Phosphofructokinase 1 (PFK 1) regulation

Phosphofructokinase 1 (PFK 1) regulation

  • Phosphofructokinase is the most important control element in the mammalian glycolytic pathway. This enzyme is the pacemaker of glycolytic pathway.
  • Phosphofructokinase (PFK) is the enzyme that controls the third step of glycolysis (rate limiting step), the conversion of fructose-6-phosphate (F6P) into fructose-1, 6-biphosphate (F1, 6BP). It works by transferring a phosphate group from ATP to F6P.
  • This step is subject to extensive regulation since it is not only highly exergonic under physiological conditions, but also because it is a committed step – the first irreversible reaction unique to the glycolytic pathway.
  • Glucose 6 phosphate is not solely a glycolytic intermediate. Before this enzyme’s reaction, glucose-6-phosphate can be oxidized via pentose phosphate pathway to form NADPH, or be converted to glucose-1-phosphate for glycogenesis.

PFK 1 is an allosteric enzyme made up of 4 subunits. In addition to substrate binding sites, PFK 1 has several binding sites for allosteric effectors.

PFK 1 exists in 2 states: T-state (Enzymatically inactive) and R-state (Enzymatically active)

(F6P binds with a high affinity to the R state but not the T state enzyme)

ATP, citrate, and hydrogen ions, Fatty acids are allosteric inhibitors of the enzyme

How the allosteric inhibitors act?

  • The allosteric inhibitor binds to an enzyme at a regulatory site. As result the shape of the active site is altered so that the enzyme can no longer bind to its substrate. Hence they are negative allosteric modulators.
  • Allosteric inhibitors facilitate the formation of the T state, thereby inhibiting enzyme activity.

How the allosteric activators act?

 Allosteric activators bind to the allosteric site/regulatory site of the enzyme to facilitate the formation of the R state by inducing structural changes in the enzyme. By doing so they enhance the enzyme activity. They are positive allosteric modulators.

AMP, Pi and fructose 2, 6-bisphosphate are allosteric activators of the enzyme

Figure 1:Summary of the regulators affecting PFK-1 activity

Role of ATP in PFK 1 regulation

  • PFK1 is allosterically inhibited by high levels of ATP. ATP lowers the affinity of the enzyme for fructose 6-phosphate.
  • The activity of the enzyme increases when the cellular ATP/AMP ratio is lowered (Reason: AMP reverse the inhibitory action of ATP)
  • A high concentration of ATP converts the hyperbolic binding curve of fructose 6-phosphate into a sigmoidal one. ATP elicits this effect by binding to a specific regulatory site that is distinct from the catalytic site.

 

Figure 2:Allosteric regulation of muscle PFK-1 by ATP, shown by a substrate-activity curve.
  • At low concentrations of ATP, the Km for fructose 6-phosphate is relatively low, enabling the enzyme to function at a high rate at relatively low concentrations of fructose 6-phosphate.
  • When the concentration of ATP is high, Km for fructose 6-phosphate is greatly increased, as indicated by the sigmoid relationship between substrate concentration and enzyme activity.

The activity of PFK-1 increases when the energy charge of the cell is low (when there is a need for ATP) whereas it decreases when the energy charge of the cell is high (when ATP concentration in the cell is high). How?

Answer: PFK1 has two sites with different affinities for ATP which is both a substrate and an inhibitor for the enzyme.

  • When the nucleotide is produced faster than it is consumed, its cellular concentration is high.
  • Under such condition ATP, binding to its allosteric site, inhibits PFK-1 by reducing the affinity of the enzyme for fructose 6-phosphate.
  • When ATP consumption exceeds its production, ADP and AMP concentrations rises (in particular that of AMP) due to the reaction catalyzed by adenylate kinase that form ATP from ADP.

ADP + ADP ⇄ ATP + AMP

The activity of phosphofructokinase 1 depends on the cellular energy status:

  • When ATP is plentiful, enzyme activity decreases. If there is already plenty of ATP in the cell, glycolysis does not need to make more.
  • When AMP levels increase and ATP levels fall, enzyme activity increases.

Role of citrate in PFK 1 regulation

  • Glycolysis also furnishes carbon skeletons for biosyntheses and so a signal indicating whether building blocks are abundant or scarce should also regulates PFK 1.
  • PFK 1 is inhibited by citrate (a key intermediate in the aerobic oxidation of pyruvate, fatty acids, and amino acids)
  • High levels of citrate means that biosynthetic precursors are abundant and additional glucose should not be degraded for this purpose.
  • Citrate inhibits PFK 1 by enhancing the inhibitory effect of ATP.
  • Inhibition of glycolysis by citrate ensures that glucose will not be committed to these activities if the citric acid cycle is already saturated.

Effect of pH in PFK1 regulation

  • A fall in pH inhibits PFK 1 activity.
  • The inhibition of PFK 1 by H+ prevents excessive formation of lactic acids and precipitous drop in blood pH.

Role of AMP in PFK 1 regulation

  • PFK1 is allosterically activated by a high concentration of AMP.
  • When consumption of ATP is more than production, AMP concentration raises and it act allosterically to relieve enzyme inhibition by ATP.

Role of Fructose 2, 6 bisphosphate in PFK1 regulation

  • Fructose 2, 6-bisphosphate (Fru-2,6-P2) is the most potent stimulator (allosteric activator) of PFK1
  • Fructose 2, 6- bisphosphate activates phosphofructokinase by increasing its affinity for fructose 6-phosphate and diminishing the inhibitory effect of ATP.

The concentration of Fructose-2, 6 bisphosphate in cells is controlled through regulation of the synthesis and breakdown by PFK-2/FBPase-2.

The primary regulators of this are the hormones insulin, glucagon, and epinephrine which affect the enzyme through phosphorylation/dephosphorylation reactions.

 Refer the link: http://easylifescienceworld.com/fructose-2-6-bisphosphate-is-a-potent-regulator-of-glycolysis-and-gluconeogenesis/

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