Protein Kinases: Cell Signaling and Phosphorylation
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Cells receive signals from outside via receptors. When a ligand binds to its receptor, a signal is transmitted as part of the signaling cascade, which may alter the function of the cell. Protein kinases play a central role in these signaling cascades by transferring phosphate groups onto target proteins.
Protein Kinase Function
- Protein kinases are enzymes that transfer a phosphate group onto a target protein, changing its function from inactive to active.
- They typically use adenosine triphosphate (ATP) as a source of phosphate. ATP is composed of adenosine and three phosphate groups.
- The phosphorylated protein and the remaining adenosine diphosphate are subsequently released by the protein kinase.
Effects of Phosphorylation
- Phosphorylation can have three possible effects on the protein: activation, deactivation, or serving as a signal for other proteins to interact with the target and pass on the signal.
Signal Switching Off
- Phosphatases are enzymes that remove the phosphate group from the target protein, leading to dephosphorylation and switching off the signal. The balance between kinase and phosphatase activity regulates phosphorylation as a reversible protein modification.
Importance of Protein Kinases
- Protein kinases are driving forces for many cellular processes, with over 500 different identified kinases.
- They are classified into various groups, families, and subfamilies based on structure and function.
- Kinases can be broadly classified according to their phosphorylation targets: tyrosine, serine, or threonine residues.
- Serine/threonine kinases can transfer phosphate groups to both serine and threonine residues.
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