G Proteins

G Protein Complex

1. Binding of ligand stimulates alpha subunit to release GDP and bind GTP

2. Activated by GTP bound alpha subunit dissociates from beta gamma complex

3. Both alpha and beta gamma can now regulate downstream effectors

G Proteins

1. Binding of ligand induces conformational change in receptor

2. Activated receptor binds alpha subunit

3. Activated receptor causes conformational change in alpha subunit, triggering dissociation of GDP (Guanine nucleotide exchange)

4. Binding of GTP to alpha subunit tirggers dissociation of alpha subunit both from receptor and beta and gamma units

5. Hormones dissociates from receptor; alpha subunit binds to effector, activating it

6. Hydrolysis of GTP to GDP causes alpha to dissociate from effector and reassociate with beta and gamma

Adenylyl Cyclase

cAMP is second messenger (increased) with beta adrenergic receptors (alpha s)

G alpha s

Adenylyl Cyclase K+ channel (beta gamma subunit activates effector), with cAMP decreases changes membrane potential using a muscarinic ACh receptor

Second Messengers

Diffusible molecules are used to relay a signal within a cell and are synthesized or released by specific enzymatic reactions performed by “effectors”. Short-lived and have localized production, permitting transient signal transduction

cAMP in Liver

Increase in conversion of glycogen to glucose; inhibition of glycogen synthesis; increase aa acid uptake; increase in gluconeogenesis (glucagon)

cAMP in Cardiac Muscle

Increase in contraction rate

cAMP in Intestine

Fluid secretion

cAMP Function

Stimulate protein kinase transfer of gamma phosphate from ATP to AA (serine, threonine, or tyrosine)

AA Capable of Esterification

Serine, threonine, or tyrosine (available OH group)

cAMP

Increases the activity of cAMP-dependent Protein Kinase (PKA)

Protein Kinase A

Inhibits glycogen synthase and activates phosphorylase kinase

Phosphorylase Kinase

Activates phosphorylase (b)

Phosphorylase (b)

Breaks down glycogen

Glucagon Breakdown

1. cAMP activates protein kinase A

2. Protein Kinase A activates phosphorylase kinase and inhibits glycogen synthase

3. Phosphorylase kinase activates phosphylase a

4. Phosphorylase a breaks down glycogen.

SNS (e) and Glucagon

Both stimulate glycogen breakdown to glucose through G_a-cAMP-PKA

Bacterial Toxins

Cholera toxin (CT) and pertussis toxin (PT) targets G_as and G_ai to regulate adenylyl cyclase and cAMp levels

Pertussis Toxin

Blocks dissociation of GDP from alpha i subunit

Cholera Toxin

Inhibits GTPase activity of the alpha s subunit, therefore not allowing for the release of GTP and reforming units with beta gamma subunits.

Pertussis Toxin

Blocks dissociation of GDP from alpha i, causing an increase in cAMP. G_ai usually decreases cAMP but it is inhibited in this pathway. 

G_aq

Effector enzyme phospholipase C with second messenger that is IP3, DAG (increased), with a alpha-1-adrenergic receptor.

Diacylglycerol

Activates protein kinase C

IP3

Releases Ca2+ from the ER

Phospholipase 

Cleaves and produces diacylglycerol and IP3

Gq Complex

1. Signal molecule activates GPCR

2. Activates Gq protein

3. Activates phospholipase C, which makes diacylglycerol and IP3

4. IP3 opens Ca2+ channels and diacylglycerol binds protein kinase C

5. Protein kinase C activated

ACh

Stimulates secretion from salivary glands and epithelial cells through G-protein coupled receptor linked to G_q.

Endothelin-A

Reduced receptor signaling results in ariculo-condylar syndrome (ARCND)

Tyrosine Kinase Activity

Receptors signal machines that emerged in multicellular organisms. 

Epidermal Growth Factor (EGF)

EGF receptors stimulate cell survival, growth, proliferation, or differentiation of various cell types.

Insulin

Insulin receptor stimulating carbohydrate utilization of protein synthesis 

Fibroblast Growth Factor (FGF)

FGF receptors stimulate the proliferation of various cell types; inhibit the differentiation of some precursor cells, acting as inductive signals in development

Receptor Tyrosine Kinase (RTK)

Signal molecules (could be dimer) dimmers with receptor becoming a dimer.

Phospho-Tyrosine Residue

Cross-phosphorylation by activated kinase domains, serves as signal to recruit next level signaling.

Tyrosine Phosphates

Intracellular signaling proteins bind and activate, relaying signal downstream

Cycling of Ras

1. Ras is inactive with GDP bound

2. Upstream stimulatory signal and Ras are activated by GEF (receptor tyrosine kinase). GEF releases GDP

3. Cause GTP to bind Ras (active) causing downstream signal

4. GAP induces GTP hydrolysis and Ras inactivation (bound to GDP)

Guanine Nucleotide Exchange Factor (GEF)

Removes GDP from Ras to activate it

GTPase Activating Protein (GAP)

Assists Ras GTP hydrolysis to inactivate it by GDP binding.

MAP Kinase Cascade

1. Active Ras stimulates MAP-kinase³, which activates MAP-kinase² (ATP → ADP), which activates MAP-kinase

2. Changes protein activity or gene expression

Type I Diabetes

Loss of pancreatic beta cells (insulin-secreting), resulting in loss of insulin, loss of control of blood glucose 

Type II Diabetes

Mediated by insulin resistance, loss of control of blood glucose, and insulin rates are high but beta cells in pancreas are burned out

FGFR Inhibitor

Decrease salivary gland growth

Mutations in Fibroblast Growth

Activation causes craniosynostosis syndrome as well as dwarfism. FGFR mutations fix receptor kinase activity in the “on” position. Proliferation of chondrocytes is inhibited, hypertrophy accelerated and the growth plates do not expand normally.

Missense PDGFRA

Nonsyndromic cleft palate

SNPs in PDGFC Regulatory Region

Repress transcriptional activty of promoter associated with cleft lip and palate

Mutations in PDGFRB

Causes kosaki overgrowth syndrome and pentinnen syndrome.

Noonan Syndorame

Related syndromes result from excessive RAS and MAPK signaling.

NF1

RAS-GAP

Conserved Themes in Reversible Signaling

Signaling by phosphorylation (signaling in)

Signaling by GTP-binding (singal in)