In particular, I want you to draw the states and weights diagram for the ligand binding case and then to ﬁnd Z . With Z in hand, use both of the formulas above to obtain the average occupancy and show they yield the same result. Plot your result as a function of concentration. To make the plots, assume that the binding energy gained upon binding is -10 k B T. 3. Opening Probability of Ligand-Gated Channels. In class I gave a discussion of the behavior of ligand-gated ion chan-nels. Give a brief description of the nicotinic acetylcholine receptor. Then, draw a states and weights diagram (explain all of your assumptions, explain all of the multiplicity factors, etc.) and use the Monod-Wyman-Changeux (MWC) model philosophy (i.e. that there are two states each of which can be occupied or unoccupied by ligands) to compute p open ( c ), where c is the concentration of ligand. Make a plot of this open probability as a function of concentration with a log scale on the x-axis and for several choices of pa-rameters: ± open-± closed = 2 , 5 k B T and K closed d = fK open d with f = 1 , 10 , 100. Explain the signiﬁcance of these choices of f . What is p open at zero ligand concentration? What is p open at saturating concentrations of ligand? 3
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