Ligand Binding: Dissociation Constants and Receptor Occupancy · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Half full sets the number

Science · Organisms & Life Processes · ages 19-20
Name ______________________   Date ____________

  1. Kd is 10 nanomolar and the ligand concentration is 10 nanomolar. Type the filled fraction.

    Answer: ______________

  2. What is the dissociation constant, Kd?

    • The ligand concentration filling half the receptors
    • The total number of receptors on the cell
    • The speed at which ligands arrive

  3. A smaller dissociation constant means tighter binding.

    Circle one:   True   False

  4. Two ligands are plotted on the same axes. Which one binds tighter?

    • The curve that flattens first
    • The curve with the higher top level
    • The curve that rises earlier and hits half first

  5. Kd is 10 nanomolar and the ligand concentration is 90 nanomolar. Type the filled fraction.

    Answer: ______________

  6. A binding curve passes half full at 30 nanomolar. What is the Kd?

    • 3 nanomolar
    • 30 nanomolar
    • 60 nanomolar

  7. Sam says that at ligand equals Kd, 90 percent of receptors are filled. What is wrong?

    • 90 percent, since Kd means nearly full
    • 10 percent, since Kd means nearly empty
    • 50 percent, since Kd means exactly half full

  8. Raising the ligand from 10 times Kd to 20 times Kd moves occupancy from about 0.91 to 0.95. Why so little?

    • Almost all receptors are already filled, so few empty ones remain
    • The ligand starts blocking its own receptor
    • The Kd grows as concentration rises
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Half full sets the number W1-mt_i-v79OaM0n-s1

  1. 0.5 · Filled equals ligand divided by Kd plus ligand, so 10 divided by 20 is 0.5.
  2. The ligand concentration filling half the receptors · Kd is read as the half-full concentration.
  3. True · Half occupancy comes sooner for a tighter grip, which is the smaller Kd.
  4. The curve that rises earlier and hits half first · Earlier half-full means a smaller Kd and tighter binding.
  5. 0.9 · 90 divided by 10 plus 90 is 90 divided by 100, or 0.9.
  6. 30 nanomolar · Kd is the concentration where the curve passes half full.
  7. 50 percent, since Kd means exactly half full · Kd is defined as the half-full concentration, so Sam overread it.
  8. Almost all receptors are already filled, so few empty ones remain · Near the flat top, doubling the ligand fills only the few receptors left.
Worksheet · LightMySky