Practice - Gluconeogenesis - Video Tutorials & Practice Problems
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Practice - Gluconeogenesis
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If you haven't answered question 31 yet, pause the video now. An enzyme that's used by both glycolysis and gluconeogenesis is 3 phosphoglycerate kinase. This is the first enzyme in the of the glycolytic pathway to generate ATP from ADP and this is the enzyme that takes 13 bisphosphoglycerate and turns it into 3 phosphoglycerate and in the process, it generates an ATP. This is, step number 7 of glycolysis. Now, these other enzymes are restricted to one pathway. Phosphofructokinase 1 is step 3 of glycolysis. Is step 1 of glycolysis and pyruvate kinase is step 10 of glycolysis. Glucose 6 Phosphatase is the last step of gluconeogenesis and that's the enzyme that removes the phosphate group from glucose and is only present in liver cells you might recall. Now, an enzyme used in gluconeogenesis that reverses the action of the glycolytic enzyme phosphofructokinase is fructose 16 bisphosphatase. And you might remember the whole pattern of phosphatases reversing the action of kinases. And phosphofructokinase produces fructose 16 bisphosphate in the 3rd step of glycolysis. And so fructose 16 bisphosphatase undoes the action of phosphofructokinase and actually removes a phosphate group from fructose 16 bisphosphate. Moving on to question 33, which of the following statements about gluconeogenesis is false? And the answer is that it consists entirely of glycolytic that are remember, there are those 3 glycolytic reactions that cannot be reversed. And so we have to use different enzymes. And actually it takes 4 reactions to undo, those 3 because the last reaction of glycolysis, that of pyruvate kinase has to be undone by 2 reactions in gluconeogenesis. And speaking of that, let's move on to question 34 because, pyruvate cannot be directly converted to phosphoenolpyruvate, right? That's pyruvate kinase takes PEP and turns into pyruvate and we have to undo that in 2 steps in gluconeogenesis. So first, we have to convert pyruvate to oxaloacetate and B is actually wrong though because it costs an ATP. B is actually wrong though because it costs an ATP and a GTP. Remember that converting oxaloacetate to PEP cost GTP, not ATP. Little tricky point there. C is also correct though. The reason that we have to undo it in 2 steps is because the action of pyruvate kinase is so energetically favorable, it cannot simply be undone by one reaction. So, the answer here is actually E both A and C. Now, last question about, gluconeogenesis is, why glycolysis and gluconeogenesis are so tightly controlled. What's the point of all this? And the point is to prevent a futile futile cycle, right? We don't want that. If gluconeogenesis and glycolysis could occur at the same time, they would just undo each other, right? We'd be wasting energy because glycolysis goes in one direction, gluconeogenesis goes in the other. We would just be flip flopping. We'd just be pointlessly carrying out reactions. We wouldn't actually, be producing any products really from either reaction because the products of one reaction are the substrates of the other and they just kind of be flip flopping back and forth. So, when glycolysis is in action, gluconeogenesis is not happening and when gluconeogenesis is in action, glycolysis is not happening and remember that most of the enzymes are readily reversible from those 2 reactions and they're also shared by those 2 reactions. So really, it's just controlling the few enzymes that carry out the irreversible steps that allow you to, regulate glycolysis and gluconeogenesis. Alright. Let's flip the page and finish up with some questions on the pentose phosphate pathway.