Session 3: Intracellular signal transduction Flashcards
Define the physiological concept “G protein”.
A nucleotide regulatory binding protein which has inherent GTPase activity and is a signal transducer.
Classify G proteins into two main groups and briefly describe the physiological function of these two groups.
- Monomeric G proteins - growth factor signal
cascades, vesicle fusion, and regulation of actin
cytoskeleton - Heterotrimeric G proteins - signal transducers
Identify (at least) 5 families of heterotrimeric G proteins.
- Gs - the alpha sub-unit activates adenyl cyclase
- Gi - mediators of hormonal inhibition of adenyl
cyclase - Gq - involved in the phospholipase C-DAG-IP3 signal
transduction pathway - Gt (transducin) - major retinal G protein that activates
cGMP phopsphodiesterase - Ga - hydrolyses GTP to GDP + Pi
Name the subunits (with their respective functions) of a typical heterotrimeric G protein.
Alpha subunit - the separated alpha subunit brings about many biological effects such as the activation of phospholipase C and adenyl cyclase.
Beta-gamma sub-unit - modulate the activity of the alpha subunit.
Briefly discuss the physiological importance of G proteins.
They act as signal transducers which convert a sensory, neural or ligand-mediated message (signal or stimulus) into an action potential or an activated signaling pathway in a sensory receptor or cell-specific receptor bearing target cell.
Name factors that influence G-protein function (regulation/control).
1. Helper proteins: Most G (GTP-binding) proteins depend on helper proteins, e.g. GAPs (GTPase Activating Proteins) promote GTP hydrolysis
- Guanine nucleotide exchange factors:
Promote GDP/GTP exchange, example
* an activated receptor
* other receptor-independent activating proteins
Briefly describe the common structural characteristics of serpentine receptors.
- Amino terminal outside the cell: modified by different
N-coupled oligosaccharides; binding of negatively
charged ligands, ligand-binding domain, often heavily
glycosylated - Hydrophobic alpha-helices: hydrophobic clusters of
amino acid residues that form hydrophobic pockets
into which small ligands fit. Amino acid residues in the
third cytoplasmic loop of the alpha helices interact
with G proteins - Cytoplasmic domain: consists of an intracellular
carboxyl terminal that contains sites where
phosphorylation can occur
List 5 examples of ligands that interact with G protein-coupled (i.e. serpentine) receptors.
- Neurotransmitters (e.g. adrenaline, noradrenaline and
dopamine, adenosine and opioids) - Peptides (e.g. glucagon)
- Glycoprotein hormones (e.g. FSH, LH, AND TSH)
- Arachidonic acid derivatives (e.g. prostaglandins)
NAP, G
List 4 specific examples of serpentine receptors.
- A1
- A2a
- A2b
- A3
Indicate the respective broad/ general functional roles of the ligand, the receptor, the G protein and the enzymes in these signal pathways.
Ligand - signal
Receptor - signal discriminator
G protein - signal transducer
Enzymes - signal amplification
List 4 membrane enzymes that can be activated in association with these pathways.
- Adenyl cyclase
- Guanyl cyclase
- Phosphodiesterase
- Phospholipase C
The G APP
State the general function of protein kinases.
They catalyse the phosphorylation of intracellular functional proteins.
Name and briefly describe the three clinically most important G protein-coupled receptor-associated signal pathways.
- Adenyl cyclase-cAMP system:
- cAMP activates protein kinase A
- a beta adrenergic receptor is coupled to a stimulatory
Gs protein which activates adenyl cyclase and
increases cAMP levels
- the alpha2 receptor is always coupled to an inhibitory
Gi protein, which inhibits adenyl cyclase and
decreases cAMP levels - Phospholipase C-glycerol-Ca++ system:
- phospholipase is activated
- PLC splits PIP2 into IP3 and DAG, which act as second
messengers
- IP3 binds to an IP3 receptor on the SR membrane and
and causes Ca++ release from the SR
- DAG and Ca++ additively activate protein kinase C
which catalyses protein phosphorylation on the serine
and threonine residue, thus eliciting a response - Ca++- calmodulin system
- The Ca-calmodulin complex can activate kinase II,
which causes phosphorylation of serine and threonine
residues
- enzymes modulated are myosin light chain kinase,
phosphodiesterases, and phosphorylase kinase
- intracellular Ca content is restored by the Ca++/H+
ATPase pump (one Ca++, 2 H+)
CAP!!!
Describe the 10 steps of activation and mechanism of action of the cAMP second messenger system and the consequences of cAMP formation.
- Ligand interacts w/ serpentine (7-helix) GPCR
- Activated receptor subject to conformational (3D)
change - Conformational in associated G protein occurs
- Nucleotide-binding site on alpha sub-unit of G
protein becomes more accessible to cytosol, where
the GTP [ ] is higher than the GDP [ ] - Alpha subunit of the G protein releases GDP which is
replaced by GTP - Substitution of GDP by GTP causes another
conformational change in the alpha subunit of the G
protein - The alpha subunit of the G protein dissociates from
the inhibitory beta-gamma complex - The dissociated (energized) alpha subunit of the G
protein can now move away to interact w/ and
activate adenylate cyclase by phosphorylating it - Adenylate cyclase can now catalyse the breakdown
of ATP to form the second messenger, cAMP - cAMP activates protein kinase A, which catalyses
the phosphorylation of various functional cellular
proteins, which alters their activity and usually
activates them by adding a high energy phosphate
group
Name the respective functions of adenyl cyclase and phosphodiesterase in signal pathways.
Adenyl cyclase: catalyses the formation of cAMP from ATP
Phosphodiesterase: catalyses the conversion of cAMP to physiologically inactive 5’AMP
