Carbohydrate Metabolism › Gluconeogenesis

Gluconeogenesis Control

Notes

Gluconeogenesis Control

Sections

MECHANISMS OF REGULATION

  • Allosteric regulation
  • Hormonal regulation
  • Substrate availability

ALLOSTERIC REGULATION

Pyruvate carboxylase

  • 2Pyruvate + 2CO2 + 2ATP --> 2Oxaloacetate + 2ADP
  • Activated by Acetyl CoA (product of FA breakdown, marker of energy abundance & low blood glucose)

Phosphoenolpyruvate carboxykinase (PEPCK)

  • 2Oxaloacetate + 2GTP --> 2Phosphoenolpyruvate (PEP) + 2GDP + 2CO2

Corresponding glycolytic reaction

  • Pyruvate kinase: 2PEP + 2ADP --> 2Pyruvate + 2ATP
  • Inhibited by Acetyl CoA

Fructose 1,6-bisphosphatase-1 (FBP-1)

  • Fructose 1,6-BP + H2O --> Fructose 6-P + Pi
  • Activated by Citrate (CAC intermediate & marker of energy abundance)
  • Inhibited by AMP (marker of low energy) & fructose 2,6-BP (hormonally regulated)

Corresponding glycolytic reaction

  • PFK-1: Fructose 6-P + ATP --> Fructose 1,6-BP + ADP
  • Inhibited by Citrate
  • Activated by AMP & fructose 2,6-BP

SUBSTRATE AVAILABILITY

Glucose 6-phosphatase

  • Glucose-6-phosphate + H2O --> Glucose + Pi
  • Not allosterically regulated because Km >>> [glucose 6-phosphate]
  • Substrate level control

Corresponding glycolytic reaction

  • Glucokinase: Glucose + ATP --> Glucose 6-phosphate + ADP

HORMONAL REGULATION

  • FBP-2 & PFK-2 are hormonally regulated (PFK-2 inactive when phosphorylated)
  • High blood glucose = increased Insulin: glucagon ratio = PFK-2 active
    = increased fructose 2,6-BP = promote glycolysis and inhibits gluconeogenesis
  • Low blood glucose = decreased insulin: glucagon ratio = PFK-2 phosphorylated & inactive
    = decreased fructose 2,6-BP = slows glycolysis and removes inhibition from gluconeogenesis
  • INSULIN: promotes glycolysis
  • GLUCAGON: promotes gluconeogenesis

Full-Length Text

  • Here, we will learn the reciprocal control of glycolysis, glucose breakdown, and gluconeogenesis, the synthesis of glucose from non-carbohydrate precursors.
  • To begin, start a table to learn the key mechanisms of gluconeogenesis regulation.
  • Denote that they are:
    • Allosteric regulation
    • Hormonal regulation
    • Substrate availability

In order to learn these mechanisms, let's introduce the 4 regulated reactions in gluconeogenesis.

  • Start with 2 pyruvate molecules.
  • Reaction 1: pyruvate carboxylase converts the pyruvate to oxaloacetate; this process consumes ATP and carbon dioxide.
  • Reaction 2: Phosphoenolpyruvate carboxykinase (PEPCK) converts two oxaloacetates into two 3-carbon phosphoenolpyruvates (PEP's).
    • Indicate that 2 GTP's are consumed.

Five reversible reactions follow, but we will not draw them, here.

  • Reaction 3: fructose 1,6-bisphosphatase-1 dephosphorylates fructose 1,6-bisphosphate to fructose 6-phosphate.
    • Indicate that this phosphate is released and one H2O is consumed.
  • Fructose 6-phosphate reversibly converts to glucose 6-phosphate.
  • Reaction 4 (the final reaction): glucose 6-phosphatase removes a phosphate from glucose 6-phosphate to produce the final product, glucose.
    • Indicate that again, one H2O is consumed.

Now, let's add the corresponding glycolytic steps to our diagram.

  • Reaction 1: glucokinase phosphorylates glucose, which requires 1 ATP. We use glucokinase instead of hexokinase because we are focusing on the liver, which is where gluconeogenesis primarily occurs.
  • Reaction 2: phosphofructokinase-1 (PFK-1) phosphorylates fructose 6-phosphate, which again consumes 1 ATP.
  • Reaction 3 is a one step reversal of the first two reactions in gluconeogenesis.
  • Show that pyruvate kinase dephosphorylates phosphoenolpyruvate to produce pyruvate.
    • Indicate that this reaction produces ATP.

Now, let's illustrate how these irreversible steps are regulated. Let's start with allosteric regulation.

  • Indicate that acetyl CoA is an allosteric activator of the first enzyme in gluconeogenesis: pyruvate carboxylase.
  • Indicate that acetyl CoA is a product of fatty acid breakdown, which releases energy via beta-oxidation.
  • Write that acetyl CoA is a marker of energy abundance and low blood glucose.
    • Don't confuse energy levels with glucose levels!
  • Next, show that acetyl CoA inhibits pyruvate kinase, the first enzyme in glycolysis.
    • An abundance of energy and low blood glucose inhibits glycolysis.
  • Now, show that the following are allosteric inhibitors of fructose 1,6 bisphosphatase-1: AMP and fructose 2,6-bisphosphate.
  • Show that citrate is an allosteric activator of FBP-ase-1.
  • Indicate that AMP is a marker of low intracellular energy.
    • Gluconeogenesis is a very expensive process, and requires an abundance of energy to proceed.
  • Show that AMP activates PFK-1, the rate-limiting step in glycolysis.
    • Glycolysis supplies energy under these conditions.
  • Indicate that citrate is a citric acid cycle intermediate, and a marker of energy abundance.
  • Show that it inhibits PFK-1.
  • Now, show that fructose 2,6-bisphosphate activates PFK-1.

Let's take a closer look at fructose 2,6-bisphosphate, which integrates both allosteric and hormonal control in the liver.

  • Draw the enzyme that reversibly produces fructose 2,6-bisphosphate: PFK-2/FBP-2.
    • Notice that these enzymes parallel PFK-1 of glycolysis and FBP-1 in gluconeogenesis, but act on the second carbon instead of the first.
  • Redraw the enzyme with PFK-2 phosphorylated.
    • Glucagon promotes PFK-2 phosphorylation while insulin promotes its activation.
  • Thus, fructose 2,6-bisphosphate concentrations are under hormonal control.

Let's illustrate this.

  • Draw a vessel above the phosphorylated and dephosphorylated enzymes.
  • Write fed state above the dephosphorylated enzyme.
  • Show that blood glucose is high and the insulin: glucagon ratio is high.
    • This promotes the dephosphorylation, and activation, of PFK-2.
  • Show that fructose 2,6-bisphosphate concentrations increase.
    • As we have seen, this promotes glycolysis and inhibits gluconeogenesis.
  • Now, show that in the fasting state, blood glucose is low and the insulin:glucagon ratio is low.
    • This promotes PFK-2 phosphorylation.
  • Indicate that fructose 2,6-bisphosphate concentrations decrease.
    • This promotes gluconeogenesis by decreasing inhibition on FBP-1.

Insulin and glucagon also control the remaining enzymes in our diagram.

  • Show that glucagon stimulates all the key enzymes unique to gluconeogenesis.
  • Now, indicate that insulin stimulates the enzymes of glycolysis: glucokinase, PFK-1 and pyruvate kinase activity.
  • Reciprocal control is modulated by the blood insulin: glucagon ratio.

Now, let's illustrate the final mode of regulation: substrate availability.

  • Write that glucose 6-phosphatase is not allosterically regulated.
    • Indicate that instead, it experiences substrate-level control.

What does this mean?

  • Show that the Km of glucose 6-phosphatase, the concentration at which the enzyme is half-saturated, is much greater than the physiological concentration of glucose 6-phosphate in the cell.
    • Thus, its activity is directly dependent on substrate availability.
  • Note that we do not include all the activators and inhibitors of glycolysis in this tutorial, but just those that are significant in reciprocal control.