Class 11 Biology — New Syllabus 2025-26

ENZYME
Q1. Describe the structure of enzymes, explaining the role and component parts of the active site.

Definition: Enzymes are specific globular proteins that speed up specific chemical reactions by lowering the required activation energy, but remain unaltered in the process — also known as biocatalysts. Rates of enzyme-catalysed reactions may be 10⁸ to 10¹⁰ times greater than corresponding uncatalyzed reactions.

Structure: Enzymes are three-dimensional globular proteins, made of polypeptide chains coiled upon themselves.

Active site: A small cleft or depression on the surface of the globular enzyme molecule, consisting of only a few amino acids — the location where catalysis occurs.

Shape: The shape of each enzyme's active site is very specific, so only a certain substrate molecule can fit into it. It is three-dimensional and bears a specific charge.

Regions: Active site has two distinct regions:

(i) Binding site — substrate molecule fits in by weak chemical forces such as hydrogen bonds.
(ii) Catalytic site — catalyses the reaction and the substrate is transformed into products.

COFACTORS AND COENZYMES
Q2. Differentiate among the three types of co-factors by giving examples.

Cofactors: Many enzymes use additional non-protein chemical components to aid in catalysis, called cofactors. There are three kinds:

(i) Metal ions: Many enzymes use metal ions such as Ca²⁺, Mg²⁺, Mn²⁺, Cu²⁺, and Zn²⁺ as cofactors. These change non-functional active sites into functional sites, and change the shape of the enzyme to allow substrate binding. Trace elements like molybdenum and manganese are also used as cofactors.

(ii) Prosthetic Groups: Some cofactors form covalent bonds with the enzyme — e.g., hematin (heme), part of haemoglobin/myoglobin, acts as a prosthetic group in enzymes like catalase and peroxidases.

(iii) Coenzyme: When the cofactor is a non-protein organic molecule loosely attached to the enzyme, it is called a coenzyme. Coenzymes transport electrons (hydrogen atoms) from one enzyme to another. Many vitamins (niacin/B3, riboflavin/B2) function as coenzymes. The most important coenzyme in the cell is the hydrogen acceptor NAD⁺ — when it acquires a hydrogen atom it reduces to NADH, carrying energy from food oxidation.

Q3. Define activation energy and explain through a graph how an enzyme speeds up a reaction by lowering activation energy.

The speed of a chemical reaction depends on the amount of activation energy required to initiate it. Activation energy is the energy which works to destabilize existing chemical bonds.

Effect of enzymes: Enzymes bring reactants together in the correct orientation and stress particular chemical bonds of reactants, lowering activation energy and speeding up the reaction rate.

Effect on end product: The presence of enzymes does not affect the nature or properties of the end products — e.g., sucrose will always hydrolyse into glucose and fructose whether sucrase is present or not.

Enzyme-Substrate Complex: Due to its specificity, an enzyme recognizes a specific substrate. The substrate binds to the active site, forming an enzyme-substrate (ES) complex, and the catalytic site is activated. E + S → ES → E + P.

Q4. Explain the mechanism of enzyme action through the induced fit model. Compare it with the lock and key model.

1. Lock and Key Model

History: In 1894, German chemist Emil Fischer proposed the lock-and-key model. "As a specific key can open only a specific lock, in the same manner a specific enzyme can transform only one specific substrate into products."

Postulates: Active site is a rigid structure — no modification or flexibility before, during, or after enzyme action.

2. Induced Fit Model (Modified form of Lock and Key Model)

History: Later studies did not support the lock-and-key model in all reactions. American biochemist Daniel Koshland presented the induced fit model in 1958. "When a substrate combines with the binding site of an enzyme, it induces changes in enzyme structure. These changes enable the enzyme to perform its catalytic activity more effectively."

Postulates: Active site is NOT a rigid structure — capable of modification and flexibility before enzyme action starts.

Comparison

Active site: rigid (lock-and-key) vs flexible (induced fit).
Enzyme regulation: non-allosteric enzymes (lock-and-key) vs allosteric enzymes (induced fit).
Reactions: simple one-step reactions (lock-and-key) vs complex reactions allowing feedback regulation (induced fit).

FACTORS AFFECTING THE RATE OF ENZYME ACTION
Q5. Describe the effect of temperature on the rate of enzyme action. Compare enzymes of humans and thermophilic bacteria.

The shape of a protein is determined by hydrogen bonds and hydrophobic interactions holding its polypeptide chains in position — both easily disrupted by slight temperature changes.

Optimum Temperature: Every enzyme works at its maximum rate at a specific optimum temperature — 37°C for human enzymes.

At Low Temperature: bonds determining enzyme shape become less flexible — do not permit the induced change necessary for enzyme action, so reaction rate is slow.

At High Temperature (up to a limit): heat adds to kinetic energy, molecules move/collide more frequently — reaction rate increases.

Denaturation: Well above optimum temperature, heat energy increases vibrations too violently — bonds cannot hold polypeptide chains in position, globular structure is lost — rapid decrease in enzyme action, may be blocked completely.

★ Thermophilic bacteria: live in hot springs; have proteins with stronger bonding between polypeptide chains and can function at 70°C or higher — more thermostable than human enzymes.

Q6. How does pH affect the rate of enzyme action? Write the optimum pH of various human enzymes.

All enzymes work at maximum rate within a narrow pH range — the optimum pH. Pepsin is active in acidic medium; trypsin in alkaline medium. Papain (from green papaya) works in both acidic and alkaline media.

Effect of changed pH: Polypeptide chains are held by bonds between oppositely charged amino acids (e.g., glutamic acid − and lysine +), sensitive to H⁺ concentration. Changes in pH change ionization of amino acids at the active site (and of the substrate). A slight change retards or blocks enzyme activity; extreme change can break bonds, causing denaturation.

Q7. Describe how the concentration of enzyme affects the rate of enzyme action.

Enzymes are very efficient in low concentrations — a small number of molecules can catalyse reactions of a large amount of substrate. The overall rate depends directly on the amount of enzyme present (if substrate is unlimited).

Effect of increase: more enzyme molecules = more active sites = more substrate transformed into products. But once substrate concentration is fixed, further enzyme increase doesn't help — rate plateaus.

Q8. Explain the effect of substrate concentration on the rate of enzyme action.

Low substrate concentration: free enzyme molecules bind new substrate — rate increases.
High substrate concentration: all active sites occupied — additional substrate finds no free sites, so rate no longer increases. This point is called saturation.

ENZYME INHIBITION
Q9. Describe enzymatic inhibition, its types and its significance.

Inhibitor: A chemical that interferes with and blocks an enzyme's activity.
Inhibition: Inhibitors attach with enzymes but are not transformed into products, blocking active sites temporarily or permanently. The final products of complex enzymatic reactions also act as inhibitors of the first enzyme (allosteric inhibition).

1. Competitive Inhibitors

Resembles the substrate structurally, competes for the same binding site. Example: succinic dehydrogenase catalyses oxidation of succinic acid to fumaric acid; malonic acid resembles succinic acid and blocks the active site.

2. Non-competitive Inhibitors

No structural similarity to substrate — binds elsewhere, altering the enzyme's shape so the active site no longer fits the substrate. Example: succinyl-CoA (product) inhibits succinyl-CoA synthetase.

3. Reversible and Irreversible Inhibitors

Irreversible: covalent bonds, cannot be released by dilution/dialysis/increasing substrate — e.g., penicillin permanently disables bacterial cell wall enzymes.
Reversible: weak (hydrogen) bonds, released by increasing substrate concentration — e.g., malonate reversibly inhibits succinate dehydrogenase.

4. Significance of Enzyme Inhibition

(i) Regulates metabolic pathways. (ii) Many drugs work as inhibitors (antibiotics, cancer drugs). (iii) Used to manage medical conditions (anticoagulants). (iv) Understanding toxin/poison inhibition is critical in treatment. (v) Valuable tool in pharmaceutical research and studying enzyme kinetics.

Q10. Explain feedback inhibition of enzymes.

Definition: Feedback inhibition is the phenomenon where the product of a process controls the process itself, often limiting production of more products.

In metabolic pathways, the product of one reaction becomes the substrate for the next. At the pathway's end, a desired product is synthesized. To regulate its concentration, the pathway must shut down — the final product acts as an inhibitor, reacting with an initial enzyme and changing its conformation so it can no longer bind its substrate.

Example: When a cell has more ATP than required, ATP itself acts as a non-competitive inhibitor, blocking the enzyme that catalyses ATP synthesis.

CLASSIFICATION OF ENZYMES
Q11. Classify enzymes on the basis of reactions catalyzed.

According to the general type of reaction, enzymes are classified into six classes:

1. Oxidoreductases — catalyse oxidation/reduction, add/remove H⁺ ions or electrons. E.g., cytochrome oxidase.
2. Transferases — catalyse transfer of a functional group (methyl, acyl, amino, phosphate) between substrates. E.g., hexokinase transfers phosphate from ATP to glucose.
3. Hydrolases — catalyse hydrolysis, breaking substrates into monomers by adding water. E.g., lipase, amylase, peptidase.
4. Lyases — catalyse non-hydrolytic addition/removal of groups (CO₂, NH₂) from substrates. E.g., pyruvate decarboxylase removes CO₂ from pyruvic acid.
5. Isomerases — catalyse intra-molecular rearrangement (one isomer → another). E.g., hexose isomerase converts glucose to fructose.
6. Ligases — catalyse joining of two molecules via new C-C, C-N, C-O, or C-S bonds, using ATP energy. E.g., DNA/RNA polymerase joining nucleotides.

Q12. Classify enzymes on the basis of substrates. Give examples of enzyme naming according to substrates.

1. Proteases: catalyse breakdown of proteins. E.g., pepsin/trypsin break large polypeptides into smaller ones; aminopeptidases further break these into dipeptides; erypsin breaks dipeptides into amino acids.

2. Lipases: act upon lipids. E.g., pancreatic lipase hydrolyses lipids into fatty acids and glycerol.

3. Carbohydrases: act upon carbohydrates. Amylase (starch/glycogen→maltose), Cellulase (cellulose→cellobiose/glucose), Maltase (maltose→glucose), Sucrase (sucrose→glucose+fructose), Lactase (lactose→glucose+galactose).

4. Nucleases: act upon nucleic acids. E.g., RNAase, DNAase, ATPase — breakdown of RNA, DNA and ATP respectively.

Q13. Classify enzymes in table form on the basis of reactions they catalyze. Also write their important subclasses.