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Phosphofructokinase (PFK) Regulation

Major regulatory enzyme in glycolysis. Regulation occurs through multiple mechanisms:

Allosteric Control

  • Activators: AMP, ADP, Pi (inorganic phosphate)

  • Inhibitors: ATP, Citrate, Fatty acids, Acetyl-CoA

  • Pasteur Effect: Allosteric control of glycolysis. Lack of oxygen leads to increased glycolysis.

  • ATP Utilization: Muscle contraction increases ATP demand, affecting PFK activity.

  • Alternate Fuels: Fatty acid oxidation influences PFK activity.

  • Fructose-2,6-bisphosphate (F26BP): Co-ordination of PFK and Fructose-1,6-bisphosphatase (F16BPase) activity.

Isoenzymes

  • PFK Isoenzymes: Cytosolic; found in all cells.

  • PFK-1: Catalyzes the reaction Fructose-6-phosphate (F6P) to Fructose-1,6-bisphosphate (F16BP).

  • Tissue Localization: Different tissues express different isoenzymes.

  • Differential Control: Isoenzymes are subject to differential control mechanisms.

Fructose-2,6-Bisphosphate (F26BP) Regulation

  • F26BP Role: Key regulator coordinating glycolysis and gluconeogenesis.

  • Increased F26BP:

    • Activates PFK-1, thus promoting glycolysis

    • Inhibits F16BPase, thus reducing gluconeogenesis

  • Decreased F26BP:

    • Inhibits PFK-1, thus reducing glycolysis

    • Activates F16BPase, thus promoting gluconeogenesis

PFK-2 Regulation

  • PFK-2: Catalyzes the reaction F6P to F26BP.

  • Multiple PFK-2 isoforms:

    • Tissue-specific expression.

    • Activity modulated by phosphorylation.

    • Activity modulated at the level of expression.

  • Bifunctional Enzymes: Single polypeptide chain with two enzymatic activities.

    • PFK-2 activity: F6P → F26BP.

    • F26BPase activity: F26BP → F6P.

Pyruvate Kinase (PK) Regulation

Cytosolic enzyme that catalyzes the reaction:

PEP+ADP→ATP+Pyruvate\text{PEP} + \text{ADP} \rightarrow \text{ATP} + \text{Pyruvate}

Isoenzymes

  • Tetramer with identical subunits

  • Two Genes: Each gene derives two transcripts via alternative transcription start sites or alternative splicing.

    • PKL Gene

      • R (Erythrocytes)

      • L (Liver, kidney, intestinal mucosa, pancreatic (\beta) cell)

    • PKM Gene

      • M1 (Muscle, heart, brain)

      • M2 (Most tissues, minor form, tumour cells, foetal form)

Kinetic and Regulatory Properties

Property

R

L

M1

M2

Kinetics wrt [PEP]

Sigmoidal

Sigmoidal

Hyperbolic

Sigmoidal

Activation by F16BP

Yes

Yes

No

Yes

Inhibition by ATP

Yes

Yes

Weak/no

Weak/no

Inhibition by amino acids

Yes

Yes

No

No

Phosphorylation by PKA

Yes

Yes

No

No

Regulation

  • Allosteric Control

    • Feedforward Activation: F16BP activates PK.

    • Feedback Inhibition: ATP, amino acids inhibit PK.

  • Phosphorylation

    • Protein Kinase A (PKA) phosphorylates and inhibits R and L isoforms.

  • Coordination of Energy Balance:

    • Regulation balances glucose use versus alternative fuels for ATP generation.

R and L Isoenzymes

  • Feedforward Activation: Glucose (\rightarrow) F6P (\rightarrow) F16BP (\rightarrow) PEP (\rightarrow) Pyruvate.

  • Feedback Inhibition: Fatty acids and amino acids inhibit.

L Isoform: Long-Term Control

  • Decreased in Starvation and Diabetes: Protein levels of the L-isoform decrease.

  • Transcriptional Control: Linked to insulin's role in maintaining the expression of transcription factor SREBP-1c.

    • SREBP-1c: Key transactivating factor in the regulation of genes involved in glucose catabolism and utilisation (e.g., glucokinase).

  • Starvation Effects: Allosteric regulation, phosphorylation, and transcriptional control (through SREBP-1c) decrease L-type pyruvate kinase activity.

Metabolic States

Fed State

During the fed state, glucose is readily available, promoting glycolysis and pathways utilizing glucose-derived intermediates such as:

  • Glycogen synthesis.

  • Pentose phosphate pathway for pentose sugar production.

  • Lipid synthesis utilizing acetyl-CoA.

  • Amino acid synthesis

  • Citric acid cycle for energy production.

Starved State

Characterized by:

  • Gluconeogenesis: Maximized conversion of amino acids to glucose.

  • Muscle Proteolysis: Increased breakdown of muscle protein to provide amino acids for gluconeogenesis.

  • Elevated Glucagon: High glucagon levels stimulate gluconeogenesis and inhibit glycolysis.

  • Decreased Insulin: Reduced insulin levels diminish the expression of enzymes involved in glucose utilization.

Hypoxia

Under hypoxic conditions, glycolysis is up-regulated to produce ATP in the absence of oxygen, this is also known as the Pasteur effect.