[bronzedglow]

Hello,

I am a biology major and I am currently taking an MBB class and I'm a little stumped by one of my problem set questions. The question reads:

Consider the binding reaction L + R --> LR
where L is a ligand, and R is a receptor

when 1*10^-2M L is added to a solution containing 5*10^-2M R, 90% of the L binds to form LR

What is the Keq of the reaction?


Now it's been a while since my last chemistry class, but I seem to remember Keq reactions being solved using an ICE table, but I also remember that neither solids nor liquids factor into the equation. Now a ligand is a soluble molecule(usually a hormone) and the receptor is essentially a protein which is a solid and to be frank - I have no idea where to start so any input would be really appreciated.

All I have is:

Keq = [LR] / [L][R]

[Borek]

All I have is:

Keq = [LR] / [L][R]

And that's all you need, simply plug numbers into.

[sdekivit]

strange question, since with a ligand binding to an enzyme or receptor reacting like described above, usually you speak of the dissociation constant K_d: [L][R] / [LR]. This can be rewritten in terms of the fractional receptor occupance r:

Use Kd = [L][R]/[RL] and [R]_total = [R]_free + [RL] and plug this into the rewritten expression of Kd: Kd/[L]= [R]_free / [R]_free.

This will yield [LR] / [R]_total = r = [L] / (Kd + [L]). This a very useful equation to study receptor antagonists.

Here you can calculate [RL], [L] and [R] at equilibrium since concentrations of L and R are known.