ETC Complex II KA question

[basophilic]

A question was asking about what would be the consequences of inhibiting complex II of ETC. This was one of KA's explanations: "FADH_2 donates its electrons at Complex II. Notably, it is also downstream from Complex I (where NADH donates its electrons). Thus, it will impair electron flow from both NADH- and FADH_2 donated electrons. Therefore, "Impaired oxidative phosphorylation" is a correct choice since ATP production is dependent on electron flow through the complexes."

Isn't this wrong? Complex II may be downstream but it's in PARALLEL to COMPLEX I, not in SERIES, isnt it? In other words, I would expect oxidative phosphorylation to not be affected much except for the fact that fewer ATP will be produced for each fuel molecule.

Also, the proton gradient would be relatively unchanged, right? My reasons: 1) Complex II doesn't directly contribute to the proton gradient and 2) whatever electrons FADH2 can't provide through Complex II will be added by the next NADH waiting in line, correct?

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[bee17]

You are correct: Complexes I and II are not in series. Electrons from NADH never flow through Complex II. However, the fact that fewer ATP will be produced per glucose does mean that oxidative phosphorylation will be impaired. I agree with their answer, but not their explanation.