1. As we discussed, NAD+ and NADP+ are essentially the same molecule (save the a

Question

1. As we discussed, NAD+ and NADP+ are essentially the same molecule (save the additional phosphate in NADP+) and serve similar purposes in that they act as electron transfer agents in oxidation/reduction reactions. However, they act in very different sets of reactions. NAD+ is found in the catabolic reactions of glycolysis whereas NADP+ is found in nearly all the anabolic reactions of the cell. Intriguingly, NAD+ and NADP+ are maintained at very different cellular concentrations [NAD+] >> [NADP+]. Think about it! Why does the cell maintain these two molecules at such different levels?

2. Both NADP+ and NAD+ could be considered cofactors or co-substrates. Provide an explanation for why they are able to be involved in separate reactions. That is, why does NADH not substitute for NADPH in reactions involving NADPH? Be specific.

3. In glycolysis, steps 4,5, and 6 have a positive delta G under standard conditions.

a. How, then, are these reactions able to proceed without being coupled to the breakdown of an energy carrier molecule? (there is a simple answer here – don’t over think it)

b. Step 6 is an especially interesting example, because it has a positive delta G of 1.5 kcal/mol yet it releases an electron to NAD+. Provide an explanation for how this happens.

c. Think about the previous questions, under what conditions might they run in reverse?

d. Likewise, we know that step 1,3 and 10 of glycolysis are not reversible. Why are steps 6 and 7 fully reversible?

4. Fermentation leads to the build-up of products that can have negative effects on cells: pyruvate (acidifies cells) and ethanol (cellular poison). So if it is poisonous why do cells do it? That is, what benefits does it serve to the cell?

5. Creatine phosphate is generated as a high-energy storage compound in skeletal muscle. Upon bursts of activity, it can be used to generate ATP before aerobic respiration begins to generate large amounts of ATP.

a. What is the free energy of creatine phosphate?

b. Is the deltaG of creatine phosphate enough to drive ATP synthesis?

c. Under what conditions would the reverse reaction ATP hydrolysis used to drive the synthesis of creatine phosphate occur?

d. If you think about the free energy change in the breakdown of creatine phosphate to generate ATP, it seems odd (to me anyway) that there is this additional step needed before the energy can be used in cellular processes. Thinking about the evolution of enzymes, why don’t cells use creatine phosphate directly?

6. Insulin binds to the insulin receptor on many cells. The direct outcome of this is that the number of glucose transporters in the plasma membrane increases leading to increased glucose uptake. Signaling through the insulin receptor also triggers activation of enzymes associated with glycolysis (sugar breakdown) as well as glycogen synthesis (sugar synthesis and storage) and fatty acid synthesis (this should seem odd). Why does this make sense? That is, provide an explanation that reconciles these observations.

Explanation / Answer

According to chegg rules, we can answer only the first question.

NAD is used for catabolic reactions while NADP is used for anabolic reactions. The NAD/NADH ratio is always high in the cell as catabolic reactions are needed for release of energy. The ATP released is needed for the other important processes of the cell. In contrast, the NADP/NADPH ratio is kept low and is just sufficient for the anabolic reactions of the cell.

So the different concentrations are needed since they participate in different reactions of the cell.

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