pyruvate fate

[Lolaaa]

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Two questions:

Pyruvate decarboxylation uses the enzyme pyruvate dehydrogenase to covert pyruvate to acetyl CoA. Is it the same as pyruvate oxidation? Does this process occur in the cytoplasm or mitochondria?

Pyruvate carboxylase converts pyruvate to OOA and PECK converts OOA to PEP. I know this is part of gluconeogenesis which occurs in the cytoplasm but does this process also occur in the cytoplasm?

Please help! I'm confused

Also does anyone have a table of where each reaction occur like their reaction sites?

 

[JimKimSlim]

I hope this answers your question. Once pyruvate is formed, it gets transported to the mitochondria, where pyruvate dehydrogenase converts pyruvate to acetyl CoA. This step is oxidation, hence pyruvate oxidation. Don't believe me? Compare the oxidation number of pyruvate to acetyl CoA.

During Kreb cycle, when malate is formed, it gets transported to the cytoplasm and gets converted to OAA --> PEP, which also occurs in the cytoplasm. Only malate is able to cross the mitochondrial barrier through its shuttle mechanism. That's why OAA during krebs cycle "reverses" to malate, which crosses the membrane into the cytoplasm, and gets oxidized into OAA again in the cytoplasm. Biology can be beautifully weird...

 

[Lolaaa]

I hope this answers your question. Once pyruvate is formed, it gets transported to the mitochondria, where pyruvate dehydrogenase converts pyruvate to acetyl CoA. This step is oxidation, hence pyruvate oxidation. Don't believe me? Compare the oxidation number of pyruvate to acetyl CoA.

During Kreb cycle, when malate is formed, it gets transported to the cytoplasm and gets converted to OAA --> PEP, which also occurs in the cytoplasm. Only malate is able to cross the mitochondrial barrier through its shuttle mechanism. That's why OAA during krebs cycle "reverses" to malate, which crosses the membrane into the cytoplasm, and gets oxidized into OAA again in the cytoplasm. Biology can be beautifully weird...

Yes thank you!! It's just that the terminology is confusing me alittle. Since it's a decarboxylation reaction why isn't pyruvate decarboxylase enzyme instead of dehydrogenase? and Is removing a carboxyl group leads to an oxidation reaction? I feel like I need to understand how these terms are linked instead of just memorizing them. Could you explain to me how these terms are connected?

 

[JimKimSlim]

Pyruvate dehydrogenase is a multi enzyme complex, and pyruvate decarboxylase is one of the first enzymes that decarboxylate pyruvate with TPP. Removing carboxyl group doesn’t necessarily mean oxidation, but you need to see the bigger picture. think acetyl CoA as acetate (which is one of the most oxidative compounds since CO2). PDH complex is one of the most fascinating enzymes, and I suggest drawing the mechanism of each step by hands.

 

[FrameshiftAndrew]

and Is removing a carboxyl group leads to an oxidation reaction? I feel like I need to understand how these terms are linked instead of just memorizing them. Could you explain to me how these terms are connected?

This is an excellent question with a short answer and a long answer. The short answer is that decarboxylation reactions in which the carboxyl group is lost as CO2 involve oxidation. You can confirm this by calculating the oxidation states for carbon. In a carboxyl group, C has an oxidation state of +3. In CO2, C has an oxidation state of +4. The oxidation state increasing means, by definition, that the carbon was oxidized. If you search the scientific literature, you can find examples of non-oxidative decarboxylation, but they involve different processes.

On a broader note, this points to the importance of being able to calculate the oxidation state of carbon in various organic compounds. There are some solid tutorials online about how to do this, and it's a good skill to have, because the MCAT does expect you to understand the oxidation/reduction hierarchy of organic compounds and be able to recognize quickly, for example, that alcohols are reduced relative to ketones, or that alkenes are oxidized relative to alkanes. You're unlikely to encounter a question that straight-up asks you, say, what is the oxidation state of carbon in CH4, or CH3OH, or whatever (although it's not impossible!), but in principle you should be able to figure it out as a way to double-check your work if you come across some kind of biological redox reaction.

In terms of the terminology regarding pyruvate oxidation, one of the reasons why this term is used is that it aligns with the overall logic of carbohydrate (and fatty-acid) metabolism—which, in a nutshell, is that reduced biomolecules are oxidized to generate electrons, which are pushed to the ETC by the electron carriers FAD/FADH2 and NAD+/NADH. This, by the way, is a high-level concept that the AAMC emphasizes multiple times in the content outline, so it's good to see how every individual step fits into the picture.