Step six of the citric acid cycle transfers two hydrogen atoms to a carrier. Which carrier accepts them, and why that one?
AFAD, since the electron energy is too low for NAD plus
BNAD plus, since it accepts hydrogen more readily
CCoenzyme A, since it is bound to the enzyme
DUbiquinone, since it moves through the membrane
Answer & Solution
Correct answer: A. FAD, since the electron energy is too low for NAD plus
1. Step six is a dehydration process that converts succinate into fumarate.
2. Two hydrogen atoms are transferred to FAD in the process.
3. The energy contained in the electrons of these atoms is insufficient to reduce NAD plus but adequate to reduce FAD.
4. So the choice of carrier is set by the energy of the electrons, not by convenience.
5. The reduced form of this carrier stays attached to the enzyme and passes electrons to the chain directly.
6. Ubiquinone does move through the membrane, but it collects electrons later rather than accepting them here.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.3 Oxidation of Pyruvate and the Citric Acid Cycle_
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