Q.45 An organism uses only the glycerophosphate shunt pathway to transport cytosolic
NADH to mitochondria. For every two electrons transported, complex I, complex
III, and complex IV of the electron transport chain in this organism transport 2.5,
1.5, and 2.0 protons (H+), respectively. The H+ to ATP ratio of FOF1–ATPase of this
organism is 4.0. Terminal electron acceptor is oxygen.
The number of ATP molecules synthesized by oxidizing NADH from glycolysis is
__________ (rounded off to two decimal places).
Answer: 1.75
The glycerophosphate shuttle transfers electrons from cytosolic NADH into mitochondria via FADH₂-like entry at ubiquinone, bypassing Complex I. For every two electrons transported, Complex III pumps 1.5 × 2 = 3 H⁺ and Complex IV pumps 2.0 × 2 = 4 H⁺, totaling 7 H⁺. With an H⁺/ATP ratio of 4.0 for FOF₁-ATPase, ATP yield is 7 ÷ 4.0 = 1.75 per two electrons (one NADH).
Introduction to Glycerophosphate Shuttle Mechanism
The glycerophosphate shuttle pathway (also called glycerol-3-phosphate shuttle) reoxidizes cytosolic NADH from glycolysis by converting dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate in the cytosol. This glycerol-3-phosphate diffuses to the mitochondrial inner membrane, where mitochondrial glycerol-3-phosphate dehydrogenase transfers electrons to FAD, producing FADH₂ and DHAP. These electrons enter the electron transport chain (ETC) at ubiquinone (CoQ), bypassing Complex I and mimicking FADH₂ oxidation.
This shuttle is crucial in tissues like skeletal muscle and brain, yielding fewer ATP than the malate-aspartate shuttle (which feeds electrons to Complex I) due to skipped proton pumping at Complex I.
Electron Entry and Proton Pumping
In this organism using only the glycerophosphate shunt, two electrons from one cytosolic NADH bypass Complex I (0 H⁺ pumped). They flow through:
- Complex III: Pumps 1.5 H⁺ per electron, so 1.5 × 2 = 3 H⁺.
- Complex IV: Pumps 2.0 H⁺ per electron (4 H⁺ total for O₂ reduction), so 2.0 × 2 = 4 H⁺.
Total protons per two electrons: 0 + 3 + 4 = 7 H⁺.
Shuttle Comparison Table
| Shuttle/Entry Point | Complexes Active | H⁺ Pumped (per 2e⁻) | Typical ATP Yield |
|---|---|---|---|
| Malate-Aspartate (NADH → Complex I) | I, III, IV | 10 (standard: 4+2+4) | ~2.5 |
| Glycerophosphate (FADH₂-like → CoQ) | III, IV | 6 (standard: 0+4+2) | ~1.5 |
| This Organism (Custom Ratios) | III, IV | 7 (0+3+4) | 1.75 |
ATP Synthesis via FOF₁-ATPase
The proton gradient drives FOF₁-ATPase (ATP synthase). Here, the H⁺ to ATP ratio is 4.0, higher than mammalian ~3.3 (c-ring 10/3) or chloroplast ~4.0, reflecting organism-specific c-subunit stoichiometry.
ATP Calculation
- Total H⁺ translocated = 7
- ATP per 2e⁻ = 7 H⁺ ÷ 4 H⁺/ATP = 1.75 ATP
One glycolytic NADH (two electrons to O₂ → H₂O) thus synthesizes 1.75 ATP molecules, rounded to two decimal places.
CSIR NET Exam Relevance
For CSIR NET Life Sciences, master shuttle differences, custom proton stoichiometries (not standard 4/2/4 H⁺), and non-integer ATP yields. Glycolysis nets 2 NADH; via this shuttle: 2 × 1.75 = 3.5 ATP (vs. 5 via malate-aspartate). Terminal acceptor oxygen confirms full ETC reduction.
Key Exam Points:
- Glycerophosphate shuttle bypasses Complex I → FADH₂-like entry
- Custom proton ratios: Complex III (1.5 H⁺/e⁻), Complex IV (2.0 H⁺/e⁻)
- FOF₁-ATPase H⁺/ATP = 4.0 (organism-specific)
- Total: 7 H⁺ → 1.75 ATP per NADH
