PS271 Lecture Notes - Peptide, The Shortcut, Cytosol

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1 Feb 2013
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The Basic Structure of Transmitter-Gated Channels
Four different types of polypeptides are used as subunits for the nicotinic receptor, and they are designated as: β, α, δ,
and γ.
A complete mature channel is made from two α subunits, and one each of β, δ and γ.
There is one ACh binding site on each of the α subunits; the simultaneous binding of ACh to both sites is required for the
channel to open
The structure of glutamate receptors resembles that of potassium channels, and this has inspired the surprising
hypothesis that glutamate receptors and potassium channels evolved from a common ancestral ion channel
Amino Acid-Gated Channels
Amino acid-gated channels mediate most of the fast synaptic transmission in the CNS
Several properties of these channels distinguish them from one another:
o Pharmacology of their binding sites describes which transmitters affect them and how drugs interact with them
o Kinetics of the transmitter binding process and channel gating determine the duration of their effect
o Selectivity of the ion channels determines whether they produce excitation or inhibition and whether Ca2+
enters the cell in significant amounts
o Conductance of open channels helps determine the magnitude of their effects
Glutamate-Gated Channels:
3 Glutamate Receptor Subtypes: AMPA, NMDA, and Kainate
AMPA-gated and NMDA-gated channels mediate the bulk of fast excitatory synaptic transmission in the brain
AMPA-gated channels are permeable to both sodium and potassium but not calcium
AMPA channels can be activated by a rapid increase in sodium ions into the cell, thus depolarizing the cells
AMPA receptors coexist with NMDA receptors at many synapses in the brain, so most glutamate-mediated EPSPs have
components contributed by both
NMDA-gated channels also cause excitation of a cell by admitting sodium, but they differ from AMPA receptors in two
very important ways:
o NMDA-gated channels are permeable to calcium
o Inward ionic current through NMDA-gated channels is voltage dependent
If excessive amounts of calcium are released, it can trigger the death of a cell
Activation of NMDA receptors can cause widespread and lasting changes in the postsynaptic neuron
Inward ionic current through the NMDA channel is voltage dependent
G-Protein-Coupled Receptors and Effectors
Transmission at the G-protein-coupled receptors involves 3 steps:
o Binding of the neurotransmitter to the receptor protein
o Activation of G-proteins
o Activation of effector systems
The Basic Structure of G-Protein-Coupled Receptors
Most G-protein-coupled receptors are simple variations on a common plan, consisting of a single polypeptide containing
seven membrane-spanning alpha helices
Two of the extracellular loops of the polypeptide form the transmitter binding sites
Structural variations in this region determine which neurotransmitters, agonists, and antagonists bind to the receptor
Two of the intracellular loops can bind to and activate G-proteins and, consequently, which effector systems are
activated in response to transmitter binding
The Ubiquitous G-Proteins:
G-Protein = Guanosine Triphosphate (GTP) binding protein
Basic Mode of operation of a G-Protein [Fig. 6.24, Pg. 159]
o In the normal inactive state, a GDP from the cytosol binds to the alpha subunit of the G-Protein.
o This GDP binding causes the G-Protein to bind to a coupled receptor which in turn causes the G-Protein to
release the GDP and exchange it for a GTP from the cytosol.
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Document Summary

Four different types of polypeptides are used as subunits for the nicotinic receptor, and they are designated as: , , , and . A complete mature channel is made from two subunits, and one each of , and . There is one ach binding site on each of the subunits; the simultaneous binding of ach to both sites is required for the channel to open. The structure of glutamate receptors resembles that of potassium channels, and this has inspired the surprising hypothesis that glutamate receptors and potassium channels evolved from a common ancestral ion channel. Amino acid-gated channels mediate most of the fast synaptic transmission in the cns. 3 glutamate receptor subtypes: ampa, nmda, and kainate. Ampa-gated and nmda-gated channels mediate the bulk of fast excitatory synaptic transmission in the brain. Ampa-gated channels are permeable to both sodium and potassium but not calcium.

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