Gluconeogenesis' first enzyme

[Jengreef]

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I know that gluconeogenesis begins by carboxylating pyruvate. However, I just hit a practice passage in TBR that asked for the enzyme of the first step of gluconeogenesis in the liver. My first thought is that lactate is first converted into pyruvate before it can be carboxylated. If you don't have pyruvate from lactate, you can't have gluconeogenesis. With that in mind, wouldn't lactate dehydrogenase technically be the first enzyme for gluconeogenesis?

Or is lactate dehydrogenase too specific an enzyme for this pathway? I know there are multiple precursor molecules that can be converted to pyruvate for gluconeogenesis besides lactate, but I don't know whether or not those molecules are converted to lactate first before hitting the liver (TBR doesn't specify one way or another).

If multiple molecules are converted directly into pyruvate, then lactate dehydrogenase wouldn't qualify as the first official enzyme for this particular pathway since there would be multiple other enzymes that would take equal importance during this step. But if these other molecules must become lactate before hitting the liver, then the first official step once you've reached the liver would be to change lactose into pyruvate, not carboxylating a non-existent pyruvate molecule.

Any help?

 

[MedPR]

Don't need to know for the MCAT, and I certainly don't know the answer. I assume it wasn't in the passage?

 

[Jengreef]

The passage gave me a bare-bones diagram of the Cori cycle, which made it look like gluconeogenesis began with lactate, not pyruvate, which would make lactate dehydrogenase the first enzyme (the the answer given in the book for this particular question disagrees with me).

I didn't do too hot on my first run through the MCAT, so my approach this time around is to prefer to overprepare than to underprepare. So if I hit any passage from my home-study books, I try to aim for mastery of the materials and concepts of that passage if possible.

Anyway, if anyone could help me on this, it would be greatly appreciated.

 

[ezsanche]

I know that gluconeogenesis begins by carboxylating pyruvate. However, I just hit a practice passage in TBR that asked for the enzyme of the first step of gluconeogenesis in the liver. My first thought is that lactate is first converted into pyruvate before it can be carboxylated. If you don't have pyruvate from lactate, you can't have gluconeogenesis. With that in mind, wouldn't lactate dehydrogenase technically be the first enzyme for gluconeogenesis?

Or is lactate dehydrogenase too specific an enzyme for this pathway? I know there are multiple precursor molecules that can be converted to pyruvate for gluconeogenesis besides lactate, but I don't know whether or not those molecules are converted to lactate first before hitting the liver (TBR doesn't specify one way or another).

If multiple molecules are converted directly into pyruvate, then lactate dehydrogenase wouldn't qualify as the first official enzyme for this particular pathway since there would be multiple other enzymes that would take equal importance during this step. But if these other molecules must become lactate before hitting the liver, then the first official step once you've reached the liver would be to change lactose into pyruvate, not carboxylating a non-existent pyruvate molecule.

Any help?

I think you are thinking along the right line here! Lactate is only ONE percursor that can be converted to pyruvate. Off the top of my head I know analine can also be converted directly to pyruvate via a transaminase process.