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(4 pts) A sample of cells has a total receptor concentration of 25 mM. Ninety percent of the receptors have bound ligand and
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Answer #1

Binding of ligand to receptor protein can be represented by the chemical equilibrium

P + L <=KfKr==> PL where P = protein , L = Ligand

Kf = forward rate , Kr = reverse rate

The apparant dissociation constant

Kd = Kr / Kf = [P] [L] / [PL] -------(1)  

=> [PL] = [P] [L] / Kd

the fraction of protein receptors occupied by ligand = (occupied protein / total protein) = [PL] / ([P] + [PL])

= ([P] [L] / Kd) / ([P] + ( [P] [L] / Kd))

= [P] [L] / (Kd[P] + [P] [L])

= [L] / (Kd + [L]) ----------(2)

Now when 90% of receptors have bound to ligands then θ = 0.9 & given [L] = 125μM = 0.125 mM

=> 0.9 = 0.125 / (Kd + 0.125)

=> 0.9 Kd + 0.1125 = 0.125

=> Kd = (0.0125 / 0.9)

=> Kd = 0.0139

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