Biochem Topic #6 (Lectures #17-19)

PTM Function

Post Translational Modifications turn proteins on/off

Phosphorylation

PO4 group added to serine threonine tyrosine (Papa always brings alcohol on his trips)

Oxidation (non-enzymatic)

Redox reactions involving Cys Sulfur oxidation

Ubiquitination

Adds protein marker for degradation in proteosome

Role of Cys Redox PTMS

Redox signaling

Protection from irreversible oxidation

Activate certain proteins

Proteins that remove ROS

Superoxide Dismutase (SOD)

Catalase (Kat)

Peroxiredoxins(Prx)

Glutathione (GSH)

Protein that reduces H2O2

Kinases

Adds phosphate

Phosphatases

Removes phosphate

G Proteins summary

Heterotrimeric

Bind GTP (guanosine nucleotides)

Active-GTP

Inactive GDP

Adrenergic signaling

Epinephrine binds to GPCRs

G protein binds to adenyl cyclase (effector enzyme)

Adenyl cyclase produces secondary messenger cAMP

cAMP activates PKA (protein kinase A)

PKA phosphorylates other (target) proteins that trigger adrenergic response ie. activation of glycogen phosphorylase-break down of stored glucose

GPCR Signaling Summary

Ligand Binds to GPCR

G Protein becomes activated

Binds to an effector enzyme that produces second messenger

Second messenger continues activation cycle

PKA Gene Transcription

PKA Activates CREB that supports transcription of certain genes

cAMP phosphodiesterase 

Removes cAMP to AMP (involved in turning off signal of G protein)

Gi

Inhibitory G protein that binds to adenyl cyclase stopping cAMP production

Self inactivation of GTPase activity 

Alpha sub unit of G protein is activated with GTP but can self hydrolyze GTP to GDP to turn itself off 

Cholera and Pertussis toxins

Lead to constant activation of adenyl cyclase

cAMP is produced which triggers sodium ions to be pumped into intestinal lumen

Additon of water and other electrolytes are also excreted to reduce sodium conc

BARK

Used to desensitize proteins in b-adrenergic signaling

B-arrestin

Removes GPCR from the cell surface to prevent over activation

RTKs function and mechanism

Similar cell signaling to GPCRs

After 2 ligand binding occurs on 2 RTK extracellular domains self-phosphorylation of kinase domain occurs

Triggers additional phosphorylation of cytosolic tyrosines

Insulin signaling pathway

Insulin receptor (RTK) binds insulin and undergoes autophosphorylation

IRS-1 is phosphorylated on Tyr residues

Phosphorylation cascade continues until Ras is activated by GTP

Ras activates Raf(activates Raf-1)

Raf-1 phosphorylates MEK

MEK activates ERK 

ERK enters nucleus and phosphorylates transcription factors like ELk1 to alter cell function 

FOXO

“Fasting Nuclear Transcription factor” looks to promote things like stored glucose breakdown and lipid utilization needs to be deactivated with presence of insulin

Insulin Signaling termination

Triggered by activation of protein tyrosine phosphatases that remove phosphorylation of INSR and IRS

Antagonists that block IRS binding