Mechanism of Action

Mechanism of Action of Enzymes

Enzymes accelerate reactions by providing an alternative reaction pathway with a lower activation energy (Ea). The enzyme is not consumed — it is regenerated at the end of each catalytic cycle.

Active Site

A specific 3D cleft or pocket on the enzyme surface where substrate binds. Comprises only a small portion of total amino acid residues. Two regions: Binding site (recognizes substrate) and Catalytic site (performs chemistry).

Models of Enzyme-Substrate Interaction

  • Lock and Key Model (Emil Fischer, 1894): Active site has a rigid, pre-formed complementary shape to the substrate. Explains high specificity but not all enzyme behavior.
  • Induced Fit Model (Koshland, 1958): Active site is flexible; undergoes conformational change upon substrate binding to achieve proper alignment of catalytic groups. Better explains enzyme behavior (e.g., Hexokinase closes around glucose, excluding water).

Chemical Mechanisms of Catalysis

  • Acid-Base Catalysis: Active site amino acids (His, Asp, Glu) donate or accept protons to facilitate reaction (most common mechanism).
  • Covalent Catalysis: Transient covalent bond forms between enzyme and substrate. Example: Serine proteases (trypsin, chymotrypsin) form acyl-enzyme intermediate. Nucleophilic serine at active site.
  • Metal Ion Catalysis: Metal ions stabilize negative charges on transition state, mediate redox reactions, or orient substrate. Examples: Zn²⁺ in Carbonic anhydrase, Mg²⁺ in kinases.
  • Proximity and Orientation Effects: Enzyme brings reactants into close proximity and correct orientation — major contributor to rate enhancement.
  • Transition State Stabilization: Enzyme binds the transition state more tightly than substrate or product → lowers Ea.

Serine Proteases — Classic Example

Active site has a catalytic triad: Ser, His, Asp. Mechanism: Ser-OH acts as nucleophile → forms acyl-enzyme intermediate → water hydrolyzes the bond → enzyme regenerated. Examples: Trypsin (cleaves after Arg/Lys), Chymotrypsin (after Phe/Trp/Tyr), Elastase (after small residues).

Enzyme Action — Energetics

  • Enzymes do NOT change ΔG of reaction (equilibrium position)
  • They only speed up the rate to reach equilibrium
  • They lower Ea for both forward AND reverse reactions
  • Transition state analogs are powerful enzyme inhibitors (used as drugs)

Regulatory Sites

Allosteric enzymes have a regulatory site distinct from the active site. Binding of allosteric effectors causes conformational change that alters catalytic activity. Examples: Phosphofructokinase-1 (key regulator of glycolysis), ATCase (pyrimidine synthesis).

Quiz - Exam Preparation Strategy

When studying Quiz for your final board exams, it is critical to focus on the core concepts and fundamental formulas. Relying strictly on NCERT textbook solutions and practicing previous year questions (PYQs) is the proven methodology for scoring high marks. Avoid rote memorization and instead focus on the logical application of the theories presented in this chapter.

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

How can I quickly memorize the concepts of Quiz?

The most effective way is to create short, handwritten revision notes and continuously test your knowledge using our interactive Mock Tests. Spaced repetition and active recall are much better than passive reading.

What type of questions are most commonly asked from Quiz?

Board exams tend to favor conceptual application questions and direct formula-based derivations from the NCERT syllabus. Ensure you have solved every single exercise in the official textbook.

Is reading the NCERT book enough for this chapter?

Yes, the NCERT textbook is the absolute gold standard for board exams. However, to improve your speed and accuracy during the actual exam, you must supplement your reading by solving timed mock tests and objective questions.