Week 6: Hormonal Regulation and Signal Transduction Flashcards
What are the steps in insulin formation?
Maturation of insulin occurs while the polypeptide moves to the ER during translation
(1) Preproinsulin forms, with C, A and B peptides and a signal sequence
(2) The signal peptide is cleaved to create Proinsulin. Cysteine bonds form between the A and B peptides.
(3) The C peptide is cleaved, leaving the mature form of A and B chains linked by cysteine bonds
What are the key effects of insulin?
(1) Stimulates glycogen synthesis
(2) Uptake of glucose by muscle cells, adipocytes, brain cells, etc
(3) Promotes liver FA synthesis
(4) Promotes uptake of branched AAs, favoring muscle protein buildup
(5) Stimulates protein synthesis, inhibits intracellular protein degradation
What are the key effects of glucagon?
(1) Glycogen breakdown
(2) Inhibition of glycogen synthesis by phosphorylating glycogen synthase and phosphorylase in liver
(3) Inhibition of FA synthesis in liver by ydiminishing pyruvate production, lowering acetyl-CoA carboxylase
(4) Stimulates glucose synthesis by lowering F-2,6-BP (a proxy for F-1,6-BP that usually signals for TCA to continue when high)
(5) Activates lipase
What are the two main catecholamines?
Epinephrine and norepinephrine
What are the effects of catecholamines?
(1) Mobilization of glycogen breakdown in liver
(2) Secretion of glucagon and inhibition of insulin
(3) Inhibition of glucose uptake by muscle–switch to FAs
(4) Increase gluconeogenesis
Explain the five steps that lead to insulin activation by the beta cells of the pancreas
(1) Glucose enters pancreatic β-cells and is converted to G6P by hexokinase
(2) ATP increases from glycolysis, the TCA cycle and oxidative phosphorylation
(3) ATP blocks the K+ leak channels of the β-cell membrane, causing a buildup of positive charge inside of the cell and depolarizing it.
(4) V-gated Ca2+ channels open on the membrane, allowing Ca2+ to rush into the cell, activating insulin release from trans-Golgi membrane vesicles
What receptor is involved in insulin-resistant diabetes? Explain the basic functions of this pathway in it’s normal functioning setup.
Normally…
(1) Insulin binds to receptor tyrosine kinases on the membrane, which undergo autophosphorylation on the C-terminal tyrosine residues
(2) The receptor phosphorylates second messengers like IRS-1
(3) Signaling cascade activates transcription factors that increase expression of glucose uptake channels on cell membrane
How does exercise increase glucose uptake into cells?
As ATP is depleted during exercise and converted to AMP, AMPK (AMP kinase) becomes active, and signals to second messenger molecules and, ultimately, transcription factors, to increase GLUT4 expression INDEPENDENT of insulin. This makes the body more efficient at taking in and metabolizing glucose during exercise.
What second messenger is formed from glucagon binding to cells, and what enzyme mediates the formation of the molecule?
cAMP forms from ATP, the formation of which is mediated by adenylyl cyclase
What are the second messenger steps in the Gq pathway of GCPR activation, and what molecules are formed along the way?
(1) Gαq is released from the receptor upon ligand binding
(2) α subunit activates phospholipase C (PLC) in the cell membrane, which moves through the membrane to cleave membrane-bound PIP2 into DAG (diacylclycerol, a glyceride molecule) and IP3 (inositol trisphosphate, a sugar)
(3) DAG goes on to activate protein kinase C (PKC) which can then phosphorylate downstream effectors
(4) IP3 can move to the ER and activate Ca2+-dependent Ca2+ channels, which self-activate at low Km and self-inhibit at high Km
Explain how PKA is is constitutively inactive, and how it becomes activated
PKA is usually bound in autoinhibitory regulatory subunits, which have binding sites for cAMP. cAMP binds to the two substrate-binding sites within the regulatory subunits, releasing the regulatory proteins from the catalytic, active subunits. This opens up the substrate-binding sites and allows PKA to phosphorylate target proteins.
Explain how signal amplification works in cells
(1) Hormone/ligand binds to extracellular domain
(2) intracellular signaling from a single molecule/enzyme (adenylate cyclase), creates many (20+) molecules of a signaling protein like cAMP (x20)
(3) those 20 molecules of cAMP go on to activate many PKA enzymes (x10)
(4) PKA activates 100 molecules of phosphorylase b kinase (x100)
(5) PBK activates many glycogen phosphorylase B molecules to active glycogen phosphorylase a (x 1000)
(6) Glycogen releases many, many glucose molecules (10,000+)
What does the conversion of phosphorylase kinase b to phosphorylase kinase a do to glycogen phosphorylase and glycogen synthase enzymes?
Phosphorylation of phosphorylase kinase b to phosphorylase kinase a by PKA ACTIVATES glycogen phosphorylase a, which begins breaking down glycogen molecules into individual glucose molecules.
SIMULTANEOUSLY, it phosphorylates glycogen synthase a to glycogen synthase b, INACTIVATING it to prevent glycogen from being made
What does PKA activation of phosphoprotein phosphatase inhibitor-1 do?
Converts phosphoprotein phosphatase inhibitor-1 b to phosphoprotein phosphatase inhibitor-1 a, which inactivates phosphoprotein phosphatase
What does removal of phosphoprotein phosphatase inhibitor-1 a from phosphoprotein phosphatase-1 do?
It activates phosphoprotein phosphatase-1, which inactivates phosphorylase kinase a and activates glycogen synthase (b to a conversion) to help cells stop breaking down glycogen and start making it
What do norepinephrin and epinephrine do to adipocytes during exercise/stress?
(1) activate PKA through cAMP and adenylate second messengers
(2) PKA phosphorylates paralipin, which exposes FA droplet to the environment
(3) PKA also activates hormone-sensitive lipases, which break off and release free FAs into the bloodstream, allowing them to associate with albumin proteins and circulate to muscles and liver tissues through the bloodstream
What happens to Ac-CoA carboxylase during exercise and rest periods?
During exercise, formation of malonyl-CoA by Ac-CoA carboxylase is inhibited by cAMP phosphoylation (activated by NE/Epi). This promotes uptake of FAs by liver, and prevents FA synthesis by blocking the normal pathway by which malonyl-CoA blocks the CAT1 fatty acyl-CoA uptake channel in liver mitochondrion. This allows FA oxidation during exercise.
During rest/digest, insulin dephosphorylation of Ac-CoA carboxylase allows it to form malonyl-CoA, which blocks CAT1 channels, allowing for FA synthesis and preventing FA oxidation during in the well-fed state.
