3. In cell-cell signaling a ligand binds to its receptor. This binding is very specific. For example, the insulin receptor binds insulin and insulin-like growth factor, but not other peptide hormones. The specificity of the receptor refers to its ability to distinguish between closely related substances. Ligand binding can usually be viewed as a simple reversible reaction: Receptor (R) + Ligand (L) RL This interaction can be described by the equation: K₁ = [R][L]/[RL] The Kd refers to the affinity of the receptor for the ligand and is similar to the Michaelis Menten (km) equation that we used above to describe enzyme kinetics. Fraction of maximal binding or Cellular response 1.0 Physiological response 0.8 0.6 0.4 Fraction of receptors bound by ligand 0.2 0 1 2 3 4 Relative concentration of ligand a. The figure above plots the extent of ligand binding and the physiological response at different ligand concentrations. What is the Kd for the ligand binding to the receptor (determine this the same way you would determine Km, except 1/2 Vmax is now the fraction of maximal ligand binding! HINT...make sure to use the correct line on the graph too!)? Kd= (0.5pts) Work shown on graph (0.5pts) b. What is the ligand binding concentration for a 50% physiological response (determine this similarly to what you did in a)? (Make sure to use the graph of the physiological response to determine your answer: 0.5pts) c. Must 100% of the receptors on a cell be occupied for a maximal physiological response? Why or why not? (Rubric: Yes or no (0.25pts). Correct explanation based on graph and knowledge of signal transduction (0.75pts) d. In order to study the EGF receptor you engineer a protein that can bind ligand but lacks the intracellular domain of the protein. If you overexpress this protein in the cell it acts like a dominant negative. Explain how it might bind with normal receptors and inhibit downstream signaling. Rubric: Explain how the dominant negative interacts with cellular components (.5) and how this interaction will overall inhibit EGFR signaling (.5).