Is pyruvate actually reduced at the beginning of pyruvate oxidation?

[David513]

Hi all,

Hope you had a wonderful Thanksgiving!

I understand why pyruvate oxidation is an oxidation of pyruvate overall: you transfer electrons from pyruvate to NAD+. However, in the first step of pyruvate oxidation, you get rid of pyruvate's carboxy group (i.e. remove two oxygens) and then turn the remaining ketone into an alcohol (i.e. add an H). It seems like removing oxygens and adding an H would be reduction in this first step, yet Kaplan says that pyruvate is oxidized here. Is Kaplan wrong? Can someone explain why this is still considered oxidation?

Thank you very much in advance!

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[David513]

Bump. Help me! (Pretty please )

 

[Necr0sis713]

Well, the transfer of electrons to NAD+ to make NADH is an oxidation; NAD+ is the oxidizing agent and is reduced, because the H+ takes the electrons with it. Considering that you already mentioned that, the only reason I can think of is because adding coenzyme CoA attaches an electronegative sulfur to the carbon that lost its carboxyl group. So...since sulfur is more electronegative than carbon, the carbon is oxidized because the sulfur takes the electron density.

I don't see the ketone turning into an alcohol though...I don't know where you got that from. Pyruvate gets turned into acetyl-CoA in pyruvate processing.

 

[David513]

Thank you @O731 for responding!

I'm talking specifically about the first step of the pyruvate dehydrogenase complex involving only pyruvate dehydrogenase, not dihydrolipoyl transacetylase. The ketone seems to be converted into an alcohol, see this diagram: https://classconnection.s3.amazonaws.com/519/flashcards/2297519/png/untitled1354154979189.png

The step that forms acetyl-CoA involves dihydrolipoyl transacetylase, which comes after the step in question.

 

[Necr0sis713]

The ketone to alcohol does seem like a reduction. I think the molecule with the OH is just an intermediate, so the reaction is considered an oxidation in reference to the bigger picture. If you look at the next step, the ketone is back on and there's even a more electro negative sulfur (as opposed to the carbon of the CO2 group).

Maybe they're referring to the bigger picture? In general, dehydrogenase enzymes are known to be oxidizing enzymes

 

[David513]

I think Kaplan's wording is unclear ... perhaps they did mean that overall the pyruvate dehydrogenase complex is an oxidizing process. I took what they said to mean the initial pyruvate dehydrogenase step is an oxidation step which is why I was confused.

Thanks for the response! (Here's hoping the MCAT doesn't test on such things in intense detail haha.)

 

[JoeCaputo]

Which carbon are you talking about? The first carbon that starts as COO- has an oxidation state of +3, and leaves as CO2 having an oxidation state of +4.

The second carbon, of the carbonyl starts at +2, loses the bond to O which drops it to +1, but TPP has a terminal phosphate group. If the O on the TPP is linked to the carbonyl carbon it would remain +2 oxidation, unchanged. This is only if TPP indeed connects via an O.

 

[David513]

@JoeCaputo I was talking about the carbonyl carbon of pyruvate (the carbon that comes after the departing carboxyl carbon). I think your answer makes sense... I have to digest it a bit more. Thank you for responding!