Lecture 19Synaptic transmission, vesicle fusion and cyclingWhy selectivity may not be important?Thermodynamics of channel gating by ligandStructure of Acetylcholine-binding protein (AChBP)Latest about nicotinic Ach receptor

Erest

DepolarizationEPSP

HyperpolarizationIPSP

mutation in Dynamin

The Docking Complex

vesicle

plasma membrane

SNARE proteins form coiled-coils

from A. Brunger

Postsynaptic ionotropic receptors:Nicotinic ACh receptor (nAChR)

Glutamate receptor (GluR)AMPA, NMDA subtypes

GABAA receptor

Glycine receptor

excitatory (cation channels)

inhibitory (Cl- channels)

What is driving force?

K+Cl-

K+

Cl-

I

V

EK < 0

)( KKK EEgI

driving

force

K+Cl-

K+Cl-

I

V

EK = 0

E (mV)

E – actual membrane potential

ENa = +60 mV

EK = -90 mV

Erest = -70 mV

- it is known that Na+ influx causes depolarization, and K+ efflux causes hyperpolarization. Yes.

Hmmm, …but nAChR passes both of ions! Why does it lead to excitation?

Ethreshold

ACh ACh

nACh R (Na+, K+ channel)

R AR A2R A2R*closed opengate

from Unwin

A2D

desensitized

L0, LA – equilibrium constants

KA, JA – binding constants

C

OLO

ΔGC→O = -RT ln([O]/[C])=-RT lnL0

In the absence of ligand the O state is unfavorable, openings are rare

C

OLO

ΔGC→O = -RT ln([O]/[C])=-RT lnL0

In the absence of ligand the O state is unfavorable, openings are rare

ΔGC→AC = RT ln([AC]/[A][C])=RT lnKA

Initially binding of the ligand occurs with low affinity

ΔGAC→AO = -RT ln([AO]/[AC])=-RT lnLo

Binding makes the AO state more favorable than AC, openings are frequent

ΔGAC→OC = -RT ln([AO]/[AC])=-RT lnLA

Binding makes the AO state more favorable than AC, openings are frequent

AOAA JRTLRTLRTKRT lnlnlnln

AOAA JLLK A

A

O

AKJ

LL

The change in equilibrium constant between the states equals to the change in affinity to the ligand that occurs with opening.

Consider what should be strong agonist, weak agonist or competitive blocker

AChBP is released upon ACh release and then sequesters ACh

In Molluscs:

from Sixma et al.

AChBP: 210 aa per subunit, pentamer

from Sixma et al.

nicotine carbamylcholine