Question

1. Explain how the three classes of receptors initiate a signal transduction cascade and describe how they are activated and the downstream proteins/molecules activated.

Answer 1

Generally receptors are transmembrane proteins which bind to signaling molecules (growth factors, hormones, neurotransmitters, and extracellular matrix components are some of this chemical signals) outside the cell and transmit the signal to internal signaling pathways.

There are three major classes of membrane receptors: G-protein-coupled receptors, ion channel receptors, and enzyme-linked receptors. They use different mechanisms to transform external signals into internals: protein action, ion channel opening or enzyme activation.

After a receptor receives a signal, it undergoes a conformational change and launches signal transduction cascades which amplify the message. Also the activation of receptors can trigger the synthesis of second messengers (like cyclic AMP,cAMP) which coordinate intracellular signaling pathways.

The enzyme adenylyl cyclase synthesizes cAMP from ATP. The activation of adenylyl cyclase can result in the synthesis of hundreds or thousands of cAMP molecules. These cAMP molecules activate the enzyme protein kinase A (PKA), which then phosphorylates protein substrates by adding phosphate groups to them. The addition of phosphate groups causes a conformational change in the enzymes activating or inhibiting the enzyme activity.

Each step in the cascade further amplifies the initial signal, and the phosphorylation reactions mediate short and long term responses in the cell. cAMP stop signaling when it is degraded by the enzyme phosphodiesterase.