Study Notes on Enzyme Coupled Receptors

Enzyme-coupled receptors play a key role in cellular signal transduction, allowing cells to react to external signals.

Transmembrane Domain: Each subunit of the receptor contains one transmembrane domain.
Largest Class: The most prominent type of enzyme-coupled receptor is the Receptor Tyrosine Kinases (RTKs), characterized by their intrinsic enzyme activity.
Types of Enzyme Activity: RTKs exhibit intrinsic enzyme activity and can associate with other enzyme activity.

Signal Protein Family and Corresponding Receptors:
- Epidermal Growth Factor (EGF): EGF receptors - Stimulates cell survival, growth, proliferation, or differentiation; acts as inductive signal in development.
- Insulin: Insulin receptors - Stimulates carbohydrate utilization and protein synthesis.
- Insulin-like Growth Factor (IGF1): IGF receptor-1 - Stimulates cell growth and survival in many cell types.
- Nerve Growth Factor (NGF): Trk receptors - Stimulates survival and growth of some neurons.
- Platelet-Derived Growth Factor (PDGF): PDGF receptors - Stimulates survival, growth, proliferation, and migration of various cell types.
- Macrophage Colony-Stimulating Factor (MCSF): MCSF receptor - Stimulates monocyte/macrophage proliferation and differentiation.
- Fibroblast Growth Factor (FGF): FGF receptors - Stimulates proliferation of various cell types; inhibits differentiation of some precursor cells; acts as an inductive signal in development.
- Vascular Endothelial Growth Factor (VEGF): VEGF receptors - Stimulates angiogenesis.
- Ephrin: Eph receptors - Stimulates angiogenesis; guides cell and axon migration.

Ligand-Mediated Dimerization: Binding of a ligand causes the receptors to form dimers.
Receptor-Mediated Dimerization: The inactive RTKs can also dimerize without a ligand.
Trans-autophosphorylation: Upon dimerization, the kinase domains undergo autophosphorylation, which activates them and generates binding sites for signaling proteins, ultimately relaying the signal.

Inactive Monomers: Initially, RTKs exist as inactive monomers.
Ligand Binding: The ligand promotes dimerization.
Active Dimers: The dimerized receptors now show increased activity.
Trans-autophosphorylation: The receptor kinases phosphorylate each other, leading to active dimers and subsequent signaling.

Proteins associated with signaling through RTKs possess specialized interaction domains, including:
- SH2 (Src Homology 2) Domain: Most common in various signaling proteins.
- PTB (Phosphotyrosine Binding) Domain: Another domain interacting with phosphorylated tyrosines.
Assembly of Signaling Complexes: Includes adaptor proteins, docking proteins, signaling enzymes, and transcription factors.

Formation of Signaling Complex: Dominated by transient interactions where some proteins propagate the signal while others act as scaffolds.
Multiple Responses: Signal transmission can lead to a variety of biochemical changes across different cellular processes.

Dephosphorylation: Mediated by tyrosine phosphatases reduces the signaling intensity.
Internalization: Mechanisms may involve receptor internalization often through ubiquitin tagging (Cbl ubiquitinates the active receptor).

Mutations affecting c-Cbl-dependent downregulation can lead to prolonged signaling, implicated in cancer development.

Ras-MAP Kinase Cascade: Key pathway activated by adaptor proteins like Grb2 (growth factor receptor-bound protein 2), leading to the activation of MAP kinases.
Role of MAP Kinase: Initially phosphorylated and activated by upstream kinases, resulting in significant cellular responses such as changes in gene expression and activity of various proteins.
Specificity and Crosstalk: Achieved through spatial localization of components, minimizing unintended interactions (crosstalk) among different signaling pathways.

PI3K Activation: Insulin signaling leads to the activation of PI3-kinase, which produces PIP3 from PIP2, resulting in the activation of Akt.
Function of Akt: Promotes cell growth and survival, inhibits apoptosis through phosphorylation events.
Roles of PTEN: A tumor suppressor gene that can deactivate PIP3, therefore negatively regulating AKT signaling. Mutations in PTEN are associated with various cancers.
mtor Activation: Akt indirectly activates mTOR, contributing to growth and metabolism through regulation of protein synthesis.

Cytokine receptors lack intrinsic enzymatic activity; instead, they associate with Janus kinases (JAKs) to propagate signaling.
JAKs phosphorylate receptors, recruiting STAT proteins that then dimerize and enter the nucleus to regulate transcription.
Regulation of JAK/STAT: Inhibitors such as the PIAS and SOCS can down-regulate this signaling, ensuring responses are finely tuned.

Transforming Growth Factor Beta (TGF-β): Activation leads to different signaling mechanisms mainly involving serine/threonine kinases.
Non-canonical vs Canonical Signaling: Signaling pathways diverge depending on receptor activation and resulting pathways impacting cellular functions such as growth, differentiation, and apoptosis.

There is significant cross-talk among pathways (JAK/STAT, TGF-β, etc.), where components may interact to modulate processes such as gene transcription and cellular responses.
- Interactions can involve complex cascades, recruitment of additional factors leading to multifaceted cellular responses.

Enzyme-coupled receptors, primarily RTKs, are crucial in signal transduction pathways affecting multiple biological processes, and their dysregulation can lead to severe pathologies, including cancer.