Exam 2 Flashcards
Signal transduction pathway
When extracellular signal molecule binds to the receptor protein, it activates one or more intracellular signaling pathways or systems.
System depend on intracellular signaling proteins. Their function is to process the signal inside receiving cell and distribute to intracellular targets (effector proteins).
The effector protein is altered in some way by the signal and implement appropriate change in cell behavior.
Examples of effector proteins: transcription regulators, ion channels, components of metabolic pathway or parts of the cytoskeleton.
Contact-dependent signaling
Development of complex structured (eye, immune response)
Paracrine signaling
Secretion of local mediators rhat act on cells in the local environment.
Autocrine signaling (respond to its own signal). E.g. used to reinforce developmental decision. (Cancer cells)
Synaptic signaling
Limited to neurons
Endocrine signaling
Long-distance; distribution of hormones via bloodstream.
Hormones are a signaling molecule secreted by the endocrine cell.
Describe the two major classes of signaling molecules and how signal characteristics affect receptor location
Different signal characteristicd drive receptor type
Hydrophilic signals, unable to cross the PM, use Cell-surface receptors.
Small hydrophobic signals (steroid hormones) diffuse across the plasma membrane and can activate intracellular receptors
Binding specificity
The ability to distinguish closely related substances
Agonist
Drug that binds to a receptor and activates the receptor to produce a biological response
Antagonist
Drug that binds to receptor, does not induce a response: prevents a receptor activation
Binding affinity
Defines the strength of the interaction beteeen the ligand (signaling molecule) and its receptor
Kd
Equilibrium binding constant =Kd
Kd = dissociation constant
It is a measure of the affinity if the receptor for its lifand
Kd = [Rfree][Lfree]/[RL]
Kd is = the ligand concentration at 1/2 max binding
Lower the Kd, higher the affinity
Do cells need signals to survive?
Yes
If a cell has no signal it will go through apoptosis
What is special about acetylcholine relating to cell signaling?
Acrtylcholine can have different effects on different target cells
If they:
1. Use the same receptor
Or
2. Different receptors in use but they both have the signal binding domain
Remember: An extracellular signal has little information content
How are responses to a signal molecule programmed
- Through the receptors that the target cells carry
2. Through the internal machinery to which the receptors are coupled
What are the requirements fir a signal to bind to a cell-surface receptor
Must be a transmembrane protein
Must have both extracellular and intracellular domains
Binding to the extracellular domain must lead to a molecular change inside the cell that can be recognized
What are the 3 major classes of cell-surface receptors?
- Ion-channel coupled receptors
- gated ion channels - G-protein couples receptors.
- work via trimerix G-proteins
- associated with vision and smell - Enzyme coupled receptors
- enzyme = kinase
What are the 3 main types of molecular switches
- Ligand binding: allostery, allosteric proteins. (Structural change occurs = functional change)
- Phosphorylation
- The protein kinase adds phosphate groups
- The protein phosphatase removes phosphate groups - GTP-binding proteins
What are the two main groups of protein kinases?
Serine/threonine kinases
Tyrosine kinases
What are the two major types of G proteins?
Large trimeric G-proteins: help relay signals from the G-protein receptors that activate them
Small monomeric G-protein: help relay signals from many classes of cell-surface receptors.
What is the function of Guanine nucleotide-dissociation inhibitors (GDIs)
Can hold G-protein inactive by inhibiting the release of a bound GDP
Function of GTPase-activating proteins
Regulators of G protein signaling (RGS).
Promote dephosphorylation to GDP
Function of Guanine nucleotide-exchange factors (GEFs)
Promote bonding of GTP
Function of scaffolding proteins
Bring groups of interacting signaling proteins together.
This decreases the possibility of activating the wrong protein
Function of modular interaction domains
They mediate interaction between intracellular signaling proteins
Describe the major characteristics of a G-protein coupled receptor
Largest family of cell surface receptors
Examples: light, odorants, peptides (neurotransmitters), hormones
Structure is single polypeptide that is a multi pass transmembrane protein with 7 transmembrane domains
Activation of the receptor creates binding site for trimetic G-protein on intercellular side
Interaction of the G-protein with the receptor activates the G protein
Describe the major characteristics of Trimeric G-Protein
Composed of 3 polypeptides (subunits) 
Function in a similar fashion to mono medic GTP-binding proteins
Covalently attached lipids found on both the alpha and y subunits
Process of G-protein activation
- Ligand (signaling molecule) binds receptor activating it which causes conformational change that exposes a binding site for G-protein
- G-protein binds: Receptor acts as the GEF (GDP is ejected, GTP binds)
- GTP binding activated the alpha subunit causing it to dissociate from the Beta/gamma subunit. This activated the beta/gamma.
- Both subunits can activate target proteins
- GTPase activity of alpha subunit, hydrolyses GRP to GDP, inactivates alpha subunit, which reassociates with beta gamma subunit
Key concept: both alpha subunit and beta/gamma subunit can relay a signal
I’m GPCR pathways, effector proteins are either….
Enzymes that catalyze the formation of second messengers or ion channels
Describe the role of Adenylyl Cyclase
It is an effector protein. An enzyme that catalyzes formation of 2 messengers
Produces cAMP
Membrane bound enzyme
Converts ATP into cAMP which is a ubiquitous intercellular messenger (2nd messenger)
cAMP works as a ligand to activate other target proteins
In order to function as a messenger cells need to be able to quickly change its concentration therefore rapid synthesis and rapid degradation can occur
What mediates the effects of cAMP
Protein Kinase A (PKA)
Tetramers: 2 regulatory and 2 catalytic domains
Binding of cAMP causes dissociation of the two catalytic domains
Unmasks the active sites: activated
Binding of cAMP to the regulatory subunits is cooperative. Binding of the first cAMP lowers the Kd for binding of the second!
This means that small changes in level of cytosolic cAMP can cause proportionately large changes in the amount of active PKA
PKA
is a serine/threonine Kinase
Will phosphorylate different proteins in different cell types
Characteristics of signal that binds to cell surface receptor
Transmembrane
Extra cellular and intracellular domain
Causes molecular change that can be recognized inside cell
The 3 major classes of cell surface receptors
Ion channel receptor
G protein coupled receptor
Enzyme coupled receptor
GPCR
Single poly peptide
Transmembrane multi pass
Activation of it creates binding site for trimeric G protein
Interaction of the G protein with receptor activated the G protein
Trimeric G protein
3 polypeptides (alpha subunit and beta/gamma sub unit)
Covalently attached lipids found on both the alpha and gamma subunits
Effector proteins of GPCR pathways
Enzymes that catalyze formation of second messengers or ion channels
Adenylate Cyclase
Produces cAMP
Membrane bound
Converts ATP into cAMP
cAMP
2nd messenger
Works as ligand to activate other target proteins
PKA
binding of cAMP causes dissociation of the two catalytic domains: becomes active
Small changes in level of cAMP can cause large changes in amount of active PKA
PKA is a regulator of cAMP
Phospholipase C
Enzyme that cleaves PI 4,5 biphosphate to produce IP3 and DAG
IP3 and DAG
2nd messengers
IP3 opens Ca++ channels
DAG activates protein kinase C
Ca++
Acts as an allosteric effector (ligand)
Calmodulin
Enzymes
ION pumps
Protein kinases
Dark adapted cells
Have open channels that are depolarized
Increased levels of cGMP
RTKs
Important in responses to signals that promote growth
Signal binding (activation) causes the receptors to dimerize. Diners can cross-phosphorylate
Phosphorylated tyrosines are docking sites for proteins with SH2 and PTB
SH3 domains: protein/protein
Adaptor proteins
Composed almost entirely of SH2 and SH3
Explain how Ras is activated (RTK)
Active when bound to GTP
activation of RAS by RTK
The activated receptor is bound by Grb2 (adaptor protein)
grb2 is bound by the RAS-gef (sos)
SOS activates RAS
RAS can activate MAPkinase modules
Mitogens are signals that stimulate cell proliferation
Kinase cascades allow signal amplification
Akt
Critical for cell proliferation and survival
