Chapter 5

Detection of Extracellular signals: the role of receptors

Objectives

• What are the main types of receptors• How to those receptors function in general• Why is ligand binding important• How do we measure ligand binding

Role of the Receptor

• Specificity, only one signal detected• High affinity, usually low concentrations 10-8 M• Must propagate message• Must turn off

Types of Receptors - G-protein Coupled Receptors

Gγ

Gβ Gα

Signal propagation

Signal propagation

Extracellular

Intracellular

–P P–

Types of Receptors – Enzyme Activity

Extracellular

Intracellular

Types of Receptors – Ligand Gated Channel

Extracellular

Intracellular

acetylcholine

sodium

GCPR

β2 Adrenergic Receptor

GPCR

• 7 Transmembrane helices• Signal through heterotrimeric G-proteins or

associated proteins• Mediate signaling for hormones,

neuropeptides, light, smell, and taste• Can involve phosphorylation

Ligand Gated Receptors

• Usually 5 membrane helices• Create a channel for ions• Rapid open / close• Function in neuronal

signaling

Enzyme Receptors

• Largest Group Receptor Tyrosine Kinases– Autophosphorylation– Dimers to be active

• Some Receptors phosphorylate other proteins

Intracellular Receptors

NPC

Hsp56

Hsp70

Hsp90

PromotorDNA

Nucleus

Cytosol

Extracellular

Ligand binding to Receptors

• Agonist – ligand that binds and activates a receptor

• Antagonist – ligand that binds receptor but does not activate it

• Constitutively Active – a receptor that is active in the absence of ligand

• Consider Ligand/Receptor binding as very similar to Ligand/Protein binding

Ligand binding to Receptors

Equations:

Ligand binding to Receptors

• How to measure [LR]– Fluorescence labeling

• Hard to quantify

– Radioisotope• 131I, 125I, 3H

– Influences• Time, pH, temperature, other ligands present…

Ligand binding to Receptors

• How much ligand binds nonspecifically– Compete with

unlabeled ligand– Subtract

• Kd

Total binding

Specific binding

Nonspecific binding

Scatchard Plot

Plot: vs

Scatchard Plots

Receptor Sensitivity

• Receptor Density– More receptors = better chance of detection– Less receptors = desensitization

• Clathrin coated vesicles and endocytosis

• Refractory Response– Homologous desensitization– Heterologous desensitization

Homologous desensitization