Biochem exam 4

Signal transduction

process by which a cell converts a signal into a cellular response

receptor proteins

a protein that stimulates a cellular response after protein structural changes initiated by a ligand

effects of activating a receptor protein involves one of these responses

1. covalent protein modifications, such as phosphorylation or dephosphorylation reactions

2. protein conformational changes from high-affinity non covalent binding

3. altered rates of protein expression; can be at transcriptional or translational level

first messenger

extracellular signal that binds to a receptor ex: epinephrine

receptor

detects the signal and initiates signaling ex: B-2 adrenergic receptor

intracellular signaling

relays or regulates the signal ex: G protein or protein kinase

target protein

changes activity to produce a response ex: metabolic enzyme

upstream events

early steps in signaling pathway

downstream events

later steps, such as target protein modifications

endocrine

travels through bloodstream to distant target cells

paracrine

acts on nearby cells

autocrine

acts on the cell that released it

hormones

biologically active compounds that are released into the circulatory system, are considered first messengers because they initiate the receptor-activating signal that gives rise to a physiological response

first messengers can be:

peptide hormones, soluble gasses, or neurotransmitters

nitric oxide

NO→GC(GTP→cGMP→PKG→vasodilation

(cGMP→GMP) (cGMP PDE inhibits this)

second messengers

small, nonprotein intracellular molecules that amplify receptor-generated signals

ex: cyclic GMP (cGMP), cAMP, DAG, IP₃, Ca²⁺

AC Proteins

Adenylate cyclase

enzyme that produces cAMP

β2-AR Protein

β2-adrenergic receptor

membrane-spanning receptor that binds epinephrine

βARK protein

B-adrenergic receptor kinase

specific G protein-coupled receptor kinase enzyme that acts on β-adrenergic receptors

Gα proteins

Gα protein

G protein subunit with GTPase activity

Gαβγ proteins

Heterotrimeric G protein

complex of Ga, GB, and GY subunits associated with GPCRs

GAP proteins

GTPase activating protein

protein that stimulates GTPase activity of G proteins to inhibit signaling

accelerates GTP hydrolysis

GEF proteins

Guanine nucleotide exchange factor

protein that stimulates GDP-GTP exchange to activate signaling

GPCR protein

G protein-coupled receptor

membrane spanning receptor associated with intracellular G proteins

GRK protein

G protein-coupled receptor kinase

Enzyme that phosphorylates GPCRs

PDE protein

Phosphodiesterase

Enzyme that breaks down cAMP or cGMP

PH protein

Pleckstrin homology domain

adaptor domain that binds membrane lipids

PKA protein

Protein kinase A

cAMP-dependent enzyme with kinase activity

PKC protein

Protein kinase C

Lipid-activated enzyme with kinase activity

PLC protein

Phospholipase C

Enzyme that produces diaglycerol and inositil triphosphate

RGS protein

Regulator of G protein signaling

GTPase activating protein that is associated with GPCRs

Amplification

occurs when one activated component acts on many molecules at the next step

PIP₂

Phospholipid in plasma membrane

Phospholipase C

cleaves PIP₂ to produce DAG and IP₃

DAG

remains in membrane (nonplar)

IP₃

moves through membrane (polar)

phosphoinositide cascade

IP₃ stimulates Ca²⁺ and releases from endoplastic reticulum

Ca²⁺ binds proteins such as calmodoulin to regulate activity

DAG and Ca²⁺ together activate conventional PKC isoforms

GPCR receptor

activates heterotrimeric G proteins

Receptor tyrosine kinase

uses intrinsic kinase activity to phosphorylate tyrosine residues

TNF receptor (cell death)

recruits adapter proteins that organize downstream signaling

nuclear receptor

regulates transcription through interactions with DNA and regulatory proteins

ligand-gated ion channel

changes ion flow when a ligand binds

detection

a ligand activates an appropriate receptor

transmission

signaling proteins, second messengers, or ion movement carry the signal

response

target proteins alter metabolism, gene expression, transport, or cell behavior

amplification

one activated component can affect many downstream signals

termination

signal removal and pathway inactivation limit the response

Heterotrimeric

means three different subunits, alpha, beta and gamma

GPCRs meditate responses to hormones, neurotransmitters, and sensory stimuli by

containing seven transmembrane alpha helices

ligand binding changes the receptors conformation

the activated receptor promotes activation of an intracellular heterotrimeric G protein

β-2 adrenergic receptor (GPCR) process

epinephrine binds to receptor and conformational changes

epinephrine binds extracellular side of receptor

receptor helices change positions

intracellular surface adopts conformation that promotes G protein activation

receptor agonists

activate receptor signaling by mimicking natural ligand

receptor antagonist

bind to receptors with high affinity and block binding of physiological agonists without promoting structural changes needed for signal transduction

Glucagon signaling

receptor: glucagon receptor

signaling: G_sα

second messenger: cAMP

enzyme: adenylate cyclase

epinephrine signaling pathway 1

receptor: β2-adrenergic receptor

signaling: G_sα

second messenger: cAMP

enzyme: adenylate cyclase

epinephrine pathway 2

receptor: α1-adrenergic receptor

signaling: G_qα

second messenger: DAG, IP₃ followed by increased cytosolic Ca²⁺

^{enzyme: phospholipase C}

G_s

effector: adenylyl cyclase

increases cAMP

G_i

effector: adenylyl cyclase

decreases cAMP

G_q

effector: phospholipase C

produces IP₃ and DAG

glucagon

opposite of insulin, increases blood glucose levels

GPCR step by step activation

1. ligand binds to GPCR and changes conformation

2. the activated receptor promotes GDP release from Gα

3. GTP binds Gα

4. Gα-GTP and GβY regulate downstream effectors

5. Gα hydrolyzes GTP to GDP, ending active state

GDP-GTP exchange activates the switch, GTP hydrolysis resets it

Protein kinase A (PKA) system

cAMP activates PKA
PKA phosphorylates specific target proteins

what does PKA signaling do in liver

promotes glycogen breakdown and inhibits glycogen synthesis

activation of protein kinase A steps

-inactive PKA contains 2 regulatory and 2 catalytic subunits

-cAMP binds to regulatory subunits

-cAMP binding releases active catalytic subunits

-catalytic subunits phosphorylate target proteins

-cAMP binds regulatory subunits; ATP supplies the phosphate used by the catalytic subunit

G_s-cAMP-PKA signaling in liver steps

1. G_s activates adenylyl cyclase

2. cAMP activates PKA

3. target phosphorylation promotes glucose availability

SH1

kinase activity domain in src kinase

SH2

domain in src kinase that binds phosphorylated tyrosine

SH3

domain in src kinase that binds proline-rich sequences

GTP hydrolysis inactivates what

Gα

What removes cyclic nucleotide second messengers

Phosphodiesterases

what reverses target phosphorylation

protein phosphatases

GRK (G protein coupled receptor kinases) phosphorylate the GPCR cytoplasmic domain on what residues

Ser and Thr

receptor phosphorylation and arrestin binding reduce what?

further G protein activation

G protein activation

the receptor promotes GDP-GTP exchange on Gα

How can the same ligand produce different responses?

through different receptor subtypes and intracellular targets

βARK, a GRK

β-adrenergic receptor kinase

β-arrestin binds the phosphorylated receptor, reducing its coupling to G proteins and promotes internalization

What are RTKs

membrane receptors with an intracellular tyrosine kinase domain

what does ligand binding do in RTKs

changes receptor organization or conformation and activates kinase activity

How are many RTKs activated

through ligand-induced dimerization

EGF receptor

responds to epidermal growth factor

Insulin receptor

reponds to insulin

Growth factors binding to RTK steps

1. ligand binding promotes receptor activation

2. receptor subunits phosphorylate one another

3. signaling proteins bind phosphorylated sites and initiate downstream pathways. phosphorylation can both regulate kinase activity and create docking sites

How does EGF activate

EGF binds to EGFR and stimulates receptor dimerization cell surface and kinase activation

what does GRB2 bind to in EGF pathway

SOS which then promotes GDP release from RAS and GTP binds RAS then RAS→RAF→MEK→ERK

what is RAS

a small GTPase anchored to the cytoplasmic surface of plasma membrane

Ras-GDP is inactive

Ras-GTP is active

what helps return Ras to inactive state

GAP proteins because they accelerate GTP hydrolysis

what are RAF MEK and ERK

phosphorylations

what are ERKs involved with

regulating proteins involved in gene expression and other cellular responses

are Ras Raf Mek and Erk kinases or GTPases

Ras is GTPase, others are kinases

what can persistent Ras activity cause

sustain growth signaling by mutations causing an impairment of returning GTP Ras to GDP-bound state