular protein, that speeds up a specific biochemical reaction without being used up. It works by lowering the activation energy of the reaction.

Key features of enzymes:

They are globular proteins (although some RNAs — ribozymes — are also catalytic; recall rRNA in Lesson 7).
They are catalysts — they increase the rate of a reaction without being changed overall. They can be reused.
Each enzyme is specific to one substrate or a small range of closely related substrates.
Enzymes catalyse reactions inside cells (intracellular enzymes, e.g. catalase, DNA polymerase) and outside cells (extracellular enzymes, e.g. the digestive enzymes amylase, lipase, trypsin).
They are reusable — a single enzyme molecule can catalyse millions of reactions per second.

2. Activation Energy and How Enzymes Lower It

Every chemical reaction requires energy to get started, even if the overall reaction is exergonic (energy-releasing). This initial energy barrier is called the activation energy (Eₐ).

Enzymes lower the activation energy required for a reaction to proceed. They do not change the overall energy change (ΔG) of the reaction. They only make it easier — and therefore faster — to reach the transition state.

How do enzymes lower activation energy?

They bring reactant molecules into the correct orientation and position to react.
They distort bonds in the substrate, weakening them.
They provide an alternative reaction pathway through the enzyme–substrate (ES) complex.
They may provide an environment with a different pH or charge distribution that favours the reaction.

Exam Tip: Never say enzymes "remove" the activation energy. They lower it. Never say enzymes "make reactions happen that otherwise wouldn't". A reaction that is energetically unfavourable (ΔG > 0) still will not proceed even with an enzyme.

3. The Active Site and Specificity

The key to enzyme function is the active site — a small region of the enzyme's surface, typically only a few amino acids in size, where the substrate binds and the reaction takes place.

Key Definition — Active site: A specific region on the surface of an enzyme, formed by a small number of amino acids, where the substrate binds and the reaction is catalysed.

The active site has a specific 3D shape that is complementary to the shape of the substrate. This shape arises from the tertiary structure of the enzyme — the overall folding of the polypeptide chain, held together by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between amino acid side chains (R groups).

Specificity

Because the active site has a specific shape and chemistry, only substrates with a complementary shape can bind. This is called enzyme specificity.

Each enzyme catalyses only one type of reaction (often just one reaction).
Examples:
- Amylase hydrolyses starch to maltose — it will not hydrolyse cellulose, even though cellulose is also a glucose polymer.
- Maltase hydrolyses maltose to glucose — it will not touch sucrose.
- Catalase breaks down hydrogen peroxide — one of the fastest enzymes known, with a turnover of ~40 million reactions per second.

Exam Tip: Specificity ultimately comes from the primary structure — the sequence of amino acids — which dictates how the protein folds into its tertiary structure and therefore the shape of the active site.

4. The Lock-and-Key Model (Fischer, 1894)

The earliest model of enzyme action was proposed by Emil Fischer in 1894. He suggested that the active site of an enzyme is a rigid shape that is exactly complementary to the substrate, just as a lock is exactly shaped to accept a specific key.

Strengths of the lock-and-key model:

Explains why enzymes are specific.
Explains why denaturation (which changes the shape of the active site) abolishes activity.

Weakness:

It is too rigid. Real enzymes often change shape slightly when the substrate binds.

5. The Induced-Fit Model (Koshland, 1958)

The induced-fit model, proposed by Daniel Koshland, is the currently accepted model. It proposes that:

The active site is not a rigid shape — it is only roughly complementary to the substrate to begin with.
When the substrate binds, the enzyme changes shape slightly so that the active site moulds more closely around the substrate.
This shape change puts strain on the substrate bonds, helping to lower the activation energy.

Strengths of the induced-fit model:

Explains the mechanism by which enzymes lower activation energy (strain on substrate bonds).
Accounts for the finding that enzymes can catalyse reactions with a range of very closely related substrates.
Explains why some enzymes can distinguish between almost-identical molecules only after contact.

Both models are in OCR A-Level Biology A and you should be able to describe and contrast them.

6. The Enzyme–Substrate Complex and the Course of a Reaction

The five key stages of a catalysed reaction are:

Substrate approaches the active site of the enzyme in solution.
Substrate binds to the active site, which is complementary in shape — the enzyme–substrate (ES) complex forms. In the induced-fit model, the active site changes shape to mould around the substrate.
Reaction takes place at the active site. Bonds in the substrate may be broken (as in hydrolysis) or new bonds formed (as in condensation). The enzyme–product complex is formed briefly.
Products are released from the active site.
The enzyme returns to its original shape and is ready to bind another substrate.

graph LR
  A[Enzyme + Substrate] --> B["Enzyme-substrate complex<br/>ES"]
  B --> C["Active site lowers Ea<br/>reaction proceeds"]
  C --> D[Enzyme-product complex]
  D --> E[Products released]
  E --> A

The enzyme is unchanged by the reaction and can immediately bind another substrate molecule. A single enzyme molecule can catalyse thousands to millions of reactions per second — this is called its turnover number.

7. Intracellular and Extracellular Enzymes

OCR requires you to know examples of both.

Type	Meaning	Examples
Intracellular	Acts inside the cell that made it	Catalase (breakdown of hydrogen peroxide in peroxisomes), DNA polymerase, Rubisco (carbon fixation in chloroplasts), respiratory enzymes
Extracellular	Secreted out of the cell to act externally	Amylase (in saliva, hydrolyses starch), trypsin (in the small intestine, hydrolyses proteins), lipase (pancreas, hydrolyses triglycerides)

Exam Tip: Learn at least one named intracellular and one named extracellular enzyme example for OCR-style questions.

8. Evidence for Enzyme Action — Catalase

Catalase is a useful model enzyme for practical work. It catalyses the decomposition of hydrogen peroxide (a toxic metabolic by-product) into water and oxygen:

$2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$

In a school laboratory you can measure the rate of oxygen production (volume of O₂ per unit time) to quantify enzyme activity. Catalase contains Fe²⁺ at its active site as a prosthetic group (see Lesson 10).

Lesson 9: Enzyme Action — Active Sites, Specificity and Catalysis