Differentiate between Ser/Thr kinases and Tyr kinases in terms of substrate specificity.
- Ser/Thr kinases: phosphorylate Serine/Threonine residues
- Tyr kinases: phosphorylate Tyrosine residues
Serine and threonine residues have a similar shape and can use the same enzymes, while tyrosine has a bulky aromatic ring requiring different enzymes.
What are the subfamilies of protein kinases mentioned?
- Tyrosine kinases
- STE kinases (dual-specificity kinases)
Tyrosine kinases are found almost exclusively in metazoans and choanoflagellates, indicating their early emergence in multicellular organisms.
Describe the general structure of protein kinases.
- N-terminal lobe
- C-terminal lobe
- Active site between lobes
The structure is conserved across different protein kinases.
What is required for the catalytic mechanism of protein kinases?
- Divalent cations (e.g., Mg²⁺)
- Properly positioned catalytic residues
These components are essential for ATP coordination and hydrolysis.
The active site of protein kinases binds to which two components?
- ATP
- Peptide substrates
The active site is crucial for the phosphorylation reaction.
What does the activation loop do in protein kinases?
Provides a platform for the peptide substrate
Phosphorylation of the activation loop stabilizes it in an open conformation, facilitating substrate binding.
What are the products of the phosphorylation reaction catalyzed by protein kinases?
- Phosphorylated protein
- MgADP
The reaction involves the removal of the gamma phosphate from ATP.
How do kinases recognize the correct substrate?
- Specific sequence motifs
- Docking sites away from the active site
- Modular binding domains
- Accessory proteins
These mechanisms help kinases identify and bind to their target substrates.
What are the subfamilies of protein phosphatases mentioned?
- Tyrosine phosphatases
- Serine/threonine phosphatases
The majority of protein phosphatases are tyrosine phosphatases.
What is the mechanism of Ser/Thr phosphatases?
- Bimetal active site
- Transfer of phosphoryl group to water
Metal ions coordinate and orientate the substrate for effective dephosphorylation.
Describe the reaction mechanism of tyrosine phosphatases.
- Cysteine acts as the initial nucleophile
- Formation of a covalent thiophosphate intermediate
- Nucleophilic attack by water releases phosphate
This mechanism is characteristic of Class I (Cys-based) tyrosine phosphatases.
What can Dual Specificity Phosphatases (DUSPs) dephosphorylate?
- Phospho-tyrosine
- Phospho-serine
- Phospho-threonine
- Non-protein substrates (e.g., lipids)
DUSPs play a versatile role in cellular signaling.
What is the role of PTEN in cellular signaling?
Negatively regulates the Akt/protein kinase B pathway
PTEN acts as a tumor suppressor and is often mutated in cancers.
True or false: Kinases are referred to as the ‘eraser’ in the context of phosphorylation.
FALSE
Kinases are the ‘writers’ that add phosphate groups, while phosphatases are the ‘erasers’ that remove them.
What does the Raf V600E mutation mimic?
Phosphorylation
The negative charge on glutamic acid (E) in this mutation simulates the effect of phosphorylation.
