🔬 What are Enzymes?

Enzymes are biological catalysts that speed up biochemical reactions occurring inside living cells.

  • Nature: Almost all enzymes are proteins (exception: Ribozymes, which are RNA enzymes).
  • Catalyst meaning: A catalyst increases the rate of reaction without being consumed or permanently changed.
  • Importance: Without enzymes, most cellular reactions would occur too slowly to sustain life.

⭐ Key Point to Remember: Enzymes make life possible by speeding up metabolism.

🧩 Enzyme Structure – The Active Site

  • Enzymes possess a specific three-dimensional (tertiary) structure.
  • A small region on the enzyme surface called the active site binds the substrate (S).
  • Temporary binding forms an enzyme–substrate (E–S) complex.
  • Substrate is converted into product (P) and released, while the enzyme remains unchanged.

Reaction sequence: [ E + S ⇌ ES → EP → E + P ]

🧠 Memory Aid: Enzyme is reusable like a machine.

🔐 Models of Enzyme Action

1️⃣ Lock and Key Model (Emil Fischer, 1894)

  • Active site is rigid.
  • Substrate fits exactly like a key into a lock.
  • Explains specificity, but not flexibility.

2️⃣ Induced Fit Model (Daniel Koshland, 1958)

  • Active site is flexible.
  • Substrate binding induces shape change in enzyme.
  • Better explains real enzyme behavior.

⭐ Exam Favourite: Induced Fit Model is the accepted model.

⚡ How Enzymes Speed Up Reactions

Activation Energy (Ea)

  • Minimum energy required to start a reaction.
  • Enzymes lower activation energy by stabilizing the transition state.

Important Facts:

  • Enzymes do not change:
  • Initial energy of substrate
  • Final energy of product
  • Free energy change (ΔG)
  • Enzymes only increase rate, not equilibrium.

🧠 One-liner: Enzymes provide a shortcut for reactions.

🌡️ Factors Affecting Enzyme Activity

1️⃣ Temperature

  • Each enzyme has an optimum temperature.
  • High temperature → Denaturation (loss of 3D structure).

2️⃣ pH

  • Each enzyme has an optimum pH.
  • Examples:
  • Pepsin: pH ~2 (stomach)
  • Trypsin: pH ~8 (intestine)

3️⃣ Substrate Concentration

  • Rate increases with [S] until Vmax is reached.
  • At Vmax, enzyme is saturated.

4️⃣ Enzyme Inhibition

  • Competitive inhibition:

  • Inhibitor resembles substrate

  • Binds to active site

  • Overcome by increasing substrate

  • Non-competitive inhibition:

  • Binds to allosteric site

  • Changes enzyme shape

  • Cannot be overcome by substrate

📌 NCERT & NEET Highlights

  • Enzymes are globular proteins.
  • Ribozymes = RNA enzymes.
  • Denaturation is irreversible.
  • VmaxV_{max} depends on enzyme concentration, not substrate.

🧠 Memory Capsules – Enzymes

  • Enzyme = Protein catalyst (except ribozyme)
  • Active site → substrate binding region
  • Induced Fit Model → accepted model
  • Activation energy ↓ → reaction rate ↑
  • Optimum temperature & pH for each enzyme
  • Competitive inhibitor → active site
  • Non-competitive inhibitor → allosteric site
  • Vmax → enzyme saturation
  • Denaturation = loss of function

🔁 Remember: Structure decides function!

💡 Questions and Answers

Q1. What are enzymes?

A1.

Enzymes are protein molecules that act as biological catalysts, speeding up chemical reactions in living cells without being used up.

Key Points:

  • Protein nature
  • Increase reaction rate
  • Reusable

Q2. What is activation energy?

A2.

Activation energy is the minimum energy required to start a chemical reaction. Enzymes lower this energy so reactions occur faster.

Key Points:

  • Energy barrier
  • Enzymes lower Ea

Q3. Why do enzymes get denatured at high temperature?

A3. High temperature breaks weak bonds holding the enzyme’s 3D structure, causing loss of the active site and enzyme activity.

Key Points:

  • Protein nature
  • Structure destroyed

Q4. Differentiate competitive and non-competitive inhibition.

A4. Competitive inhibitors compete with substrate for the active site, while non-competitive inhibitors bind elsewhere and change enzyme shape.

Key Points:

  • Active site vs allosteric site
  • Substrate concentration effect

Q5. What is Vmax?

A5.

Vmax is the maximum rate of an enzyme reaction when all active sites are occupied by substrate.

Key Points:

  • Enzyme saturation
  • Depends on enzyme amount