ETC complexes - discrepancy between books

[663697]

My TPR book shows 3 complexes in the ETC -- NADH dehydrogenase, cytochrome C reductase, and cytochrome C oxidase, with FADH2 reducing coenzyme Q. I was checking the Voet Biochemistry textbook for more info and instead of 3 complexes it shows 4, with succinate -> fumarate reducing Complex II, rather than Coenzyme Q. Is this discrepancy likely to come up on the test, and if so which one should we go with?

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[Lawpy]

There are four complexes in the electron transport chain. Succinate dehydrogenase (i.e. Complex II) links between the citric acid cycle and the electron transport chain. Conenzyme Q shuttles electrons from Complex I and Complex II to be delivered to Complex III.

 

[663697]

There are four complexes in the electron transport chain. Succinate dehydrogenase (i.e. Complex II) links between the citric acid cycle and the electron transport chain. Conenzyme Q shuttles electrons from Complex I and Complex II to be delivered to Complex III.

Awesome diagram, thanks. Something else came to mind -- on the sample test there was a question about how many electrons cytochrome C carries, and after some searching online I found out that it only carries 1 e- due to it being a Fe(3+) center which gets reduced to Fe(2+). However I'm having a hard time understanding why this doesn't apply for the other complexes as well if they are also Fe cytochromes. I don't have the exact sources on me right now but from what I remember some of them also carry 2 e- and I was trying to make sense of why this is so.

 

[Lawpy]

Awesome diagram, thanks. Something else came to mind -- on the sample test there was a question about how many electrons cytochrome C carries, and after some searching online I found out that it only carries 1 e- due to it being a Fe(3+) center which gets reduced to Fe(2+). However I'm having a hard time understanding why this doesn't apply for the other complexes as well if they are also Fe cytochromes. I don't have the exact sources on me right now but from what I remember some of them also carry 2 e- and I was trying to make sense of why this is so.

Pretty sure all iron cytochromes transfer only one electron because iron exists in two forms in normal homeostatic conditions: Fe2+ and Fe3+. Some complexes like NADH dehydrogenase/Complex I have two-electron steps because electrons are transferred from NADH to reduce FMN to FMNH2 (FMN and FAD accept two electrons to reduce to FMNH2 and FADH2). Each electron is then transferred individually from FMNH2 to an iron-sulfur cluster and to uniquinone. That's why ubiquinone is reduced in two steps (from ubiquinone to semiquinone radical to ubiquinol). But iron can't transfer two electrons in a single step since it alternates only between Fe3+ and Fe2+ states in normal conditions.

 

[aldol16]

Awesome diagram, thanks. Something else came to mind -- on the sample test there was a question about how many electrons cytochrome C carries, and after some searching online I found out that it only carries 1 e- due to it being a Fe(3+) center which gets reduced to Fe(2+). However I'm having a hard time understanding why this doesn't apply for the other complexes as well if they are also Fe cytochromes. I don't have the exact sources on me right now but from what I remember some of them also carry 2 e- and I was trying to make sense of why this is so.

Iron heme centers can only carry one electron at a time. Ferric gets reduced to ferrous and ferrous has too low of a redox potential to take on another electron.

 

[663697]

Pretty sure all iron cytochromes transfer only one electron because iron exists in two forms in normal homeostatic conditions: Fe2+ and Fe3+. Some complexes like NADH dehydrogenase/Complex I have two-electron steps because electrons are transferred from NADH to reduce FMN to FMNH2 (FMN and FAD accept two electrons to reduce to FMNH2 and FADH2). Each electron is then transferred individually from FMNH2 to an iron-sulfur cluster and to uniquinone. That's why ubiquinone is reduced in two steps (from ubiquinone to semiquinone radical to ubiquinol). But iron can't transfer two electrons in a single step since it alternates only between Fe3+ and Fe2+ states in normal conditions.

Iron heme centers can only carry one electron at a time. Ferric gets reduced to ferrous and ferrous has too low of a redox potential to take on another electron.

Yeah, that makes sense. I think I was confusing it with Coenzyme Q. Is this the only one that carries 2 electrons?

 

[Lawpy]

A good article to check out: https://www.tamu.edu/faculty/bmiles/lectures/electrontrans.pdf

Also FAD, FMN and ubiquinone can accept one electron or two electrons at a time. They have this flexibility because of the radical intermediate involved.

 

[aldol16]

Yeah, that makes sense. I think I was confusing it with Coenzyme Q. Is this the only one that carries 2 electrons?

It can carry one or two electrons. NAD can carry two electrons at a time - it accepts what is a hydride equivalent.