Biochem Ch 23

Front 1

1. The noncarbohydrate precursors for gluconeogenesis

Back 1

lactate (transported from the muscles via the bloodstream to the liver where it is converted to pyruvate using lactate dehydrogenase)

pyruvate

glycerol - from the breakdown of fatty acids. glycerol kinase + glycerol phosphate dehydrogenase.

amino acids

TCA cycle intermediates.

Front 2

2. Physiological significance of gluconeogenesis

Back 2

Brain cells, red blood cells, and cells of the CNS are almost completely dependent upon glucose as an energy source because of blood brain barrier and because RBC do not have mitochondria.

The liver can only store enough glycogen to feed the brain for half a day.

So, glucose needs to be synthesized via gluconeogenisis.

Front 3

3. Why opposing metabolic pathways are never just the reverse of one another

Back 3

1. Both forward and reverse pathways have to be energetically favorable.

2. The pathways have to be independently controllable so that one direction can be activated while the other is inhibited.
(If the reactions of the pathways were catalyzed by the same set of enzymes acting reversibly, the flow of the carbon through these pathways would be dictated by the law of mass action, not by the cell's changing needs for energy, precursors, or macromolecules.

Front 4

4. The three bypass steps in gluconeogenesis

Back 4

Pyruvate + ATP + H2O + CO2--> oxaloacetate + ADP + Pi + 2H+

oxaloacetate + GTP --> PEP + GDP

FBP -->F6P

F6P --> G6P

Front 5

5. Be familiar with the mechanisms of pyruvate carboxylase and PEP carboxykinase

Back 5

pyruvate + H2O + CO2 + ATP --> oxaloacetate + ADP + Pi + 2H+

oxaloacetate + GTP --> PEP + GDP

Front 6

6. The two shuttle pathways for transporting oxaloacetate from mitochondria to the cytosol; which one is preferred under most situations? Why?

Back 6

oxaloacete + malate dehydrogenase + NADH + H+ --> malate

oxaloacetate + aspartate aminotransferase --> aspartate

Malate shuttle is preferred in most biological conditions because it increases the concentration of NADH which favors reductive biosynthesis.

Front 7

7. How do the above-mentioned two shuttle pathways work to “transport” the reducing power NADH from the cytosol to the mitochondrion?

Back 7

The malate dehydrogenase involves the mitochondrial oxidation of NADH followed by the cytosolic reduction of NAD+ and therefore also transfers NADH reducing equivalents from the mitochondria to the cytosol.

Front 8

8. How is lactate, taken up by the liver, used for gluconeogenesis?

Back 8

L-lactate + NAD+ lactate dehydrogenase --> pyruvate + NADH

Front 9

9. Overall reaction of gluconeogenesis

Back 9

2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 6 H2O --> glucose + 4 ADP + 2 GDP + 2 NAD+ + 2H+

Front 10

10. Sum of glycolysis and gluconeogenesis; what does this equation tell you about the importance of reciprocal regulation of these two pathways?

Back 10

2 ATP + 2 GTP + 4 H2O --> 2 ADP + 2 GDP + 4 Pi + 4H+

if glycolysis and gluconeogenesis were to proceed uncontrollably, it would lead to a futile cycle leading to the hydrolysis of four molecules of ATP.

Front 11

11. Regulation of glycolysis and gluconeogenesis; pay attention to specific allosteric regulators of the regulatory enzymes and hormonal regulation of the level of fructose-2, 6-bisphosphate in the liver; how the regulation ensures that the two pathways do not occur at the same time under any physiological condition

Back 11

phosphofructokinase:
activated by F2,6BP and AMP.
inactivated by citrate, ATP

fructose-6-phosphatase
inactivated by F2,6BP and AMP

pyruvate kinase:
activated by F1,6BP
inactivated by ATP, alanine, cAMP phosphorylate, acetyl CoA

pyruvate carboxylase
activated by acetyl CoA

Accumulation of TCA substrate acetylle CoA indicates a need of oxaloacetate thereby activating pyruvate carboxylase for glulconeogenesis pathway.

Also, acetyl CoA is produced when fatty acid is degraded for energy. (When given a choice b/w glucose and fatty acids, F.A. provide more energy...and also indicate that energy needs have been met...thereby activating gluconeogensis pathway.

increase concentrations of F2,6BP stimulates glycolysis, inhibits gluconeogensis.

The cellular concentration of the regulator F2,6BP is determined by the rates of its synthesis by phosphofructokinase-2 and fructose-2,6-phosphatase. Both of the enzymes are part of the same polypeptide chain, and both are regulated, in a reciprocal fashion by glucagon.

(glucagon is secreted when blood glucose levels are low). So glucagon activates fructose-2,6-bisphosphatase, decreasing the amount of F2,6BP, activating gluconeogensis.

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12. The primary functions of the pentose phosphate pathway

Back 12

1. to produce NADPH as a reducing power for reductive biosynthesis (especially of fatty acids and cholesterol)

2. to produce R5P for nucleotide synthesis

3. to metabolize dietary pentoses.

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13. Contrast NAD+/NADH with NADP+/NADPH; why do we need to have these two chemically similar co-enzymes for biosynthetic reactions?

Back 13

Although NADPH differs from NADH only by the addition of a phosphate group at the 2-OH group of NADH's adenosine moiety, they are not metabolically interchangable.

Cells normally maintain their [NADH] / [ NAD+] at 8 x 10^-4 in the cytosol in order to oxidize metabolites.

However, [NADPH] / [NADP+] are at about 100 in the cytosol, which favors reductive biosynthesis.

So two forms of the nicotinamide co-enzymes are required to carry out both oxidative degradation and reductive biosynthesis at the same time int he same place.

If NADH had to be used for all reductive biosynthesis, the ratio would greatly hinder these reactions.