BiologyCBSE Class 11 · Biomolecules

Enzymes: Action, Inhibition, Classes and Co-factors

Updated 17 October 2026 · Fact-checked

Enzymes are biological catalysts, mostly proteins, that lower the activation energy by forming an enzyme-substrate complex at the active site. Activity depends on temperature, pH, substrate concentration and inhibitors. Competitive inhibitors resemble the substrate. Enzymes are grouped into six classes, and many need co-factors such as prosthetic groups, coenzymes or metal ions.

Understand Enzyme Action, Inhibition and Classes

What enzymes are. Almost all enzymes are proteins (some nucleic acids called ribozymes act as enzymes). They speed up reactions without being consumed. Each enzyme has an active site, a crevice or pocket into which the substrate fits; the tertiary structure creates it.

Mechanism. Substrate (S) + enzyme (E) ⇌ enzyme-substrate complex (ES) → enzyme-product complex (EP) → enzyme + product (P). Enzymes lower the activation energy, the energy barrier between the substrate and the transition state, so reactions proceed faster at body temperature. The enzyme is released unchanged and used again.

Factors affecting activity.

Temperature: activity rises to an optimum and falls at high temperature because of denaturation; low temperature gives temporary inactivity (enzymes from thermophilic organisms stay active at high temperature).
pH: each enzyme has an optimum pH; extremes change charges and shape.
Substrate concentration: velocity of reaction rises with substrate concentration and then reaches a maximum velocity (Vmax), at which all active sites are occupied; the substrate concentration at which the velocity is half of Vmax is the Km (Michaelis constant, a measure of affinity).
Inhibitors: in competitive inhibition a molecule closely resembling the substrate competes for the active site; the activity falls but is restored by raising substrate concentration. Example: malonate inhibits succinic dehydrogenase because it resembles succinate. Many drugs act as competitive inhibitors (for example in treating bacterial infection).

Classification (six classes).

Oxidoreductases/dehydrogenases: oxidation-reduction between two substrates, S reduced + S′ oxidised → S oxidised + S′ reduced.
Transferases: transfer a group (other than hydrogen) between substrates.
Hydrolases: hydrolysis of ester, ether, peptide, glycosidic, C–C, C–halide or P–N bonds.
Lyases: remove groups from substrates by mechanisms other than hydrolysis, leaving double bonds.
Isomerases: interconversion of optical, geometric or positional isomers.
Ligases: link two compounds, e.g. enzymes forming C–O, C–S, C–N or P–O bonds, using energy (ATP).

Co-factors. Some enzymes are conjugated: the protein part is the apoenzyme; a non-protein co-factor is needed, and together they form the holoenzyme. Without the co-factor, the apoenzyme is inactive. Types: prosthetic groups (tightly bound, e.g. haem in catalase and peroxidase), coenzymes (loosely bound, often vitamin derivatives, e.g. NAD and NADP containing niacin), and metal ions (zinc for carboxypeptidase).

Graph (describe): velocity versus temperature rises, peaks at the optimum, then falls sharply.

Enzyme Action, Inhibition and Classes: key facts

Reaction: $E + S = E S - > E P - > E + P$; Enzyme is released unchanged
Km: Substrate concentration at half Vmax; Low Km = high affinity
Holoenzyme: $A p oe n z y m e (p r o t e in) + co - f a c t or = h o l oe n z y m e$; Co-factor types: prosthetic group, coenzyme, metal ion
Classes: $O x i d or e d u c t a ses ∣ t r an s f er a ses ∣ h y d r o l a ses ∣ l y a ses ∣ i so m er a ses ∣ l i g a ses$; Six classes

How to answer Enzyme Action, Inhibition and Classes questions

Questions ask for effects on activity, inhibition behaviour or class of a named reaction.

1Identify the factor: temperature, pH, substrate, inhibitor.
2For a graph, describe rise, optimum and fall, with the reason.
3For inhibition, state resemblance to the substrate and whether raising substrate concentration helps.
4For classification, ask what the reaction does: remove H, transfer, hydrolyse, split, rearrange, join.
5Identify the co-factor type from binding strength.

Enzyme Action, Inhibition and Classes in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Washing powder enzyme

Suppose a detergent labelled with a protein-digesting enzyme works well in warm water but loses activity in boiling water.

Explain, and say which class of enzyme digests protein.

Try it first, then show the solutionSolution

Warm water is near the enzyme optimum temperature.
Boiling denatures the enzyme and destroys its active site.
Protein digestion is hydrolysis, so a hydrolase (protease).

Answer: Optimum temperature then denaturation; hydrolase.

Memory hook: Warm helps; boiling kills.

Tricks to remember Enzyme Action, Inhibition and Classes

Short memory aids. Say each one aloud once, then use it on a question the same day.

Oh Tom, He Loves Idle Lakes
Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.
Use it when: Six classes
CI is a copy
Competitive inhibitor copies the substrate.
Use it when: Inhibition
Apo + Co = Holo
Protein + co-factor = holoenzyme.
Use it when: Conjugated enzymes

Enzyme Action, Inhibition and Classes: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Active site · 2 marks · Very short answerBoard level

What is the active site of an enzyme? How does it relate to the tertiary structure?

Try it first, then show the solutionSolution

A crevice or pocket on the enzyme where the substrate binds (1 mark).
It is formed by the folding of the polypeptide in tertiary structure (1 mark).

Answer: Substrate-binding pocket formed by folding.

Example 2: Temperature effect · 3 marks · Short answerBoard level

Explain the effect of temperature on enzyme activity.

Try it first, then show the solutionSolution

Activity rises as temperature rises, up to the optimum (1 mark).
At high temperature the enzyme denatures and activity falls (1 mark).
At low temperature the enzyme is temporarily inactive and recovers on warming (1 mark).

Answer: Optimum, then denaturation.

Example 3: Competitive inhibition · 3 marks · Short answerHarder board level

A reaction slows when a molecule that looks like the substrate is added, yet recovers at a very high substrate level. Identify the inhibition, illustrate with malonate, and explain the recovery.

Try it first, then show the solutionSolution

An inhibitor closely resembling the substrate competes for the active site (1 mark).
Malonate competes with succinate for succinic dehydrogenase (1 mark).
More substrate outcompetes the inhibitor and restores velocity (1 mark).

Answer: Malonate versus succinate; excess substrate overcomes.

Example 4: Classify the reaction · 4 marks · Short answerHarder board level

Name the class of enzyme for: (a) glucose-6-phosphate to fructose-6-phosphate (b) lactate + NAD to pyruvate + NADH (c) peptide hydrolysis (d) joining two molecules using ATP.

Try it first, then show the solutionSolution

(a) Isomerase (1 mark).
(b) Oxidoreductase (dehydrogenase) (1 mark).
(c) Hydrolase (1 mark).
(d) Ligase (1 mark).

Answer: Isomerase, oxidoreductase, hydrolase, ligase.

Example 5: Cofactor removal · 5 marks · Long answerHOTS / competency

A purified enzyme loses activity after dialysis but regains it when a small molecule is added back. Identify the parts, give two possible co-factor types, and say how you would tell a coenzyme from a prosthetic group.

Try it first, then show the solutionSolution

Protein part is the apoenzyme; it is inactive alone (1 mark).
Adding the co-factor restores the holoenzyme (1 mark).
Co-factor could be a coenzyme (such as NAD) or a metal ion (such as zinc) (1 mark).
Prosthetic groups are tightly bound and do not dialyse away; coenzymes are loosely bound and can be removed (1 mark).
So loss on dialysis suggests a loosely bound coenzyme or metal ion, not a prosthetic group (1 mark).

Answer: Apoenzyme + removable co-factor; dialysis points to coenzyme or metal ion.

Where students lose marks in Enzyme Action, Inhibition and Classes

The slips examiners see most often, and what to write instead.

Mistake: Enzyme used up
Catalyst not understood.
Wrong working
Enzyme is consumed in the reaction.
Right working
Enzyme is released unchanged.
Do this: Catalyst.
Usually costs: 1 mark.
Mistake: Low temperature denatures
Cold and heat mixed.
Wrong working
Low temperature denatures enzymes.
Right working
Low temperature only slows; high temperature denatures.
Do this: Cold: pause; heat: damage.
Usually costs: 1 mark.
Mistake: Activation energy raised
Direction reversed.
Wrong working
Enzymes raise activation energy.
Right working
Enzymes lower it.
Do this: Lower.
Usually costs: 1 mark.

Exam tips

Always mention activation energy.
Describe the graph shape for temperature and pH.
Name the class and the reaction type.
Distinguish prosthetic group, coenzyme and metal ion with an example each.

Practice questions on Enzyme Action, Inhibition and Classes

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
An enzyme that links two molecules using ATP belongs to
(a) lyases (b) ligases (c) hydrolases (d) isomerases
Hint
Joining enzymes.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Competitive inhibition can be reversed by adding more substrate.
Reason (R): The inhibitor and substrate compete for the same active site.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Competition for a site.
Answer
(a).
Q3 · 2 marks · Very short answerHarder board level
What is Km? How does a low Km value indicate the enzyme's affinity?
Hint
Half Vmax.
Answer
Km is the substrate concentration at which velocity is half of Vmax. A low Km means the enzyme reaches half its maximum at a small substrate concentration, indicating high affinity.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a graph shows enzyme activity rising with temperature to 37 degrees Celsius, then collapsing at 60 degrees Celsius; in another test, adding a molecule similar to the substrate lowers activity but raising the substrate restores it.
(a) Name the optimum temperature. (1)
(b) Explain the collapse at 60 degrees Celsius. (1)
(c) Name the type of inhibition and the mechanism. (2)
Hint
Denaturation; competition.
Answer
(a) 37 degrees Celsius. (b) Denaturation destroys the active site. (c) Competitive inhibition: the similar molecule competes for the active site, so more substrate overcomes it.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a cell has plenty of enzyme but a vitamin deficiency stops activity. Link the vitamin to enzyme structure and predict the effect of supplying the vitamin.
Hint
Coenzyme.
Answer
Many vitamins form coenzymes (for example niacin in NAD). Without the coenzyme the apoenzyme stays inactive, so reactions stall. Supplying the vitamin restores the coenzyme, forming holoenzyme and recovering activity.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is an apoenzyme?Show answer
The protein part of a conjugated enzyme.
2.Example of competitive inhibitor?Show answer
Malonate.
3.Which class hydrolyses?Show answer
Hydrolases.
4.Haem is a?Show answer
Prosthetic group.
5.Zinc is a co-factor for?Show answer
Carboxypeptidase.
6.Which class uses ATP to join?Show answer
Ligases.

Enzyme Action, Inhibition and Classes: frequently asked questions

How do enzymes work?

They lower activation energy through an enzyme-substrate complex.

What is Vmax?

Maximum velocity at saturating substrate.

What is a co-factor?

A non-protein part needed for activity.

What is a ribozyme?

An RNA with enzyme activity.

Why does pH matter?

Charges on the enzyme change with pH, altering shape.