Signal Transduction Pathways (G-Protein, Receptor Tyrosine Kinase, cGMP)

Signal Transduction Overview

• Signal transduction is a vital topic in biochemistry and immunology.

  • It plays a critical role in various bodily functions, including biochemistry, endocrinology, neurology, and psychiatry.

  • Understanding signal transduction is crucial for clinical applications and high-stakes exams like USMLE and COMLEX.

G Protein-Coupled Receptors (GPCRs)

Structure and Function

• GPCRs are integral membrane proteins with seven transmembrane domains.

• A signal binds to the receptor, causing a conformational change in the associated G protein, which is heterotrimeric (composed of three subunits: alpha, beta, and gamma).

Mechanism of Action

• When the signal binds:

  • The alpha subunit activates by exchanging GDP for GTP.

  • Activated alpha subunit can signal via two main pathways:

    • Gαs (Stimulatory)

      • Activates adenylyl cyclase, converting ATP to cAMP.

      • cAMP activates protein kinase A (PKA), leading to downstream effects.

    • Gαi (Inhibitory)

      • Inhibits adenylyl cyclase, preventing the formation of cAMP, thereby inhibiting PKA.

Alternative Pathway

• Gαq (Stimulatory)

  • Activates phospholipase C (PLC).

  • PLC converts PIP2 into IP3 and DAG.

  • IP3 triggers calcium release from the endoplasmic reticulum.

  • DAG activates protein kinase C (PKC), both leading to further downstream effects.

Summary of Functions

• GPCRs can activate or inhibit PKA through different pathways.

• Important for controlling various physiological processes based on receptor activation.

Hormones Associated with GPCRs

• cAMP Pathway Hormones:

  • FSH, LH, ACTH, TSH, CRH, HCG, ADH (V2), MSH, PTH, calcitonin, GHRH, glucagon, histamine (H2).

• IP3 Pathway Hormones:

  • GnRH, oxytocin, ADH (V1), TRH, histamine (H1), angiotensin II, gastrin.

Mnemonics for GPCR Pathways

• AC for craps and poker:

  • AC = adenylyl cyclase, C = cAMP, P = PKA.

Receptor Tyrosine Kinases (RTKs)

Structure and Mechanism

• RTKs are the largest class of signal transductors with intrinsic enzyme activity.

• Binding of growth factors causes dimerization of two RTKs.

Cross-Phosphorylation

• Dimerized RTKs phosphorylate each other's tyrosine residues (cross-phosphorylation).

• This creates docking sites (SH2 domains) for other signaling molecules.

Activation of RAS

• RAS binds to the SH2 domain and is activated by exchanging GDP for GTP.

• Activated RAS leads through a cascade pathway: RAS → RAF → MEK → ERK.

Amplification Mechanism

• Each kinase in the MAP kinase cascade activates further kinases, leading to an amplified response to the original signal.

Hormones Associated with RTKs

• Insulin, IGF-1, FGF, EGF.

Mnemonic for RTKs

• RTK = RAS, T (three), K (kinases)

  • Reflects RAS activation leading to three subsequent MAP kinases.

cGMP Pathway

Overview

• cGMP pathway involves nitric oxide (NO) activating guanylate cyclase.

• Guanylate cyclase converts GTP to cGMP, which activates protein kinase G (PKG).

Effects of the cGMP Pathway

• Primarily impacts smooth muscle, leading to vasodilation, which is essential in cardiovascular physiology.

Associated Hormones

• BNP and EDRF are influenced by the cGMP pathway.

Mnemonic for cGMP Pathway

• All components associated with this pathway (guanylate, cGMP, PKG) prominently feature the letter "G" except for nitric oxide, which starts the pathway.

Conclusion

• Understanding these pathways is crucial for both clinical knowledge and exam success, such as the USMLE or COMLEX. Familiarity with the mechanisms and associated hormones helps in securing high-yield points during assessments.