⚡ Enzymes and Metabolism

Energy and Metabolism

Thermodynamics in Biology:

• Free energy (ΔG): energy available to do work
• Exergonic reactions: ΔG < 0 (release energy, spontaneous)
• Endergonic reactions: ΔG > 0 (require energy input)

ATP (Adenosine Triphosphate):

• Universal energy currency
• Stores energy in phosphate bonds
• ATP → ADP + P releases ~30.5 kJ/mol

Enzymes

What are enzymes?

• Biological catalysts (usually proteins)
• Speed up reactions without being consumed
• Lower activation energy (Ea)
• Do NOT change ΔG of reaction

Structure:

• Active site: region where substrate binds
• Substrate: reactant molecule
• Specific shape determines which substrates bind

Mechanism:

1. Induced fit model:

   • Enzyme changes shape when substrate binds
   • Active site molds around substrate
   • Forms enzyme-substrate complex
   • Products released, enzyme returns to original shape

2. Enzyme + Substrate ⇌ ES complex → Enzyme + Product

Factors Affecting Enzyme Activity

1. Temperature

• Optimal temperature maximizes activity
• Too low: slow molecular movement
• Too high: denaturation (lose shape)
• Most human enzymes optimal at 37°C

2. pH

• Each enzyme has optimal pH
• Extreme pH denatures enzyme
• Examples:
  • Pepsin (stomach): pH 2
  • Trypsin (intestine): pH 8

3. Substrate Concentration

• Low [S]: activity increases with more substrate
• High [S]: enzyme saturation (plateau)
• Maximum velocity (Vmax) reached

4. Enzyme Concentration

• More enzyme = more activity
• Linear relationship (if substrate abundant)

Enzyme Regulation

Competitive Inhibition

• Inhibitor competes with substrate for active site
• Similar shape to substrate
• Can be overcome by adding more substrate

Noncompetitive Inhibition

• Inhibitor binds to allosteric site (not active site)
• Changes enzyme shape → active site altered
• Cannot be overcome by adding substrate

Allosteric Regulation

• Regulatory molecule binds to allosteric site
• Can be activator or inhibitor
• Changes enzyme shape and activity

Feedback Inhibition

• End product inhibits earlier enzyme in pathway
• Prevents overproduction
• Example: ATP inhibits glycolysis enzymes

Cofactors and Coenzymes

• Cofactors: inorganic helpers (metal ions like Zn²⁺, Fe²⁺)
• Coenzymes: organic helpers (vitamins like NAD⁺, FAD)
• Required for enzyme function

Key Concepts

1. Enzymes lower activation energy but don't change ΔG
2. Active site binds substrate with high specificity
3. Induced fit: enzyme changes shape upon binding
4. Temperature and pH affect enzyme shape and activity
5. Competitive inhibitors block active site
6. Noncompetitive inhibitors change enzyme shape
7. Feedback inhibition regulates metabolic pathways