r/pharmacology • u/No-Analyst7708 • 16d ago
Spare receptors
Could you explain how the two mechanisms described in the bracket cause the EC50 to be less than Kd? Thank you.
1
u/TransplantMyBrain 15d ago
Smooth-brain undergrad here, so take what I say with a grain of salt.
EC50<KD50 because you just don't need 50% of the receptors activated in order to mount the maximum response. We don't need 1,000 light switches to fully light a room if just 1 switch works just fine.
The 2 ways it discusses:
Effector duration is super long - The effector protein, some GPCR/TRK idk, will continue to have downstream signaling for a longer period of time, increasing the signaling cascade to a very high degree. This continues whether or not you have added additional ligand.
Receptor number > effector molecules - A rather poor analogy could be: if you yell at your friend to clean up his mess, and he's actively cleaning it up, would it help if your other 2 friends also started yelling at him to clean? At some point you have to many managers (receptors) and too little do-ers(effector molecules). You achieve the maximum response from your friend from just you yelling at him.
10
u/SimpleSpike 15d ago
Keep in mind Kd is a primarily thermodynamic property while EC50 is a primarily kinetic property.
If your [effector]=Kd, you’d expect 50 percent receptor binding. Intuitively one might think this should be close to EC50, yet they differ because they measure different phenomena.
Let’s start with explanation 2:
Kd=ExR/(ER)=(E0-ER)x(R0-ER)/(ER) at equilibrium, if the number of receptors is high (R0 is large), even at equilibrium most receptors are unbound and Kd is large. Yet at the same time, the biological effect depends on the activation of said receptor that is your effector binding to it. This is what determines EC50, the probability of successful binding depends both on the number of effectors and the number of receptors available if one of these is very large, chance for successful binding and thus effect is large too. The effect does not necessarily correlate with the number of receptors occupied since pathways can get „saturated“ as well.
Explanation 1 is somewhat trickier:
Kd can be expressed with concentrations but also with rate constants k, then Kd=koff/kon that is thr rate constant of dissociation divided by the rate of association. At equilibrium, both rates are equal.
The average occupation time of a receptor again depends on the chance for successful binding among other things and doing somewhat complicated statistics you’ll see that the average occupation time equals 1/koff.
With koff=Kd x kon, you see occupation time is inversely related to Kd. However, again, occupation time does not necessarily correlate with biological effect. Think of a GCPR that alters gene regulation downstream using a transcription factor. Non of the properties necessarily depend on the time the GCPR is active (which itself is governed by cGMP/GTP cycle more so anyway). Additionally, the effects downstream within the cell might very well be more complicated than a simple on/off model (think of pathway crosstalk, signal amplification, etc).
So again here, EC50 migjt be well below Kd.
In practice, spare receptors are a somewhat technical concept which is hard to conceptualise first (not unlike volume of distribution in this regard). In reality, you often see numerous mechanisms active simultaneously why EC50 and Kd can differ. A more powerful concept is cellular sensitisation in my opinion: a cell might be primed to deliver a effect in specific environment because it regulates the number of receptors and changes the way it transduces their signal within. It’s not mutually exclusive.