Chapter 9 Notes

Key Components of Cell Communication:
- Ligand: signaling molecule that binds to a receptor.
- Receptor Protein: the molecule that interacts with the ligand.
- This interaction initiates signal transduction, converting the information into a cellular response.

Receptors can be classified by their location:
1. Intracellular Receptors: located within the cell.
2. Cell Surface Receptors (or Membrane Receptors): found on the plasma membrane to bind ligands outside the cell.
- They are transmembrane proteins in contact with both the cytoplasm and extracellular environment.

Steroid Hormones:
- Typically nonpolar and lipid-soluble, allowing them to cross the plasma membrane.
- Binding to intracellular receptors causes the complex to translocate to the nucleus and regulate gene expression.

Hormones cross the plasma membrane and bind to cytoplasmic receptors.
Hormone binding alters the receptor conformation, exposing the DNA-binding site.
Hormone-receptor complex translocates to the nucleus.
Complex binds to DNA, influencing transcription.
The cellular response is a change in gene expression.

Steroid Hormone Receptor Domains:
1. Hormone-binding domain
2. DNA-binding domain
3. Domain interacting with coactivators impacting transcription levels.

Receptor Tyrosine Kinases (RTKs):
- Influence cell processes such as cell cycle and proliferation.
- Are associated with cancer when function is altered.
- Structure consists of a single transmembrane domain, an extracellular ligand-binding domain, and an intracellular kinase domain.
- Upon ligand binding:
- Dimerization (two receptors dimerize).
- Autophosphorylation occurs which leads to cellular responses.
- Phosphorylation event modifies the activity of response proteins.

Insulin receptors are a class of RTKs that, when activated, facilitate the phosphorylation of insulin receptor substrates. - This action increases glycogen synthase activity, converting glucose to glycogen, thus lower blood sugar levels.

G Protein-Coupled Receptors (GPCRs):
- Largest receptor category in animal cells.
- Act by coupling with G proteins which link receptors to effector proteins producing cellular responses.
- G proteins are active when bound to GTP and inactive when bound to GDP.

Ligand binds to GPCR, activating it.
Inactive G protein (with GDP) becomes active (with GTP) after binding to the activated receptor.
The active G protein dissociates and activates an effector protein, leading to cellular response.

Effector proteins activated by G proteins often produce secondary messengers, including:
1. Adenylyl cyclase: Converts ATP to $cAMP$ (cyclic AMP), which activates protein kinase A (PKA).
2. Phospholipase C: Cleaves phosphatidylinositol bisphosphate (PIP2) into $IP3$ (inositol trisphosphate) and $DAG$ (diacylglycerol). Both act as second messengers.

Ca2+ serves as a second messenger:
- Associated with various cellular responses by binding to calmodulin.
- Activity of IP3 releases calcium ions from the endoplasmic reticulum, aiding signal transduction.

Signal Amplification:
- Involves kinase cascades like the MAP kinase cascade, where one activated protein kinase can stimulate multiple downstream proteins.
Scaffold Proteins:
- Organize kinases into protein complexes for efficiency but can reduce amplification because each kinase can only activate the downstream molecules bound.

Receptor Subtypes: A signaling molecule can have a different effect based on the receptor type; for instance, epinephrine has 9 isoforms affecting various cellular responses via different G proteins.
RTKs and GPCRs: Both can activate the MAP kinase cascade, showcasing the flexibility of signaling pathways.

The principles of cell signaling are foundational in understanding illnesses such as cancer, diabetes, and hormonal imbalances, showcasing the importance of detailed pathways in therapeutic interventions.