B/B 03 · Hormonal control of liver glucose metabolism
Switching the Liver Between Fed and Fasted
Use two tables to trace how glucagon, cAMP and F2,6BP shift the liver between glycolysis and gluconeogenesis.
Passage
The liver makes glucose from non-carbohydrate precursors (gluconeogenesis) and breaks glucose down (glycolysis). The two pathways share most of their enzymes, but they differ at a few irreversible steps. One of those steps is the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase-1 (PFK-1) in glycolysis, and the reverse reaction is catalyzed by fructose-1,6-bisphosphatase (FBPase-1) in gluconeogenesis. If both enzymes ran at full speed together, the cell would only waste ATP, so the cell must control which pathway is active.
An important regulator of this control is fructose-2,6-bisphosphate (F2,6BP). This molecule is not an intermediate in either pathway. It allosterically activates PFK-1 and inhibits FBPase-1. F2,6BP is made and broken down by a single bifunctional enzyme that has two catalytic domains, a kinase domain that makes F2,6BP from fructose-6-phosphate and a phosphatase domain that breaks it down. Hormones that act on the liver change the balance between these two activities.
After a meal, blood glucose rises and the pancreas releases insulin. The liver takes up glucose, stores some of it as glycogen and burns some of it by glycolysis. During a fast, blood glucose falls, the pancreas releases glucagon, and the liver instead releases glucose to protect the brain and other tissues that depend on it. The liver’s two modes of operation therefore have to be switched by hormones, and the switch has to act quickly on enzymes that are already present in the cell.
A researcher compares the livers of fasting and recently fed animals, and then uses isolated liver cells (hepatocytes) to study the signaling pathway behind the difference. Table 1 shows measurements in whole animals.
Table 1. Hormone levels and liver measurements in fed and fasted animals.
| Measurement | Fed (after a meal) | Fasted (16 hours) |
|---|---|---|
| Blood insulin | High | Low |
| Blood glucagon | Low | High |
| Liver F2,6BP (µM) | 10 | 2 |
| Liver glycolysis (relative rate) | 100 | 20 |
| Net glucose release by liver (relative rate) | 5 | 60 |
Hepatocytes are isolated from fed animals and kept in a defined medium for several hours. In the experiments below, F2,6BP is measured in cell extracts, glycolysis is followed by the release of labeled water from glucose that carries a radioactive tracer, and glucose output is measured with a glucose assay of the medium. All values are expressed relative to the control condition.
In the second experiment, hepatocytes are placed in a medium that contains a high concentration of glucose and insulin, similar to the fed state. The researcher then adds a cell-permeable analog of cyclic AMP (cAMP), which activates protein kinase A (PKA) directly, or glucagon together with a PKA inhibitor. Table 2 shows the results.
Table 2. Effects of cAMP signaling in isolated hepatocytes.
| Condition | Intracellular cAMP | F2,6BP (µM) | Glycolysis (relative rate) | Glucose output (relative rate) |
|---|---|---|---|---|
| Control (glucose and insulin) | Low | 10 | 100 | 5 |
| Control + cAMP analog | High | 2 | 25 | 60 |
| Control + glucagon + PKA inhibitor | High | 9 | 95 | 8 |
Glucose that the liver releases leaves the cell after glucose-6-phosphate has been converted to free glucose by glucose-6-phosphatase, an enzyme that is found in the liver and kidney but not in skeletal muscle.
Questions
Question 1 of 6
Which statement is best supported by Table 1?
The table shows the opposite: F2,6BP falls from 10 to 2 µM in the fasted state, and glycolysis falls with it.
The measured values are 10 µM and 2 µM, a fivefold difference.
F2,6BP is a regulator, not a substrate. The liver releases little glucose in the fed state.
Why the answer is what it is
In fasting, glucagon is high and insulin is low, and the liver’s F2,6BP drops from 10 to 2 µM. Glycolysis falls and glucose release rises, so low F2,6BP is associated with a shift toward gluconeogenesis.
Review: Glycolysis · Metabolic Regulation & Integration · Fed vs Fasted States · Glucagon & Counterregulatory Hormones · Insulin · The Liver · Allosteric Regulation & Metabolic Control
Question 2 of 6
Given that F2,6BP activates PFK-1 and inhibits FBPase-1, what is the most likely effect of the drop in F2,6BP in the fasted liver?
This describes the fed liver, in which F2,6BP is high.
Lower F2,6BP removes an activator of PFK-1 and an inhibitor of FBPase-1, so the two enzymes are moved in opposite directions, not in the same one.
The fasted liver actively produces glucose (relative output of 60), so gluconeogenesis is running.
Why the answer is what it is
With less F2,6BP, PFK-1 loses its activator, so glycolysis slows. FBPase-1 loses its inhibitor, so gluconeogenesis speeds up. The shift matches the higher glucose output in Table 1.
Review: Glycolysis · Metabolic Regulation & Integration · Fed vs Fasted States · Glucagon & Counterregulatory Hormones · Insulin · The Liver · Allosteric Regulation & Metabolic Control
Question 3 of 6
Which conclusion about the signaling pathway is best supported by Table 2?
Both hormones act on hepatocytes, and these cells were exposed to both in the experiment.
The inhibitor blocks the action of PKA, not the production of cAMP. High cAMP together with normal F2,6BP shows that PKA is needed for the effect.
The cAMP analog lowered F2,6BP and glycolysis, the opposite of this statement.
Why the answer is what it is
The cAMP analog, which activates PKA, lowers F2,6BP (10 to 2) and glycolysis (100 to 25) and raises glucose output. When PKA is inhibited, glucagon no longer lowers F2,6BP (9) even though cAMP is high. PKA is therefore a required step between cAMP and the change in F2,6BP.
Review: Glycolysis · Metabolic Regulation & Integration · Fed vs Fasted States · Glucagon & Counterregulatory Hormones · Insulin · The Liver · Allosteric Regulation & Metabolic Control
Question 4 of 6
Based on the data, which activity of the bifunctional enzyme is most likely favored when PKA phosphorylates it?
More F2,6BP would raise glycolysis. PKA activation lowered F2,6BP and glycolysis.
The fall in F2,6BP depends on PKA, a kinase, so phosphorylation of some protein in the pathway must be involved.
If both were increased equally the F2,6BP level would not need to change. The data show a clear fall.
Why the answer is what it is
Active PKA lowers F2,6BP, so the reaction that destroys it (the phosphatase activity) must be favored over the one that makes it (the kinase activity). In the liver, phosphorylation by PKA shifts the bifunctional enzyme toward its phosphatase activity.
Review: Glycolysis · Metabolic Regulation & Integration · Fed vs Fasted States · Glucagon & Counterregulatory Hormones · Insulin · The Liver · Allosteric Regulation & Metabolic Control
Question 5 of 6
A researcher proposes that skeletal muscle could release glucose into the blood during fasting in the same way as the liver. Which fact in the passage argues against this?
The passage gives no information about this, and it is not the step that limits release of free glucose from the cell.
Muscle can use glucose-6-phosphate for glycolysis. The question is about releasing glucose.
Muscle responds to insulin, which stimulates glucose uptake by muscle.
Why the answer is what it is
The passage states that free glucose is made from glucose-6-phosphate by glucose-6-phosphatase, an enzyme found in the liver and kidney but not in muscle. Without it, muscle keeps glucose-6-phosphate inside the cell for its own energy needs.
Review: Glycolysis · Metabolic Regulation & Integration · Fed vs Fasted States · Glucagon & Counterregulatory Hormones · Insulin · The Liver · Allosteric Regulation & Metabolic Control
Question 6 of 6
Which combination of findings would be expected in the liver of a recently fed animal?
This is the fasted profile.
Insulin and glucagon rise in opposite states, and net glucose release is a fasting response.
Low F2,6BP removes the activation of PFK-1, so glycolysis would not be high.
Why the answer is what it is
After a meal, insulin is high and glucagon is low. F2,6BP is high (10 µM in Table 1), PFK-1 is active, FBPase-1 is inhibited and the liver releases little glucose.
Review: Glycolysis · Metabolic Regulation & Integration · Fed vs Fasted States · Glucagon & Counterregulatory Hormones · Insulin · The Liver · Allosteric Regulation & Metabolic Control