G-Coupled Receptor Signaling and Clinical Connections

G-Coupled Receptor Signaling

• A receptor alteration that constitutively binds GTP affects the alpha subunit.
• The immediate downstream consequence is a change in Adenylate cyclase that is irreversible.
• The final downstream component of G-coupled receptor signaling is CREB Transcription.
• If CREB transcription activation following receptor activation is not activated, possible alterations include:
  • PKC: DAG or Calcium release
  • PKA: cAMP
  • Alpha subunit: Receptor activation
  • CREB: Failure to translocate active subunit or PKA to nucleus; failure to phosphorylate CREB, or activate PKA.

Big Picture Clinical Connections

• Alport Syndrome involves altered type IV collagen which increases sensitivity to proteolytic enzymes.
• Beta-Thalassemia involves altered hemoglobin and increased probability of hemolytic anemia.
• Ehler's-Danlos Syndrome involves impaired collagen function and unstable collagen with hypermotility
• Menkes disease involves reduced copper absorption by the liver.
• Muscular Dystrophy involves impaired association of laminins and collagen.
• Parkinson's Disease has toxic build-up of copper.
• Wilson's Disease involves lipid metabolism and elevated sphingomyelin.

Heme Synthesis

• The final step to generate functional heme involves ferrochelatase activity and iron incorporation.
• Enzymes involved are synthase and synthase.
• 5-ALA synthase makes 5-ALA
• Key substrates are Succinyl CoA and Glycine
• Decarboxylation is seen after ring formation in both the cytosol and during transition into the mitochondrion.
• Common regulation points affect the activity of 5-ALA synthase; heme suppresses its activity

Wonderful World of Ions

• Diffussion to the cytoplasm is correlated with activation of NOS for Calcium.
• Diffusion to the cytoplasm via SGLT1 is correlated with glucose symport activity for Potassium.
• Copper is pumped into cells from outside via an anti-port pump system requiring energy.
• Iron is required for activation of some superoxide dismutases.
• Sodium binds oxygen in heme.
• Zinc is capable of generating a hydroxyl radical when reacting with hydrogen peroxide.

Pentose Phosphate Pathway

• Necessary enzymes for NADPH production include 6-phosphogluconate dehydrogenase.
• The step where Ribulose 5-phosphate is generated is considered irreversible.
• Because energetics favor progression over reversal the reversible enzymes are considered bidirectional.
• It should be expected that cells needing energy and necessary sugars for glycolysis use this pathway.

Causes and Consequences Membrane Edition

• More Cholesterol decreases fluidity.
• Less Cholesterol increases fluidity.
• More unsaturated phospholipids increase fluidity.
• More saturated phospholipids decrease fluidity.
• Changes in fluidity can increase or decrease stabilization; however stabilization is about cellular integrity not membrane integrity

Pumps, Channels, and more

• Ionophores are chemicals which spontaneously associate with membranes and act in both facilitated diffusion and increase ion movement across membranes, essentially creating a channel and/or pore.
• Concentration gradients across a semipermeable membrane creates a gradient so ions can naturally diffuse and bring material to the desired location via symport activity and enable natural diffusion through channels or proteins that can carry out enzymatic activity.
• Ways to increase signaling intensity or the probability are, the organization of receptors in lipid rafts which ensure membrane movement of the raft to the proper location and enable organization in lipid rafts to enhance dimerization; receptor activation.

Kinetics And Transport

• Diffusion is used only for very specific molecules, including O2 and CO2.
• Facilitated diffusion can mobilize other molecules without direct energy input and rate can be increased/decreased directly due to ion concentration and rate is also determined by capabilities of the transporter.
• Active transport works against ion electrochemical gradient, requires energy, and the rate is determined by energy availability and transporter capabilities.

Oxygen Affinity

• Myoglobin has a higher affinity for oxygen.
• Anerobic conditions with energy needs and increased lactic acid production; low O2 levels decreases oxygen for myoglobin
• Low pH, increased/high hydrogen; increased/high CO2; increased/high 2,3-BPG decreases oxygen affinity for hemoglobin.
• Changing all of the above decreases oxygen affinity most efficiently because the effects on oxygen affinity are additive

Active Transport

• Requires energy either as a result of high energy bonds like ATP or as a coupling result of electrochemical gradients
• ATPases are pumps that use ATP in order to drive ion movement
• F-ATPase is a coupling factor.
• V-type is vacuolar.
• P-ATPases are phosphorylated intermediates.
• Some active transport includes ATP-binding cassette(ABC) transporters which play a role in multi-drug resistance in cancer when overexpressed and when malfuntions are associate with dsieases such cystic fibrosis.

Amino Acids and More...

• Lysines are involved in collagen and elastin crosslinking.
• Valines have weak hydrophobic interactions allowing for flexibility in elastin.
• Prolines provide bends, especially in collagen.
• Glycines is necessary for collagen alpha helix stability.
• Cysteines have sulfhydryl groups, disulfide bonds, likely important in laminins.
• Tyrosines, Serine and Threonines undergo phosphorylation.

Protein Degradation And Proteolysis

• E1 activates an enzyme and initiates the entire process.
• E2 Ubiquitin Enzyme with hold the Ubiquitin for attachment to the target.
• E3 is the ligase that delivers/targets ubiquitin from E2 to the target.
• DUB is a deubiquitylating enzyme which removes Ubiquitin for reuse before degradation.
• Regulatory particle function is recognition of the ubiquitylated target and denaturation of folded proteins so it and can be cleaved properly and proteases degrade the denatured proteins

Big Picture Clinical Connections 2

• Glucose-galactose malabsorption involves alteration to SGLT-1 and the ability of intestinal epithelial cells to absorb glucose.
• Diabetes and vascular injury changes glycine of collagen and correlated increases risk with high fracture rate.
• Unstable hemoglobin causes potential jaundice and pigmented urine.
• Osteogenesis Imperfecta is correlated with changes in glycine pf collagen and correlated increases risk with high fracture rate
• Duchenne is correlated with impaired association of due to dystrophin.
• Proteus syndrome is due to overgrowth from acquired mutations in AKT.
• Sickle Cell Anemia involves altered hemoglobin with altered cellular shape.

Reactive Oxygen SPECIES

• Superoxide, hydrogen peroxide and hydroxyl radical are the 3 ROS
• Haber Weiss and Fenton, and metal-catalyzed Haber Weiss are the reactions providing the hydroxyl radical.
• Superoxide and hydrogen peroxide are both are common precursors to the hydroxyl radical.

Big Picture Connections 3

• Pertussis toxin increases cAMP levels due to increased activity; stimulation of adenylate Cyclase.
• Cholera toxin increases cAMP levels to due to loss of activity of alpha subunits.
• Loss of fatty acid synthase activity aids the prevention of progression into S phase.
• Marfan Syndrome Fibrillin has alterations decreasing integrity elastins.
• Bullous Pemphigoid disrupts hemidesmosomes as a result of disruption of hemidesmosomes and blistering are common
• Junctional epidermolysis bullosa is associated laminin and scarring.

Ions and Channels

• When pump is inactive:

  • The Potassium is in cytoplasm
  • The Sodium is ECM/Outside Cell
  • The Calcium is in cytoplasm

• Once pump is active:

  • The Potassium is in ECM/Outside Cell
  • The Sodium is in Cytoplasm
  • The Calcium is in ECM/Outside Cell
  • The newly pumped sodium can drive glucose with them

Cellular Regulation

• Hyperphosphorylation Rb
• Activation of E2F dependent transcription
• Increased activity of CDK2/increased kinase activity to drive toward S
• Fatty acids synthase activity/fatty acid synthesis

Reactive Oxygen Species Again

• Reactive nitrogen species includes nitric oxide produced directly by nitric oxide synthase.
• Reactive nitrogen species are important for inflammation, pagocytosis, immune response.
• Reperfusion after an ischemic event is a problem because it causes an influx that production (ROS causes).
• Reperfusion promotes inflammation and creates vicious cycle of damage