Signal Transduction

Thyroxin is a steroid, and therefore it is lipophilic. It is not soluble in water.

The acetylcholine receptor at the nerve- muscle junction is an example of ion-channel receptors. The binding of ligand changes the conformation of the receptor that allows flow of specific ions through it. The ion movement thus alters the electric potential across the membrane.

Epinephrine is a small charged water soluble hormone, which is derived from amino acids and it function as hormone and neurotransmitter.

The activation of protein kinase A depends on the cyclic AMP and therefore, it is commonly known as cAMP-dependent protein kinase. The role of protein kinase A is to transfer phosphate from ATP to serine or threonine residues of various proteins in order to alter their activities.

This is not true because synthesis of cyclic AMP occurs when the epinephrine and glucagon receptors present on the liver cells are triggered by the binding of the hormones epinephrine and glucagon.

This is the correct option because the binding of the ligand to the receptor promotes the exchange of GDP for GTP and once the GTP binds to the G-proteins, the G-beta and Gama subunits dissociates and then G-alpha subunit becomes the activated stimulatory G-protein’s subunit.

This is not true because different G-proteins exist. There are some thousand heteromeric G proteins as the human genome possesses in excess of 15 genes for the alpha subunits, more than 5 genes for beta subunit and nearly 10 encoding the gama subunits. For example, G-alphaS stimulates adenylate cyclase, G-alpha i inhibits adenylate cyclase. This shows that all the G-proteins are not similar.

Guanine nucleotide Exchange Factors promote GDP/GTP binding. The G-protein coupled receptor in a G-Protein mediated cyclic AMP signal proteins cascades acts like a GEF factors by promoting the G-proteins exchange efficiency for the GTP.

These proteins activate some heterotrimeric G-Proteins without the requirement of any receptors. Such AGS have GEF activity.

GTP in addition to the G protein is essential for the signal transduction to proceed. G-alpha is the activated subunit of G-protein. The binding of a ligand like epinephrine causes conformational change in the G-protein that assist in making the nucleotide binding site of the G-protein more accessible to the cytosol where the concentration of GTP is high. The alpha unit of it releases the GDP and binds to GTP. In other words, GTP is exchanged for the GDP. The binding of GTP causes additional conformational change in G alpha subunit and then G-protein disassociates to leave beta and Gama units. G alpha-GTP complex then binds to adenylate cyclase and activates it.

The important step in the termination process is the resetting of the hormone-bound activated receptor. This is achieved by two processes. The first step marks the removal of hormone, and the receptor goes back to the inactivated state. The next step involves the phosphorylation of the serine and threonine residues in the carboxyl-terminal of the hormone-receptor complex by the receptor kinases. The final step includes the binding of B-arrestin to the phosphorylated receptor. This finally reduces the G-protein-activating ability of the receptor completely. Thus, the continuous activation of the G-protein is prevented by the phosphorylation and binding of B-arrestin that desensitize the receptor after continuous exposure to epinephrine hormone. This marks the termination of the cascade. However, if beta-adrenergic receptor kinase and Beta arrestin becomes defective, the signal termination of the signal transduction by the 7TM receptor becomes impossible.

This class of G-alpha I inhibits adenylate cyclase and the ligands to initiate signals include acetylcholine and other neurotransmitters.

In the first step, the intrinsic GTPase activity of the G-protein inactivates the stimulatory G-alpha subunit of the enzyme by forcing it to bind with GDP. After GTP hydrolysis, the phosphate is released and the G-alpha subunit reassociates with the G-beta, gama to form the heterotrimeric protein. Now, the inactivated G-protein no more activates adenylate cyclase and stops cyclic AMP formation, but this will not terminate the continuous activation of G-protein unless the ligand is permanently removed from the receptor. Therefore, the next important step is the resetting of the hormone-bound activated receptor. This is achieved by two processes. The first step marks the removal of hormone, and the receptor goes back to the inactivated state. The next step involves the phosphorylation of the serine and threonine residues in the carboxyl-terminal of the hormone-receptor complex. The final step includes the binding of B-arrestin to the phosphorylated receptor. This finally reduces the G-protein-activating ability of the receptor completely. Thus, the continuous activation of the G-protein is prevented by the phosphorylation and binding of B-arrestin that desensitize the receptor after continuous exposure to epinephrine hormone.

The conduction of an electric impulse from one nerve cell to a muscle cell either to inhibit or induce the muscle contraction happens via paracrine signaling, where in the signaling molecules secreted by a cell affect the target cells that are in close proximity to the cell that secretes them.

This statement for the protein kinases-linked receptors is incorrect because the tyrosine kinases-linked receptors lack intrinsic catalytic activity. However, the binding of the ligand prompts the formation of a dimeric receptor, which then interacts with the cytosolic enzymes.