Mechanism and significance of reversible phosphorylation in signal transduction.

Reversible phosphorylation is one of the most critical regulatory mechanisms in eukaryotic cells. It refers to the addition (phosphorylation) and removal (dephosphorylation) of phosphate groups on proteins, catalyzed by protein kinases and protein phosphatases, respectively. This post-translational modification functions as a molecular switch in numerous biological processes, particularly in signal transduction pathways.

It enables cells to rapidly transmit, amplify, and fine-tune external signals to produce precise intracellular responses. Over 30% of cellular proteins are regulated by phosphorylation, underscoring its central importance.

Basic mechanism

The phosphorylation process involves the covalent attachment of a phosphate group (PO₄³⁻) to the hydroxyl group (-OH) of specific amino acid side chains, most commonly serine, threonine, or tyrosine residues in proteins.

• Phosphorylation – Catalyzed by protein kinases, using ATP as the phosphate donor.
• Dephosphorylation – Catalyzed by protein phosphatases, which hydrolyze the phosphate group.

• Protein phosphorylation cascade.svg

  Phosphorylation cascade in signal transduction

• ATP-phosphorylation.svg

  ATP-dependent phosphorylation of proteins

Enzymes involved

• Protein kinases
  • Function: Add phosphate groups to proteins.
  • Types:
    • Serine/threonine kinases (e.g., PKA, PKC, MAPK)
    • Tyrosine kinases (e.g., insulin receptor, EGFR)
  • Activation: Often activated by second messengers like cAMP or Ca²⁺.

• Protein phosphatases
  • Function: Remove phosphate groups.
  • Types:
    • Ser/Thr phosphatases (e.g., PP1, PP2A)
    • Tyrosine phosphatases (e.g., SHP1, SHP2)
  • Regulation: Can themselves be phosphorylated or inhibited by small molecules.

Process in signal transduction

1. An external signal (e.g., hormone, growth factor) binds to a membrane receptor (e.g., GPCR, RTK).
2. The receptor activates intracellular kinases directly or via second messengers.
3. Target proteins are phosphorylated, changing their activity, location, or binding affinity.
4. A biological response is generated: enzyme activation, gene transcription, cytoskeletal rearrangement, etc.
5. Phosphatases remove the phosphate groups to terminate or regulate the signal.

Biological significance

Phosphorylation allows rapid, reversible, and energy-efficient control of protein function and cellular behavior. Key roles include:

• Signal amplification – One activated kinase can phosphorylate hundreds of downstream targets.
• Regulation of enzyme activity – Turns metabolic enzymes on/off.
• Gene expression – Activates transcription factors (e.g., CREB, STATs).
• Cell cycle control – Cyclin-dependent kinases (CDKs) regulate phase transitions.
• Apoptosis – Pro- and anti-apoptotic factors are regulated by phosphorylation.
• Cellular localization – Some proteins only enter the nucleus or bind membranes when phosphorylated.

Clinical relevance

Abnormal phosphorylation is implicated in many diseases:

• Cancer – Constitutively active tyrosine kinases (e.g., BCR-ABL fusion protein).
• Type 2 diabetes mellitus – Impaired insulin signaling due to disrupted phosphorylation.
• Alzheimer’s disease – Hyperphosphorylation of tau protein leads to neurofibrillary tangles.
• Cardiac diseases – Altered phosphorylation affects ion channel function in arrhythmias.

Many drugs are designed to target specific kinases or phosphatases:

• Imatinib – inhibits BCR-ABL in chronic myeloid leukemia.
• Trastuzumab – targets HER2, a receptor tyrosine kinase in breast cancer.

Pathways involving phosphorylation

• MAPK/ERK pathway – cell proliferation and differentiation.
• PI3K/AKT pathway – metabolism, survival.
• JAK-STAT pathway – cytokine signaling.
• cAMP pathway – regulates metabolism via PKA.

Examples

• Glycogen metabolism – Phosphorylation of glycogen synthase inactivates it, while phosphorylation of glycogen phosphorylase activates it.
• CREB activation – cAMP pathway leads to phosphorylation of CREB, a transcription factor that activates genes in the nucleus.
• RTK signaling – Growth factors activate receptor tyrosine kinases which autophosphorylate and trigger downstream cascades.

Summary table

Component	Function	Example
Protein kinase	Adds phosphate	PKA, MAPK, EGFR
Protein phosphatase	Removes phosphate	PP2A, SHP2
Second messenger	Activates kinases	cAMP, DAG, IP₃
Target protein	Biological response	Enzyme, transcription factor

References

Related articles

• Signal transduction
• Protein kinase
• Protein phosphatase
• Second messengers
• G-protein-coupled receptor
• Receptor tyrosine kinase
• Cell cycle
• MAPK/ERK pathway

Literature

• ALBERTS, Bruce. Molecular Biology of the Cell. 6. edition. New York : Garland Science, 2015. 875–900 pp. ISBN 978-0-8153-3218-3.

• BERG, Jeremy M. – TYMOCZKO, John L.. Biochemistry. 8. edition. New York : W.H. Freeman, 2015. 432–445 pp. ISBN 978-1-4641-2610-7.