Redox cofactors — What do NAD+, NADP+, and FAD (oxidized forms) all have in common in metabolism?
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AThey are reduced (already carrying maximum electrons)
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BThey have a full complement of high-energy electrons
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CThey are oxidized electron acceptors that can be reduced during catabolism or anabolism
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DThey are the direct carbon-fixing substrates in the Calvin cycle
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EThey function only in glycolysis and not elsewhere
Answer
Correct Answer: They are oxidized electron acceptors that can be reduced during catabolism or anabolism
Explanation
Introduction / Context:NAD+, NADP+, and FAD are ubiquitous redox cofactors. In their oxidized forms, they accept electrons from substrates, becoming NADH, NADPH, and FADH2. Recognizing their shared role clarifies many pathways from glycolysis to fatty acid synthesis.
Given Data / Assumptions:
- NAD+ and FAD commonly accept electrons in catabolic reactions (e.g., dehydrogenases).
- NADP+ is often used in anabolic pathways, reduced to NADPH as a reductant.
- “+” denotes oxidized form; “H/H2” denotes reduced form.
Concept / Approach:Identify the property common to all: in the listed forms they are oxidized and ready to accept electrons. They are not yet “full” of electrons; that would be NADH/NADPH/FADH2. They are not direct carbon-fixing reagents in photosynthesis.
Step-by-Step Solution:
NAD+ + 2 e− + H+ → NADH.NADP+ + 2 e− + H+ → NADPH.FAD + 2 e− + 2 H+ → FADH2.Verification / Alternative check:Enzyme names such as “dehydrogenase” indicate oxidation of a substrate with concomitant reduction of these cofactors.
Why Other Options Are Wrong:
a,b) Describe the reduced state, not the listed oxidized forms.d) Carbon fixation uses RuBP and CO2 with RuBisCO; NADPH provides reducing power but is not the carbon-accepting substrate.e) These cofactors function broadly across metabolism, not only in glycolysis.Common Pitfalls:Confusing the oxidized (NAD+, NADP+, FAD) and reduced (NADH, NADPH, FADH2) forms.
Final Answer:They are oxidized electron acceptors that become reduced during metabolism.