Q.74 The net yield of NADH in the Embden-Meyerhof pathway in E. coli is ____ .
The Embden-Meyerhof pathway (EMP), or glycolysis, produces a net yield of 2 NADH molecules per glucose molecule in Escherichia coli under standard anaerobic conditions. This holds true for the canonical pathway active in this bacterium.
Pathway Overview
The EMP pathway converts one glucose into two pyruvates through 10 enzymatic steps. NADH forms solely at the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) step, where two molecules of glyceraldehyde-3-phosphate each reduce NAD⁺ to NADH, yielding 2 NADH total. No other steps generate NADH, and E. coli lacks modifications altering this stoichiometry in the core EMP.
Step-by-Step NADH Accounting
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Glucose → glucose-6-P → fructose-6-P → fructose-1,6-BP (no NADH).
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Fructose-1,6-BP splits into dihydroxyacetone-P (DHAP) and glyceraldehyde-3-P (GAP); DHAP isomerizes to GAP.
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Two GAP → two 1,3-bisphosphoglycerate: produces 2 NADH via GAPDH.
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Subsequent steps yield 2 ATP net (4 produced, 2 consumed earlier).
Common Exam Options Explained
Multiple-choice options often test misconceptions; correct answer is 2.
| Option | Explanation | Correct? |
|---|---|---|
| 0 | Ignores GAPDH step; confuses with non-phosphorylating GapN variants in engineered strains. | No |
| 1 | Matches Entner-Doudoroff pathway, not EMP in E. coli. | No |
| 2 | Matches two GAP molecules oxidized to produce 2 NADH per glucose. | Yes |
| 4 | Double-counts or confuses with full oxidation to CO₂ (not glycolysis). | No |
E. coli Specifics
E. coli primarily uses EMP for glycolysis, yielding 2 ATP and 2 NADH anaerobically, unlike Entner-Doudoroff (1 ATP, 1 NADH + 1 NADPH). Aerobically, NADH feeds oxidative phosphorylation, but net glycolytic yield remains 2 NADH.
