Enzymes

BiologyBiomolecules (Biology)For NEET aspirants

Enzymes are the catalysts of living cells: almost all of them are proteins, and a few nucleic acids, the ribozymes, also act as enzymes. This page covers the active site, chemical reactions and their rates, activation energy, the enzyme-substrate complex and the catalytic cycle, the factors that affect enzyme activity, competitive inhibition, the six classes of enzymes and co-factors, as in the NCERT Class 11 chapter Biomolecules. NEET asks enzymes as match lists of classes and co-factors and as sequence questions on the catalytic cycle.

On this page1Nature and active site2Reactions and rates3Activation energy4Enzyme action5Factors and inhibition6Classification7Co-factors8Exam essentials9Practice

Key Points at a Glance

★ Must learn Almost all enzymes are proteins; ribozymes are nucleic acids that act as enzymes.
Active site: a crevice or pocket in the folded enzyme into which the substrate fits.
Enzymes are damaged above about 40°C; enzymes of thermophilic organisms stay active up to 80°-90°C.
$rate = \frac{δ P}{δ t}$ ; the rate doubles or halves for every 10°C change in temperature.
★ Must learn Carbonic anhydrase: about 200 molecules of $H_{2} C O_{3}$ per hour without it, about 600,000 per second with it, about 10 million times faster.
Glucose to 2 pyruvic acid takes ten enzyme-catalysed reactions; products: lactic acid (muscle, anaerobic), pyruvic acid (aerobic), ethanol (yeast).
★ Must learn Enzymes lower the activation energy; the ES complex is essential: $E + S ⇌ ES \to EP \to E + P$ .
Each enzyme has an optimum temperature and pH; low temperature inactivates it temporarily, high temperature denatures it.
Velocity rises with substrate concentration up to $V_{max}$ , when all enzyme molecules are saturated.
★ Must learn Competitive inhibitor: resembles the substrate; malonate inhibits succinic dehydrogenase (substrate: succinate).
★ Must learn Six classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.
Co-factors: prosthetic group (haem), co-enzyme (NAD, NADP with niacin), metal ion (zinc in carboxypeptidase).

1. Enzymes: Nature and Active Site

★ Exam imp Almost all enzymes are proteins. Some nucleic acids behave like enzymes; these are called ribozymes.
An enzyme can be depicted by a line diagram.
Like any protein, an enzyme has a primary structure (amino acid sequence) and secondary and tertiary structures.
In the tertiary structure, the protein chain folds upon itself and criss-crosses, making many crevices or pockets.
One such pocket is the active site.
Through their active site, enzymes catalyse reactions at a high rate.

★ Very important Active site: a crevice or pocket of the enzyme into which the substrate fits.

Inorganic catalysts

Work efficiently at high temperatures and high pressures.

Enzymes

Get damaged at high temperatures (say above 40°C).

Enzymes isolated from organisms that live at extremely high temperatures (hot vents and sulphur springs) are stable.
★ Exam imp These enzymes retain their catalytic power even at 80°-90°C.
Thermal stability is thus an important quality of enzymes isolated from thermophilic organisms.

Memory Trick

RIBOzyme = a RIBOnucleic acid that works as an enzyme. It is the main exception to "all enzymes are proteins".

Key idea

Enzymes are folded proteins (or, rarely, ribozymes) whose active-site pocket holds the substrate; most are damaged above about 40°C, unlike inorganic catalysts.

2. Chemical Reactions and Their Rates

2.1 Physical change and chemical reaction

Physical change

A change in shape without breaking of bonds.
A change in state of matter: ice melting into water, water becoming vapour.

Chemical reaction

Bonds are broken and new bonds are formed.
Inorganic and organic examples below.

Ba (OH)_{2} + H_{2} S O_{4} ⟶ BaS O_{4} + 2 H_{2} O

The reaction above is an inorganic chemical reaction.
Hydrolysis of starch into glucose is an organic chemical reaction.

2.2 Rate of a reaction

★ Exam imp Rate of a physical or chemical process: the amount of product formed per unit time.

rate = \frac{δ P}{δ t}

Rate is called velocity if the direction is specified.
Rates are influenced by temperature, among other factors.
★ Exam imp Rule of thumb: the rate doubles or decreases by half for every 10°C change in either direction.
Catalysed reactions proceed at rates vastly higher than uncatalysed ones, as enzyme-catalysed reactions show.

carbon dioxide C O_{2} + water H_{2} O ⇌ Carbonic anhydrase carbonic acid H_{2} C O_{3}

Without any enzyme, this reaction is very slow: about 200 molecules of $H_{2} C O_{3}$ form in an hour.
★ Exam imp With carbonic anhydrase, present in the cytoplasm, about 600,000 molecules form every second.
The enzyme accelerates the reaction rate by about 10 million times.

Tips and Tricks

Check the factor yourself: 600,000 per second is $6 \times 1 0^{5} \times 3600 \approx 2.2 \times 1 0^{9}$ per hour. Dividing by 200 gives about $1 0^{7}$ , that is, about 10 million times faster.

2.3 Metabolic pathways

There are thousands of types of enzymes, each catalysing a unique chemical or metabolic reaction.
★ Exam imp Metabolic pathway: a multistep chemical reaction in which each step is catalysed by the same enzyme complex or by different enzymes.
Example: glucose becomes pyruvic acid through ten different enzyme-catalysed reactions (studied in detail under respiration).

Glucose ⟶ 2 Pyruvic acid

C_{6} H_{12} O_{6} + O_{2} ⟶ 2 C_{3} H_{4} O_{3} + 2 H_{2} O

This same pathway, with one or two additional reactions, gives a variety of metabolic end products.

Condition	End product
Skeletal muscle, anaerobic conditions	Lactic acid
Normal aerobic conditions	Pyruvic acid
Yeast, during fermentation	Ethanol (alcohol)

Memory Trick

"Muscle makes Lactic, Yeast makes ethanol, Air keeps Pyruvic". Same pathway, different conditions, different products.

NEET Focus

Numbers asked directly: 200 molecules per hour (no enzyme), 600,000 per second (carbonic anhydrase), about 10 million times faster; ten reactions from glucose to pyruvic acid; rate doubles or halves for every 10°C.

Key idea

A reaction breaks and makes bonds; its rate is product formed per unit time, and enzymes raise that rate enormously.

3. How Enzymes Bring About High Rates

The chemical that is converted into a product is the substrate.
Enzymes, proteins with a three-dimensional structure including an active site, convert a substrate (S) into a product (P): $S \to P$ .
★ Exam imp The substrate must bind the enzyme at its active site, within a cleft or pocket. It diffuses towards the active site.
So there is an obligatory formation of an 'ES' complex (E stands for enzyme). This complex formation is transient.
While the substrate is bound, a new structure of the substrate, the transition state structure, is formed.
Soon after the expected bond breaking or making is complete, the product is released from the active site.
The pathway from substrate to product must pass through the transition state. There may be many more 'altered structural states' in between.
All these intermediate structural states are unstable. Stability is related to the energy status of the molecule (Figure 1).

Test yourself: hide labels

Figure 1: Activation energy is the energy gap between the substrate and the transition state. An enzyme lowers this barrier, so the substrate changes into product much faster.

The y-axis shows the potential energy content; the x-axis shows the progress of the structural change through the transition state.
If P is at a lower energy level than S, the reaction is exothermic: no energy (heating) needs to be supplied to form the product.
★ Exam imp Whether the reaction is exothermic (spontaneous) or endothermic (energy requiring), S must pass through a much higher energy state, the transition state.

★ Very important Activation energy: the difference between the average energy content of S and that of the transition state. Enzymes bring down this energy barrier, so S changes into P more easily.

Tips and Tricks

Read the graph carefully: both curves start at the same S and end at the same P; only the peak is lower with the enzyme. So an enzyme lowers the activation energy, not the energy of the substrate or the product.

Key idea

Every reaction must climb to a transition state; enzymes speed it up by lowering this climb, the activation energy.

4. Nature of Enzyme Action

Each enzyme (E) has a substrate (S) binding site, so a highly reactive enzyme-substrate complex (ES) is produced.
The ES complex is short-lived. It dissociates into the product(s) P and the unchanged enzyme.
An enzyme-product complex (EP) forms as an intermediate.
★ Exam imp The formation of the ES complex is essential for catalysis.

E + S ⇌ ES ⟶ EP ⟶ E + P

The catalytic cycle of an enzyme runs in four steps:

The substrate binds to the active site of the enzyme, fitting into it.
Binding induces the enzyme to alter its shape, so it fits more tightly around the substrate.
The active site, now in close proximity to the substrate, breaks the chemical bonds of the substrate, and the new enzyme-product complex forms.
The enzyme releases the products. The free enzyme is ready to bind another substrate molecule and run the cycle again.

Memory Trick

Catalytic cycle: Bind, Bend, Break, Release. The substrate binds, the enzyme bends around it, the bonds break, and the products are released.

Quick Recall: tap to check

What is the transient complex formed when the substrate binds the active site?

The enzyme-substrate (ES) complex.

What happens to the enzyme's shape when the substrate binds?

It changes, fitting more tightly around the substrate.

Which complex forms just before the products are released?

The enzyme-product (EP) complex.

Is the enzyme used up in the reaction?

No. It is released unchanged and runs the cycle again.

Key idea

Enzyme action is a cycle: E + S forms ES, the bound substrate becomes EP, and E is set free for the next substrate.

5. Factors Affecting Enzyme Activity

Enzyme activity changes when conditions alter the tertiary structure of the protein.
★ Exam imp The factors are temperature, pH, change in substrate concentration, and binding of specific chemicals that regulate activity.

5.1 Temperature and pH

Enzymes generally work within a narrow range of temperature and pH (Figure 2).
★ Exam imp Each enzyme shows its highest activity at a particular temperature and pH: the optimum temperature and optimum pH.
Activity declines both below and above the optimum value.
Low temperature preserves the enzyme in a temporarily inactive state.
High temperature destroys enzyme activity, because proteins are denatured by heat.

5.2 Concentration of substrate

As substrate concentration rises, the velocity of the enzymatic reaction rises at first.
★ Exam imp The reaction finally reaches a maximum velocity ( $V_{max}$ ), which is not exceeded by any further rise in substrate concentration.
Reason: enzyme molecules are fewer than substrate molecules. Once they are saturated, no free enzyme molecules remain to bind the extra substrate.

Test yourself: hide labels

Figure 2: Effect of (a) pH, (b) temperature and (c) substrate concentration on enzyme activity. Activity peaks at the optimum pH and temperature (dashed lines); velocity levels off at

V_{max}

once the enzyme is saturated.

Extra Depth: $K_{m}$ , marked on graph (c), is the substrate concentration at which the velocity reaches half of $V_{max}$ . It is called the Michaelis constant.

5.3 Inhibitors

Enzyme activity is also sensitive to specific chemicals that bind to the enzyme.
★ Exam imp When the binding of a chemical shuts off enzyme activity, the process is inhibition and the chemical is an inhibitor.
Competitive inhibitor: an inhibitor that closely resembles the substrate in molecular structure and inhibits the enzyme.
Because of this similarity, it competes with the substrate for the substrate-binding site. The substrate cannot bind, and enzyme action declines.
★ Exam imp Example: malonate inhibits succinic dehydrogenase, because malonate closely resembles the substrate succinate.
Competitive inhibitors are often used in the control of bacterial pathogens.

Memory Trick

Malonate Mimics succinate, so it Blocks the active site of Succinic dehydrogenase. The look-alike competes.

NEET Focus

Statement traps: low temperature only inactivates an enzyme temporarily; high temperature denatures it. Velocity stops rising at $V_{max}$ because the enzyme is saturated, not because the substrate runs out. The inhibitor in the classic example is malonate; succinate is the substrate.

Key idea

Temperature and pH change the folded shape, substrate supply sets the velocity up to saturation, and look-alike inhibitors block the active site.

6. Classification and Nomenclature of Enzymes

Thousands of enzymes have been discovered, isolated and studied.
Most are classified into groups based on the type of reaction they catalyse.
★ Exam imp Enzymes are divided into 6 classes, each with 4-13 subclasses, and are named by a four-digit number.

Class	Reaction catalysed
1. Oxidoreductases / dehydrogenases	Oxidoreduction between two substrates S and S′
2. Transferases	Transfer of a group G (other than hydrogen) between a pair of substrates S and S′
3. Hydrolases	Hydrolysis of ester, ether, peptide, glycosidic, C-C, C-halide or P-N bonds
4. Lyases	Removal of groups from substrates by mechanisms other than hydrolysis, leaving double bonds
5. Isomerases	Inter-conversion of optical, geometric or positional isomers
6. Ligases	Linking together of 2 compounds, e.g., joining of C-O, C-S, C-N, P-O bonds

Oxidoreductases:

S_{reduced} + S_{oxidised}^{'} ⟶ S_{oxidised} + S_{reduced}^{'}

Transferases:

S - G + S^{'} ⟶ S + S^{'} - G

Lyases:

C ∣ X - C ∣ Y ⟶ X - Y + C = C

Memory Trick

Order of the six classes: "Oh The Hungry Lion Is Lazy" = Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.

Tips and Tricks

Lyases split off groups and leave a double bond; Ligases link two compounds. Hydrolases use water to break bonds; lyases remove groups by other mechanisms. Transferases move any group except hydrogen.

Key idea

Enzymes are classed by the reaction they catalyse: six classes, each split into subclasses and named by a four-digit number.

7. Co-factors

Enzymes are composed of one or several polypeptide chains.
In many cases, non-protein constituents called co-factors are bound to the enzyme to make it catalytically active.
★ Exam imp In such enzymes, the protein portion is called the apoenzyme.
Three kinds of co-factors are identified: prosthetic groups, co-enzymes and metal ions.

Co-factor	Nature and binding	Example
Prosthetic group	Organic; tightly bound to the apoenzyme	Haem in peroxidase and catalase, which break down hydrogen peroxide to water and oxygen; haem is part of the active site
Co-enzyme	Organic; association with the apoenzyme is only transient, usually during catalysis; serves many different reactions	NAD and NADP contain the vitamin niacin
Metal ion	Forms coordination bonds with side chains at the active site and with the substrate	Zinc in the proteolytic enzyme carboxypeptidase

The essential chemical components of many co-enzymes are vitamins.
★ Exam imp Catalytic activity is lost when the co-factor is removed, which shows that co-factors play a crucial role.

★ Very important Prosthetic groups are tightly bound; co-enzymes are only transiently associated. Both are organic; the third kind of co-factor is a metal ion.

Memory Trick

Prosthetic = Permanent (tight); Co-enzyme = Comes and goes (transient). "Haem helps peroxide go", "Niacin is in NAD", "Zinc for carboxypeptidase".

Key idea

Apoenzyme + co-factor = active enzyme; the co-factor may be a tightly bound prosthetic group, a transient co-enzyme or a metal ion.

8. Exam Essentials

Pairs to Match

List I	List II
Ribozyme	Nucleic acid that acts as an enzyme
Active site	Crevice or pocket into which the substrate fits
Carbonic anhydrase	Speeds up $C O_{2} + H_{2} O ⇌ H_{2} C O_{3}$ about 10 million times
Skeletal muscle, anaerobic	Lactic acid
Yeast fermentation	Ethanol
Malonate	Competitive inhibitor of succinic dehydrogenase
Oxidoreductases	Oxidoreduction between S and S′
Transferases	Transfer of a group other than hydrogen
Hydrolases	Hydrolysis of ester, ether, peptide and glycosidic bonds
Lyases	Removal of groups, leaving double bonds
Isomerases	Inter-conversion of optical, geometric or positional isomers
Ligases	Joining of C-O, C-S, C-N and P-O bonds
Haem	Prosthetic group of peroxidase and catalase
NAD and NADP	Co-enzymes containing niacin
Zinc	Co-factor of carboxypeptidase

Exceptions

Almost all enzymes are proteins, except ribozymes, which are nucleic acids.
Enzymes are damaged above about 40°C, except those of thermophilic organisms (stable up to 80°-90°C).
Inorganic catalysts, unlike enzymes, work best at high temperature and pressure.
Low temperature does not denature an enzyme; it only keeps it temporarily inactive.
$V_{max}$ is not exceeded, however much more substrate is added.
Transferases move a group other than hydrogen.
Lyases remove groups by mechanisms other than hydrolysis.
Prosthetic groups are tightly bound, unlike co-enzymes.

Numbers to Remember

Enzymes damaged above about 40°C; thermophile enzymes active up to 80°-90°C.
Rate doubles or halves for every 10°C change.
Carbonic anhydrase: 200 per hour without enzyme; 600,000 per second with it; about 10 million times faster.
Glucose to pyruvic acid: 10 enzyme-catalysed reactions; 2 pyruvic acid per glucose.
Classes: 6; subclasses: 4-13 per class; name: four-digit number.
Catalytic cycle: 4 steps. Kinds of co-factors: 3.

Examples to Remember

Group	Examples
Enzymes named	Carbonic anhydrase, succinic dehydrogenase, peroxidase, catalase, carboxypeptidase
Habitats of thermophiles	Hot vents, sulphur springs
End products of the glucose pathway	Lactic acid, pyruvic acid, ethanol
Co-factors	Haem (prosthetic group), NAD and NADP with niacin (co-enzymes), zinc (metal ion)
Bonds broken by hydrolases	Ester, ether, peptide, glycosidic, C-C, C-halide, P-N
Bonds joined by ligases	C-O, C-S, C-N, P-O

Quick Recall: tap to check

In the activation energy graph, what is the highest point of each curve called?

The transition state.

In the same graph, which energy gap does the enzyme reduce?

The activation energy: the gap between the substrate and the transition state.

In graph (b), why does activity fall steeply above the optimum temperature?

The enzyme protein is denatured by heat.

In graph (c), what is the velocity called when it stops rising?

Maximum velocity,

V_{max}

9. Quick Revision

Almost all enzymes are proteins; ribozymes are nucleic acids with catalytic power.
The active site is a crevice or pocket of the folded enzyme into which the substrate fits.
Enzymes are damaged above about 40°C; thermophile enzymes work up to 80°-90°C.
Chemical reactions break and form bonds; rate = $δ P / δ t$ ; rate doubles or halves per 10°C.
Carbonic anhydrase speeds up carbonic acid formation about 10 million times.
A metabolic pathway is a multistep reaction; glucose to pyruvic acid takes ten steps.
Lactic acid in anaerobic muscle, pyruvic acid under aerobic conditions, ethanol in yeast.
Substrate binds the active site; the transient ES complex is obligatory; a transition state forms.
Activation energy is the gap from S to the transition state; enzymes lower it.
$E + S ⇌ ES \to EP \to E + P$ ; cycle: bind, shape change, bond breaking, release.
Optimum temperature and pH; low temperature inactivates, high temperature denatures.
Velocity rises with substrate concentration up to $V_{max}$ (saturation).
Competitive inhibitor resembles the substrate: malonate inhibits succinic dehydrogenase.
Six classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.
Co-factors: prosthetic group (haem), co-enzyme (NAD, NADP, niacin), metal ion (zinc); apoenzyme = protein part.

10. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Transferases; B. Hydrolases; C. Lyases; D. Ligases
List II: I. Removal of groups leaving double bonds; II. Joining of C-O, C-S, C-N and P-O bonds; III. Transfer of a group other than hydrogen; IV. Hydrolysis of peptide and glycosidic bonds
Choose the correct answer.
(A) A-III, B-IV, C-I, D-II
(B) A-IV, B-III, C-I, D-II
(C) A-III, B-IV, C-II, D-I
(D) A-I, B-IV, C-III, D-II

Solution:

Answer: (A). Transferases move a group other than hydrogen (III); hydrolases hydrolyse bonds (IV); lyases leave double bonds (I); ligases join bonds (II).

Solved Example 2

Read the statements about factors affecting enzyme activity.
A. Each enzyme shows its highest activity at an optimum temperature and pH.
B. Low temperature denatures enzymes permanently.
C. Velocity stops rising at

V_{max}

because all enzyme molecules are saturated.
D. A competitive inhibitor closely resembles the substrate.
E. High temperature denatures enzyme proteins.
Choose the correct answer.
(A) A, C, D and E only
(B) A, B and C only
(C) B, C and E only
(D) A, B, D and E only

Solution:

Answer: (A). B is false: low temperature keeps the enzyme only temporarily inactive. A, C, D and E are true.

Solved Example 3

Arrange the steps of the catalytic cycle of an enzyme in the correct order.
A. The enzyme releases the products
B. The substrate binds to the active site
C. The active site breaks the bonds of the substrate, forming the enzyme-product complex
D. The enzyme alters its shape to fit tightly around the substrate
Choose the correct order.
(A) B, D, C, A
(B) B, C, D, A
(C) D, B, C, A
(D) B, D, A, C

Solution:

Answer: (A). Binding, shape change, bond breaking with EP formation, then release of products.

Solved Example 4

Without an enzyme, about 200 molecules of

H_{2} C O_{3}

form in an hour. With carbonic anhydrase, about 600,000 form every second. By about how many times does the enzyme speed up the reaction?
(A) 3000 times
(B) 100,000 times
(C) 10 million times
(D) 600,000 times

Solution:

Answer: (C). $600, 000 \times 3600 \approx 2.16 \times 1 0^{9}$ per hour; dividing by 200 gives about $1.1 \times 1 0^{7}$ , about 10 million times.

Solved Example 5

Which of the following is NOT a co-factor?
(A) Haem
(B) Zinc
(C) Apoenzyme
(D) NAD

Solution:

Answer: (C). The apoenzyme is the protein portion of the enzyme. Haem (prosthetic group), zinc (metal ion) and NAD (co-enzyme) are co-factors.

Solved Example 6

Statement I: Co-enzymes are tightly bound to the apoenzyme.
Statement II: Haem is the prosthetic group of peroxidase and catalase and is part of their active site.
Choose the correct answer.
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct

Solution:

Answer: (D). Statement I is wrong: co-enzymes are only transiently associated; prosthetic groups are tightly bound. Statement II is correct.

11. Practice Questions

Practice Questions

Match List I with List II.
List I: A. Peroxidase; B. Carboxypeptidase; C. Succinic dehydrogenase; D. Carbonic anhydrase
List II: I. Zinc as co-factor; II. Inhibited by malonate; III. Haem as prosthetic group; IV. Forms carbonic acid from $C O_{2}$ and water
Choose the correct answer.
(1) A-III, B-I, C-II, D-IV
(2) A-I, B-III, C-II, D-IV
(3) A-III, B-II, C-I, D-IV
(4) A-III, B-I, C-IV, D-IIAnswer: (1). Peroxidase: haem; carboxypeptidase: zinc; succinic dehydrogenase: malonate; carbonic anhydrase: $H_{2} C O_{3}$ .
Read the statements about activation energy.
A. It is the energy gap between the substrate and the transition state.
B. Enzymes lower it.
C. Exothermic reactions do not need to pass through a transition state.
D. The transition state is a higher energy state than the substrate.
Choose the correct answer.
(1) A, B and D only
(2) A, B and C only
(3) B, C and D only
(4) A, C and D onlyAnswer: (1). C is false: even exothermic reactions pass through the higher-energy transition state.
Arrange in the order in which they occur during catalysis.
A. EP complex; B. E + P; C. ES complex; D. E + S
(1) D, C, A, B
(2) D, A, C, B
(3) C, D, A, B
(4) D, C, B, AAnswer: (1). $E + S ⇌ ES \to EP \to E + P$ .
Malonate inhibits succinic dehydrogenase because it:
(1) denatures the enzyme
(2) closely resembles the substrate succinate
(3) removes the co-factor
(4) binds a site away from the active siteAnswer: (2). It is a competitive inhibitor that competes for the substrate-binding site.
Which statement about enzymes is NOT correct?
(1) Almost all enzymes are proteins
(2) Ribozymes are nucleic acids with catalytic power
(3) Enzymes work best at high temperatures and pressures, like inorganic catalysts
(4) Enzymes of thermophiles stay active at 80°-90°CAnswer: (3). Inorganic catalysts work best at high temperature and pressure; ordinary enzymes are damaged above about 40°C.
Velocity of an enzymatic reaction stops rising at high substrate concentration because:
(1) the substrate is used up
(2) the enzyme is denatured
(3) all enzyme molecules are saturated
(4) the product inhibits the substrateAnswer: (3). Enzyme molecules are fewer than substrate molecules; once saturated, no free enzyme is left.
Statement I: Lyases catalyse the removal of groups from substrates by hydrolysis.
Statement II: Isomerases catalyse inter-conversion of optical, geometric or positional isomers.
Choose the correct answer.
(1) Both Statement I and Statement II are correct
(2) Both Statement I and Statement II are incorrect
(3) Statement I is correct but Statement II is incorrect
(4) Statement I is incorrect but Statement II is correctAnswer: (4). Lyases remove groups by mechanisms other than hydrolysis, leaving double bonds.

Review Questions

Describe the important properties of enzymes.Answer: Almost all enzymes are proteins (ribozymes are nucleic acids). They act through an active site and show substrate specificity. They work best at an optimum temperature and pH, are denatured at high temperature, and are only temporarily inactive at low temperature. They lower the activation energy and greatly raise the reaction rate, and they are released unchanged. Many need co-factors, and their activity can be blocked by inhibitors.

Common Mistakes to Avoid

Watch out

Writing that all enzymes are proteins. Ribozymes are nucleic acids that act as enzymes.
Saying low temperature denatures enzymes. It only keeps them temporarily inactive; heat denatures them.
Thinking more substrate always raises velocity. Velocity levels off at $V_{max}$ once the enzyme is saturated.
Saying enzymes change the energy of the substrate or product. They lower only the activation energy.
Mixing up lyases (remove groups, leave double bonds) and ligases (link two compounds).
Writing that transferases transfer hydrogen. They transfer a group other than hydrogen.
Calling NAD a prosthetic group. NAD is a co-enzyme; haem is the prosthetic group of peroxidase and catalase.
Swapping the pair: malonate is the inhibitor and succinate the substrate of succinic dehydrogenase.

Frequently Asked Questions

What are ribozymes?

Ribozymes are nucleic acids that behave like enzymes. Almost all enzymes are proteins, so ribozymes are the main exception. They show that catalytic power is not limited to proteins, although protein enzymes catalyse most biochemical reactions in cells. Nucleic acids serve mainly as genetic material, so ribozymes are a special case.

What is the active site of an enzyme?

When an enzyme's protein chain folds into its tertiary structure, it criss-crosses itself and forms crevices or pockets. The active site is one such pocket, into which the substrate fits. Through the active site, the enzyme catalyses the reaction at a high rate.

What is activation energy, and how do enzymes affect it?

Every reaction, exothermic or endothermic, must pass through a high-energy transition state. Activation energy is the difference between the average energy of the substrate and that of the transition state. Enzymes lower this energy barrier, so the substrate changes into product much faster.

Why do enzymes lose activity at high temperature?

Enzymes work best at an optimum temperature, and activity falls above and below it. At high temperature, the protein is denatured by heat and the enzyme is destroyed. Low temperature only keeps the enzyme temporarily inactive. Enzymes of thermophilic organisms stay active up to 80 to 90 degrees Celsius.

Why does reaction velocity stop rising at high substrate concentration?

At first, velocity rises as substrate concentration increases. Enzyme molecules are fewer than substrate molecules, so they soon become saturated. With no free enzyme left to bind extra substrate, the reaction reaches its maximum velocity, Vmax, which further substrate cannot exceed.

What is competitive inhibition? Give an example.

A competitive inhibitor closely resembles the substrate and competes with it for the substrate-binding site. The substrate cannot bind, so enzyme action declines. Malonate inhibits succinic dehydrogenase because it resembles the substrate succinate. Such inhibitors are used to control bacterial pathogens.

What are the six classes of enzymes?

Enzymes are grouped by the reaction they catalyse into six classes: oxidoreductases or dehydrogenases, transferases, hydrolases, lyases, isomerases and ligases. Each class has 4 to 13 subclasses, and each enzyme is named by a four-digit number. Hydrolases, for example, break ester, peptide and glycosidic bonds by hydrolysis, while ligases join two compounds.

How does a prosthetic group differ from a co-enzyme?

Both are organic co-factors. A prosthetic group is tightly bound to the apoenzyme, like haem in peroxidase and catalase. A co-enzyme associates only transiently, usually during catalysis, and serves many reactions; NAD and NADP, which contain niacin, are examples. Removing either kind of co-factor stops the enzyme's catalytic activity.

Previous year questions on Enzymes

11 questions from past papers, each with a step-by-step solution.

Show all 11 questions

NEET 2019, Biology Q27

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