Signal Pathways: Exhaustive Study Guide

Fundamental Principles of Signal Pathways

• Specificity: Signaling systems exhibit high specificity because signal molecules (ligands) fit into binding sites on their complementary receptors. Other signals do not fit.
• Amplification: Signal transduction involves enzymatic cascades. When an enzyme associated with a receptor is activated, it catalyzes the activation of many molecules of a second enzyme, each of which activates many molecules of a third enzyme, and so on. This results in a geometric increase in the number of affected molecules.
  • Visual progression: Signal $\rightarrow$ Receptor $\rightarrow$ Enzyme 1 $\rightarrow$ Enzyme 2 ($22$ molecules) $\rightarrow$ Enzyme 3 (represented as a vast array of active molecules).

General Mechanisms of Transmembrane Signaling

• G protein-coupled receptor (GPCR): An external ligand ($S$) binds to a receptor ($R$), which activates an intracellular G protein ($G$). This G protein then regulates an enzyme ($Enz$) that generates an intracellular second messenger.
• Receptor tyrosine kinase: Ligand binding activates the intracellular catalytic domain, which has tyrosine kinase activity. This triggers a kinase cascade.
• Receptor guanylyl cyclase: Ligand binding to the extracellular domain stimulates the formation of second messenger cyclic GMP ($cGMP$) from $GTP$.
• Gated ion channel: Opens or closes in response to the concentration of signal ligand or membrane potential.
• Adhesion receptor (integrin): Binds molecules in the extracellular matrix and changes conformation, thus altering its interaction with the cytoskeleton.
• Nuclear receptor: Hormone binding allows the receptor to regulate the expression of specific genes. This involves the hormone crossing the plasma membrane and nuclear envelope to bind the receptor, which then interacts with $DNA$ to produce $mRNA$ and eventually a new Protein.

Classification of Hormones and Signaling Molecules

• Peptide Hormones:
  • Examples: Insulin, glucagon.
  • Synthetic path: Proteolytic processing of prohormone.
  • Mode of action: Binds plasma membrane receptors; operates via second messengers.
• Catecholamines:
  • Example: Epinephrine.
  • Synthetic path: Derived from tyrosine.
  • Mode of action: Binds plasma membrane receptors; operates via second messengers.
• Eicosanoids:
  • Example: $PGE_1$ (Prostaglandin $E_1$).
  • Synthetic path: Derived from arachidonate ($20:4$ fatty acid).
  • Mode of action: Binds plasma membrane receptors; operates via second messengers.
• Steroid Hormones:
  • Example: Testosterone.
  • Synthetic path: Derived from cholesterol.
  • Mode of action: Nuclear receptors; involves transcriptional regulation.
• Vitamin D:
  • Example: $1,25$-Dihydroxycholecalciferol.
  • Synthetic path: Derived from cholesterol.
  • Mode of action: Nuclear receptors; involves transcriptional regulation.
• Retinoids:
  • Example: Retinoic acid.
  • Synthetic path: Derived from vitamin A.
  • Mode of action: Nuclear receptors; involves transcriptional regulation.
• Thyroid Hormones:
  • Example: Triiodothyronine ($T_3$).
  • Synthetic path: Derived from Tyr in thyroglobulin.
  • Mode of action: Nuclear receptors; involves transcriptional regulation.
• Nitric oxide:
  • Example: Nitric oxide.
  • Synthetic path: From arginine + $O_2$.
  • Mode of action: Cytosolic receptor (guanylyl cyclase) and second messenger ($cGMP$).

G-Protein Coupled Signaling: The Epinephrine Receptor

• Physiological Context: Epinephrine mediates the "fight or flight" response. It is produced in the adrenal glands (specifically the adrenal medulla).
• Receptor Targeting: Epinephrine binds to $\beta$-adrenergic receptors located in the liver, adipose tissue, and muscle (both skeletal and cardiac).
• Signal Components:
  • G-Protein Coupled Receptor (GPCR): An $\alpha$-helical integral membrane protein.
  • G-protein: A heterotrimeric (\alphạ\betạ̲\gamma) membrane-associated protein that binds $GTP$.

Epinephrine Receptor Structure and Metabolic Roles

• Structure: Characterized by $7$-transmembrane domains. Many analogs of epinephrine exist that can also bind to these receptors.
• Activation Effects:
  • Stimulates Glycogen breakdown.
  • Stimulates Lipolysis.
• Inhibition Effects:
  • Inhibits Glycogen synthesis.

G-Protein Signaling Mechanism

1. Epinephrine binds to the $\beta$-Adrenergic receptor.
2. The G-protein ($G_s$) exchanges $GDP$ for $GTP$.
3. The activated $\alpha$-subunit ($G_{s\alpha}$) dissociates from the \beta̲\gamma subunits.
4. The $\alpha$-subunit activates Adenylate cyclase.
5. Adenylate cyclase catalyzes the conversion of $ATP$ to Cyclic $AMP$ ($cAMP$).
6. $cAMP$ allosterically activates Protein kinase A ($PKA$).

The Ras Protein as a Prototypical G-protein

• Definition: Ras proteins act as molecular switches.
• Structural Domains:
  • P loop: Involved in phosphate binding.
  • Switch I and Switch II: Regions that change conformation based on whether $GTP$ or $GDP$ is bound.
• Key Residues and Interactions:
  • Includes $Thr35$, $Gly60$, and $Ala146$.
  • A Magnesium ion ($Mg^{2+}$) is coordinated within the binding site.
  • Binding involves the ribose and guanosine components of the nucleotide.

Activation and Amplification in G-protein Pathways

• Synthesis of cAMP: $cAMP$ acts as a secondary messenger.
• Protein Kinase A (PKA) Activation:
  • In the inactive state, PKA consists of two regulatory ($R$) subunits and two catalytic ($C$) subunits ($R_2C_2$).
  • Activation requires the binding of $4$ molecules of $cAMP$ to the $cAMP$-binding domains on the regulatory subunits.
  • This binding causes the dissociation of the regulatory subunits, releasing two active catalytic subunits ($C$).
• Quantitative Signal Amplification in Hepatocytes:
  • $1$ molecule of Epinephrine activates:
  • Adnylyl cyclase, resulting in $20$ molecules of Cyclic $AMP$.
  • $10$ molecules of Active PKA.
  • $100$ molecules of Active phosphorylase b kinase.
  • $1,000$ molecules of Active glycogen phosphorylase a.
  • This leads to the production of $10,000$ molecules of Glucose $1$-phosphate, resulting in $10,000$ molecules of Blood glucose.

Termination of the Signal (Self-Inactivation)

• GTPase Activity: The G-protein contains intrinsic $GTPase$ activity that converts bound $GTP$ back to $GDP$, thereby inactivating the $\alpha$-subunit.
• GAP (GTPase Activating Protein): Enhances the intrinsic $GTPase$ activity of the G-protein.
• cAMP Degradation: $cAMP$ is constantly hydrolyzed to terminate the signal cascade.

Insulin Signaling and Regulation

• Insulin Properties: A peptide hormone produced by the $\beta$-cells of the pancreas.
• Targets: Primarily the liver, adipose tissue, and muscle.
• Pathology: The inability to produce or sense insulin results in diabetes.
• Insulin Release Mechanism:
  • Closure of ligand-gated $K^+$ channels causes an increased positive charge inside the cell.
  • This depolarization opens $Ca^{2+}$ channels, triggering insulin release.

The Insulin Receptor Structure and Activation

• Receptor Composition:
  • $\alpha$-subunit: Located extracellularly; responsible for insulin binding.
  • $\beta$-subunit: Transmembrane; possesses tyrosine ($Tyr$) kinase activity.
• Activation Sequence:
  • Insulin binding induces receptor dimerization.
  • Conformational changes in the $\alpha$-subunit are translated to the $\beta$-subunit.
  • Autophosphorylation (cross-phosphorylation) occurs only upon dimerization.
• Kinase Domain States:
  • Inactive: The activation loop (containing $Tyr1158$, $Tyr1162$, and $Tyr1163$) blocks the substrate-binding site. $Asp1132$ is also part of this region.
  • Active: The tyrosine kinase domain becomes triply phosphorylated, moving the activation loop and allowing the target protein to enter the substrate-binding site.

The Insulin Signaling Cascade

1. Receptor Activation: Insulin binds, causing cross-phosphorylation of the receptor.
2. IRS-1 Interaction: The activated receptor phosphorylates $IRS-1$ proteins (Protein-protein interaction).
3. PI3K Activation: Phosphorylated $IRS-1$ binds and localizes Phosphoinositide $3$-kinase ($PI3K$).
4. PIP2 to PIP3 Conversion: $PI3K$ catalyzes the conversion of Phosphatidylinositol $4,5$-bisphosphate ($PIP_2$) to Phosphatidylinositol $3,4,5$-trisphosphate ($PIP_3$).
   • Chemical Equation: $PIP_2 + ATP \xrightarrow{PI3K} PIP_3 + ADP$.
5. PDK1 and Akt Activation: $PIP_3$ facilitates the interaction and activation of $PIP_3$-dependent protein kinase ($PDK1$). $PDK1$ سپس activates $Akt$ protein kinase (also known as Protein Kinase B or $PKB$).
6. Cellular Effects:
   • Activated $Akt$ leads to the phosphorylation of Glycogen synthase kinase $3$ ($GSK3$).
   • Stimulation of the $SNARE$ complex for the recruitment of $GLUT4$ glucose transporters to the cell surface.
   • Result: Increased glucose transporter expression on the cell surface and increased glucose uptake.