What are enzymes and why are they important in Molecular Biology?

Enzymes are biological catalysts that speed up chemical reactions in cells without being consumed in the process. They are crucial in Molecular Biology because they facilitate essential biochemical reactions, such as DNA replication and protein synthesis, which are vital for cell function and survival.

How do enzymes function and what is the role of the active site?

Enzymes function by lowering the activation energy of a reaction, making it easier for the reaction to occur. The active site of an enzyme is a specific region where substrate molecules bind. The shape and chemical environment of the active site allow the enzyme to interact with its substrate with high specificity, facilitating the conversion of substrates into products.

What factors affect enzyme activity and how do they influence it?

Enzyme activity is influenced by several factors including temperature, pH, and substrate concentration. Optimal temperature and pH levels are necessary for maintaining the enzyme's structure and function. Deviations can lead to denaturation or reduced activity. Substrate concentration affects the rate of reaction, with increased concentration leading to higher activity until the enzyme becomes saturated.

What is enzyme inhibition and what are its types?

Enzyme inhibition refers to a decrease in enzyme activity due to the presence of an inhibitor. There are two main types: competitive and non-competitive inhibition. Competitive inhibitors bind to the active site, blocking substrate access, while non-competitive inhibitors bind to another part of the enzyme, altering its shape and function. Both types reduce the rate of enzyme-catalyzed reactions.

How are enzymes regulated in biological systems?

Enzymes are regulated through various mechanisms to ensure metabolic pathways function efficiently. This includes allosteric regulation, where molecules bind to sites other than the active site to modulate activity, and feedback inhibition, where the end product of a pathway inhibits an enzyme involved earlier in the pathway. These regulatory mechanisms help maintain homeostasis within the cell.

Why is "Stating enzymes are 'used up' in the reaction." a common mistake in Enzymes?

Why it happens: Confusing enzymes with substrates. How to avoid it: Enzymes are catalysts — recycled, not consumed.

Why is "Showing rate continuing to increase past optimum." a common mistake in Enzymes?

Why it happens: Belief that 'more heat = more rate'. How to avoid it: Above optimum: denaturation drops rate sharply. Past denaturation rate ≈ 0.

Why is "Saying non-competitive inhibitors resemble the substrate." a common mistake in Enzymes?

Why it happens: Confusing with competitive. How to avoid it: Only COMPETITIVE inhibitors resemble substrate (to fit active site).

IB DP Biology (BI)

Enzymes

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Short Notes - Enzymes

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Detailed Study Notes

Detailed notes on Molecular Biology for IB DP Biology, covering key concepts, explanations, examples, and exam-focused revision points.

Enzymes — IB Biology SL: biological catalysts, active site, induced fit, factors affecting rate, inhibition

Maps to Theme B (Form and Function). Covers enzyme structure, lock-and-key vs induced fit models, the effect of temperature, pH and substrate concentration on enzyme activity, and competitive vs non-competitive inhibition.

At a glance

ENZYMES are protein CATALYSTS — lower activation energy without being used up.
ACTIVE SITE: region where substrate binds.
LOCK-AND-KEY: rigid complementary fit.
INDUCED FIT: substrate binding causes active site to mould around it (more modern model).
TEMPERATURE: rate doubles per 10 °C up to optimum; denatures above.
pH: each enzyme has optimum (pepsin pH 2, trypsin pH 8); extremes denature.
SUBSTRATE CONCENTRATION: increases rate until enzymes saturated (Vmax).
COMPETITIVE inhibitor binds active site; outcompeted at high substrate.
NON-COMPETITIVE binds elsewhere; changes shape of active site.

What you’ll learn

Mapped to the IB DP Biology SL subject guide (2025 onwards (first assessment May 2025)).

B1.3.1 — Define enzyme and substrate.
B1.3.2 — Compare lock-and-key and induced fit models.
B1.3.3 — Explain the effects of temperature, pH and substrate concentration on enzyme activity.
B1.3.4 — Distinguish competitive and non-competitive inhibition.

Enzymes as catalysts

Speed up reactions; lower activation energy.

Enzyme — biological catalyst, usually a globular protein (some are RNA — ribozymes).

Active site — small region of the enzyme that is complementary in shape and chemistry to the substrate (the molecule being acted on).

Two models of binding:

Model	Description	Limitation
Lock-and-key (Fischer, 1894)	Rigid active site exactly complementary to substrate	Cannot explain how enzymes accommodate multiple similar substrates
Induced fit (Koshland, 1958)	Active site moulds around substrate on binding	Now widely accepted; supported by crystal structures

How enzymes work. Bind substrate at active site → form enzyme-substrate complex → catalyse the reaction → release products. Enzymes lower the activation energy (Eₐ) of the reaction, increasing the rate without changing the equilibrium.

Properties.

Specific (only certain substrates fit).
Reusable (not consumed).
Effective in tiny amounts.
Highly affected by environmental conditions.

Enzyme + substrate → ES complex → enzyme + product.
Lower activation energy.
Induced fit more accurate than lock-and-key.
Specific, reusable, affected by environment.

Factors and inhibition

Temperature, pH, substrate, inhibitors.

Effect of temperature.

Below optimum: increasing T → more kinetic energy → more successful collisions → rate ~doubles per 10 °C.
At optimum: maximum rate.
Above optimum: H-bonds and weak interactions break → active site shape lost → DENATURATION.

Effect of pH.

Each enzyme has an optimum pH (pepsin pH 2; trypsin pH 8; salivary amylase pH 7).
Extremes of pH alter charges on R groups in the active site → bond disruption → denaturation.

Effect of substrate concentration.

At low substrate: rate ∝ substrate (more substrate → more collisions).
At high substrate: all active sites occupied → rate plateaus at Vmax.

Inhibition.

Competitive inhibitor:

Similar shape to substrate.
Binds the active site.
Blocks substrate binding.
Effect REDUCED at high substrate (substrate outcompetes inhibitor).
Example: malonate inhibits succinate dehydrogenase.

Non-competitive inhibitor:

Binds an allosteric site (not the active site).
Changes the enzyme's shape → active site can no longer fit substrate.
Effect NOT reduced by adding substrate.
Example: heavy metals (Pb²⁺, Hg²⁺) inhibit many enzymes.

	Competitive	Non-competitive
Binds	Active site	Allosteric site
Resembles substrate	Yes	No
Effect of ↑ substrate	Reverses inhibition	No change

Temperature: rises then sharply falls at denaturation.
pH: bell-shaped curve.
Substrate: hyperbolic; plateaus at Vmax.
Competitive: active site; outcompeted.
Non-competitive: allosteric; not outcompeted.

Quick recap

Enzymes lower activation energy.
Induced fit better than lock-and-key.
Temperature, pH, substrate concentration determine rate.
Competitive (active site) vs non-competitive (allosteric).

Memorise this

Verbatim phrases, formulae and definitions IB DP mark schemes credit (key for AO1 knowledge marks on Paper 1).

E + S ⇌ ES → E + P
Enzyme lowers Eₐ
Rate doubles per 10 °C below optimum
Above optimum → denatures
Competitive ≈ substrate, active site
Non-competitive = allosteric

How it’s examined

Paper 1: MCQ on inhibition type or graph interpretation. Paper 2: 'sketch and explain rate vs temperature/pH/substrate'; 'distinguish competitive and non-competitive inhibition'. IA: enzyme rate practical with controlled variables.

Sources: IB Diploma Programme Biology subject guide (IBO, 2023 — first assessment 2025). Last reviewed 2026-05-31.

Take this whole topic with you

Step-by-step worked examples — Enzymes

Step-by-step solutions to past-paper-style questions on enzymes, written exactly the way a tutor would explain them at the board.

1Sketch and explain rate vs temperature
Building confidence• temperature
Question
Sketch the graph of enzyme activity against temperature for a human enzyme. Annotate the optimum and explain the shape. (4 marks)
Step-by-step solution
1. Step 1
  Below optimum: rate rises (more kinetic energy → more successful ES collisions).
2. Step 2
  Optimum ≈ 37 °C for human enzymes — maximum rate.
3. Step 3
  Above optimum: rate falls SHARPLY as H-bonds break and active site changes shape (denaturation).
4. Step 4
  Beyond ~50 °C activity ≈ 0; denaturation is irreversible.
Answer
Rises to optimum (37 °C human), then sharp fall due to denaturation.
2Distinguish competitive vs non-competitive inhibition
Stretch• inhibition
Question
Compare competitive and non-competitive inhibition. (4 marks)
Step-by-step solution
1. Step 1
  Competitive: binds the active site directly; structurally similar to substrate.
2. Step 2
  Non-competitive: binds an allosteric site away from active site; changes enzyme shape.
3. Step 3
  Competitive: inhibition reversed by raising substrate concentration.
4. Step 4
  Non-competitive: NOT reversed by raising substrate.
Answer
Competitive: active site, similar to substrate, outcompeted by ↑[S]. Non-competitive: allosteric, NOT outcompeted.
3Why does the rate plateau?
Getting started• Vmax
Question
At high substrate concentration the rate of an enzyme-catalysed reaction plateaus. Explain. (2 marks)
Step-by-step solution
1. Step 1
  All active sites are occupied — enzymes are saturated.
2. Step 2
  Rate becomes limited by enzyme concentration (or turnover number), not substrate availability.
Answer
Enzymes saturated at Vmax — rate limited by enzyme number/turnover.

Key Definitions and Keywords — Enzymes

Definitions to memorise and the exact keywords mark schemes credit for enzymes answers — sharpened from recent examiner reports for the 2026 IB DP Biology SL sitting.

Enzyme
Examiner keyword
Biological catalyst, usually protein, that lowers the activation energy of a specific reaction.
Active site
Examiner keyword
Region of the enzyme that binds substrate and catalyses its conversion.
Induced fit model
Examiner keyword
Enzyme's active site moulds around the substrate on binding (Koshland, 1958).

Common Mistakes and Misconceptions — Enzymes

The traps other students keep falling into on enzymes questions — taken from recent IB DP Biology SL examiner reports and mark schemes — and how to avoid them.

✕Stating enzymes are 'used up' in the reaction.
Why it happens
Confusing enzymes with substrates.
How to avoid it
Enzymes are catalysts — recycled, not consumed.
✕Showing rate continuing to increase past optimum.
Why it happens
Belief that 'more heat = more rate'.
How to avoid it
Above optimum: denaturation drops rate sharply. Past denaturation rate ≈ 0.
✕Saying non-competitive inhibitors resemble the substrate.
Why it happens
Confusing with competitive.
How to avoid it
Only COMPETITIVE inhibitors resemble substrate (to fit active site).

Enzymes — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Model

Description

Limitation

Lock-and-key (Fischer, 1894)

Rigid active site exactly complementary to substrate

Cannot explain how enzymes accommodate multiple similar substrates

Induced fit (Koshland, 1958)

Active site moulds around substrate on binding

Now widely accepted; supported by crystal structures

Competitive

Non-competitive

Binds

Active site

Allosteric site

Resembles substrate

Yes

Effect of ↑ substrate

Reverses inhibition

No change

Enzymes

Short Notes - Enzymes

Detailed Study Notes

At a glance

What you’ll learn

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Sketch and explain rate vs temperature

2Distinguish competitive vs non-competitive inhibition

3Why does the rate plateau?

Enzyme

Active site

Induced fit model

✕Stating enzymes are 'used up' in the reaction.

✕Showing rate continuing to increase past optimum.

✕Saying non-competitive inhibitors resemble the substrate.

Enzymes — frequently asked questions

Enzymes

Short Notes - Enzymes

Detailed Study Notes

At a glance

What you’ll learn

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Sketch and explain rate vs temperature

2Distinguish competitive vs non-competitive inhibition

3Why does the rate plateau?

Enzyme

Active site

Induced fit model

✕Stating enzymes are 'used up' in the reaction.

✕Showing rate continuing to increase past optimum.

✕Saying non-competitive inhibitors resemble the substrate.

Enzymes — frequently asked questions