Biochemistry
MBBS Biochemistry question bank: biomolecules, enzymes, metabolism, molecular biology, vitamins, nutrition and clinical biochemistry.
12chapters144questions75High-Yield
Definition
The electron transport chain (ETC / respiratory chain) is a series of carriers in the inner mitochondrial membrane that transfer electrons from NADH/FADH₂ to oxygen, releasing energy.
Components (Complexes I–IV)
- Complex I — NADH dehydrogenase
- Complex II — succinate dehydrogenase
- Complex III — cytochrome bc1
- Complex IV — cytochrome oxidase (→ O₂)
- Mobile carriers: coenzyme Q, cytochrome c
Function
- Electrons flow down the redox gradient
- Energy pumps H⁺ → proton gradient
- Final acceptor: oxygen (→ water)
Electrons pass down the complexes to oxygen, pumping protons on the way. Complex Enzyme I NADH dehydrogenase III Cytochrome bc1 IV Cytochrome oxidase Applied
- Cyanide (blocks complex IV)
- Mitochondrial disease
🔑KEY POINTS TO REMEMBER- ETC: complexes I–IV in inner mitochondrial membrane.
- Electrons → O₂ (final acceptor → water).
- Cyanide blocks complex IV.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Oxidative phosphorylation is the synthesis of ATP coupled to electron transport; the chemiosmotic theory explains it via a proton gradient.
Chemiosmotic Theory (Mitchell)
- ETC pumps H⁺ into the intermembrane space
- Creates a proton (electrochemical) gradient
- H⁺ flows back through ATP synthase
- This drives ATP synthesis
ATP Yield
- NADH → ~2.5 ATP
- FADH₂ → ~1.5 ATP
- Total per glucose → ~30–32 ATP
The proton gradient from electron transport powers ATP synthase. Donor ATP NADH ~2.5 FADH₂ ~1.5 Applied
- Uncouplers dissipate the gradient
- Brown-fat thermogenesis
🔑KEY POINTS TO REMEMBER- Oxidative phosphorylation = ATP coupled to electron transport.
- Chemiosmotic: proton gradient → ATP synthase.
- NADH ~2.5, FADH₂ ~1.5 ATP.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
High-energy compounds release a large amount of free energy on hydrolysis; ATP is the universal energy currency.
ATP
- Adenosine triphosphate
- Two high-energy phosphate bonds
- Hydrolysis → ADP + Pi + energy (~7.3 kcal/mol)
Other High-Energy Compounds
- Creatine phosphate (muscle)
- Phosphoenolpyruvate (highest energy)
- 1,3-bisphosphoglycerate
- GTP, acetyl-CoA
ATP shuttles energy between energy-releasing and energy-requiring reactions. Compound Note PEP Highest energy ATP Energy currency Creatine-P Muscle store Applied
- ATP powers all cellular work
- Creatine phosphate buffer
🔑KEY POINTS TO REMEMBER- ATP = energy currency (2 high-energy bonds).
- PEP highest energy; creatine-P = muscle store.
- ATP ↔ ADP + Pi.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Inhibitors block electron transport at specific sites, while uncouplers dissociate electron transport from ATP synthesis.
ETC Inhibitors
- Complex I — rotenone, barbiturates
- Complex III — antimycin A
- Complex IV — cyanide, carbon monoxide, azide
Uncouplers
- Dissipate the proton gradient
- Electron transport continues, no ATP (energy → heat)
- E.g. 2,4-dinitrophenol, thermogenin
Inhibitors stop electron flow; uncouplers waste it as heat. Agent Action Cyanide Blocks complex IV 2,4-DNP Uncoupler Thermogenin Physiological uncoupler Applied
- Cyanide poisoning
- Uncoupling → hyperthermia
🔑KEY POINTS TO REMEMBER- Inhibitors: rotenone (I), antimycin (III), cyanide (IV).
- Uncouplers (2,4-DNP, thermogenin) → heat, no ATP.
- Cyanide poisoning blocks complex IV.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Biological oxidation is the removal of electrons/hydrogen from substrates to release energy; the redox potential measures the tendency to gain/lose electrons.
Biological Oxidation
- Loss of electrons / hydrogen (oxidation)
- Coupled to reduction of coenzymes (NAD, FAD)
- Energy captured as ATP
- Involves oxidases, dehydrogenases, oxygenases
Redox Potential (E₀)
- Tendency to accept electrons
- More negative → tends to lose electrons
- Electrons flow from low to high potential
Oxidation transfers electrons to coenzymes, then down the redox gradient. Enzyme Function Dehydrogenase Removes hydrogen Oxidase Uses O₂ Oxygenase Adds O₂ Applied
- Free-radical generation
- Antioxidant defence
🔑KEY POINTS TO REMEMBER- Biological oxidation = loss of H/electrons → energy.
- Enzymes: dehydrogenases, oxidases, oxygenases.
- Redox potential sets electron-flow direction.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Cytochromes are haem-containing electron carriers in the electron transport chain that transfer electrons via reversible Fe²⁺ / Fe³⁺ changes.
Types
- Cytochromes a, b, c
- Cytochrome oxidase (a + a3) — final, reacts with O₂
- Contain iron in a haem group
Function
- Carry electrons (not protons)
- Fe³⁺ ↔ Fe²⁺ (redox cycling)
Cytochromes shuttle single electrons by cycling their haem iron. Cytochrome Role b, c1 Complex III c Mobile carrier a + a3 Complex IV (O₂) Applied
- Cyanide binds cytochrome a3
- P450 in detoxification
🔑KEY POINTS TO REMEMBER- Cytochromes = haem electron carriers (Fe²⁺/Fe³⁺).
- a + a3 = cytochrome oxidase (reacts with O₂).
- Cyanide binds cytochrome a3.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
The P/O ratio is the number of ATP molecules synthesised per atom of oxygen reduced (per pair of electrons through the ETC).
Values
- NADH → P/O ≈ 2.5
- FADH₂ → P/O ≈ 1.5
- Measures efficiency of oxidative phosphorylation
The P/O ratio quantifies ATP made per oxygen consumed. Donor P/O NADH ~2.5 FADH₂ ~1.5 Applied
- ↓ by uncouplers
- A measure of efficiency
🔑KEY POINTS TO REMEMBER- P/O = ATP per oxygen atom reduced.
- NADH ~2.5, FADH₂ ~1.5.
- Lowered by uncouplers.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Shuttle systems transfer reducing equivalents from cytoplasmic NADH into the mitochondria, since NADH cannot cross the inner membrane.
Two Shuttles
- Malate-aspartate shuttle (liver, heart) — yields NADH (2.5 ATP)
- Glycerophosphate shuttle (muscle, brain) — yields FADH₂ (1.5 ATP)
Shuttles carry reducing power into mitochondria via different carriers. Shuttle Yield Malate-aspartate NADH (2.5) Glycerophosphate FADH₂ (1.5) Applied
- Explains variable ATP yield
- Tissue differences
🔑KEY POINTS TO REMEMBER- Shuttles move cytoplasmic NADH into mitochondria.
- Malate-aspartate → NADH; glycerophosphate → FADH₂.
- Explains ATP-yield differences.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Brown adipose tissue generates heat (non-shivering thermogenesis) using the uncoupling protein thermogenin (UCP-1).
Mechanism
- Rich in mitochondria (brown colour)
- Thermogenin uncouples the ETC from ATP synthesis
- Proton gradient dissipated → heat
- Important in newborns and hibernators
Thermogenin lets protons leak back, releasing energy as heat, not ATP. Feature Detail Protein Thermogenin (UCP-1) Result Heat (no ATP) Applied
- Neonatal thermoregulation
- Cold adaptation
🔑KEY POINTS TO REMEMBER- Brown fat generates heat via thermogenin (UCP-1).
- Uncouples ETC → heat, not ATP.
- Important in newborns.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
ATP (adenosine triphosphate) is the universal energy currency of the cell, storing and releasing energy through its phosphate bonds.
Structure & Function
- Adenine + ribose + three phosphates
- Two high-energy phosphoanhydride bonds
- ATP ↔ ADP + Pi (energy exchange)
- Continuously recycled
ATP is made when energy is available and split when work is needed. Reaction Energy ATP → ADP Releases ADP → ATP Requires Applied
- Muscle contraction, transport, synthesis
- Recycled thousands of times per day
🔑KEY POINTS TO REMEMBER- ATP = energy currency; two high-energy bonds.
- ATP ↔ ADP + Pi.
- Powers all cellular work.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Creatine phosphate (phosphocreatine) is a high-energy phosphate store in muscle that rapidly regenerates ATP.
Function
- Stores high-energy phosphate
- Creatine phosphate + ADP → ATP + creatine (creatine kinase)
- Buffers ATP during sudden demand
- Broken down to creatinine (excreted)
Creatine phosphate quickly tops up ATP when muscle demand spikes. Feature Detail Enzyme Creatine kinase Waste Creatinine Applied
- Creatinine = renal-function marker
- CK-MB in myocardial infarction
🔑KEY POINTS TO REMEMBER- Creatine phosphate = rapid ATP store in muscle.
- Creatine kinase regenerates ATP.
- Breaks down to creatinine (renal marker).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Redox (oxidation-reduction) potential is a measure of a substance’s tendency to gain or lose electrons, determining the direction of electron flow.
Principle
- Measured in volts (E₀)
- More negative → stronger reducing agent (loses e⁻)
- More positive → stronger oxidising agent
- Electrons flow from negative to positive
Electrons move from more negative to more positive redox couples. Couple E₀ (V) NAD⁺/NADH –0.32 O₂/H₂O +0.82 Applied
- Determines the ETC sequence
- Energy released ∝ potential difference
🔑KEY POINTS TO REMEMBER- Redox potential = tendency to gain/lose electrons.
- Electrons flow from negative to positive.
- Sets the ETC order (NADH → O₂).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).