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

Depolarization EPSP Erest Hyperpolarization IPSP

mutation in Dynamin

The Docking Complex vesicle plasma membrane SNARE proteins form coiled-coils plasma membrane 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)

K+ Cl- K+ Cl- What is driving force? E – actual membrane potential I V EK = 0 E (mV) K+ Cl- I V EK < 0 driving force E – actual membrane potential

Hmmm, …but nAChR passes both of ions! Why does it lead to excitation? - 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? ENa = +60 mV EK = -90 mV Erest = -70 mV Ethreshold

nACh R (Na+, K+ channel) R AR A2R A2R* A2D open gate closed desensitized from Unwin

L0, LA – equilibrium constants KA, JA – binding constants

C O LO ΔGC→O = -RT ln([O]/[C])=-RT lnL0 In the absence of ligand the O state is unfavorable, openings are rare ΔGC→O = -RT ln([O]/[C])=-RT lnL0

C O LO ΔGC→O = -RT ln([O]/[C])=-RT lnL0 In the absence of ligand the O state is unfavorable, openings are rare ΔGC→O = -RT ln([O]/[C])=-RT lnL0

ΔGC→AC = RT ln([AC]/[A][C])=RT lnKA Initially binding of the ligand occurs with low affinity ΔGC→AC = RT ln([AC]/[A][C])=RT lnKA

ΔGAC→AO = -RT ln([AO]/[AC])=-RT lnLo Binding makes the AO state more favorable than AC, openings are frequent ΔGAC→AO = -RT ln([AO]/[AC])=-RT lnLo

ΔGAC→OC = -RT ln([AO]/[AC])=-RT lnLA Binding makes the AO state more favorable than AC, openings are frequent ΔGAC→OC = -RT ln([AO]/[AC])=-RT lnLA

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

In Molluscs: AChBP is released upon ACh release and then sequesters ACh from Sixma et al.

AChBP: 210 aa per subunit, pentamer from Sixma et al.

nicotine carbamylcholine