Question

The activation of receptor tyrosine kinases is characterized by

(A) dimerization and phosphorylation.

(B) dimerization and IP3 binding.

(C) a phosphorylation cascade.

(D) GTP hydrolysis.

Short answer

(A) The option “dimerization and phosphorylation” is true.

(B) The option “dimerization and IP3 binding” is false.

(C) The option “a phosphorylation cascade” is false.

(D)The option “GTP hydrolysis” is false.

Step-by-step solution

Step 1: Receptor tyrosine kinases:

Receptor tyrosine kinases are extra-membrane receptors that work as protein kinases. They are enzymes that can phosphorylate other proteins by transferring the phosphate group from the ATP. The cytoplasmic side domain of these receptors can phosphorylate the tyrosine of the substrate protein.

Explanation of option ‘(A)’

Receptor tyrosine kinase exists as monomers before binding with a ligand with an extracellular ligand-binding site and an intracellular tail containing many tyrosine amino acids.

When a ligand binds with the receptor, then two receptor monomers make dimer by the dimerization process. It activates the tyrosine kinase, and it phosphorylates the tyrosine present in the tail of the monomer.

Thus,activation of receptor tyrosine kinase involves dimerization and phosphorylation.

Therefore, the given option is true.

Explanation of option ‘(B)’

Activation of receptor tyrosine kinase makes the dimer of monomers that were present before activation. IP3 binding occurs due to receptor binding with G-protein that activates phospholipase C. It breaks the membrane phospholipids and yields IP3 that bind with the IP3 gated calcium channel in the endoplasmic reticulum.

Thus,IP3 binding does not involve the receptor tyrosine kinases.

Therefore, the given option is false.

Explanation of option ‘(C)’

In a phosphorylation cascade, many cytoplasmic protein kinases act on different proteins, unlike receptor tyrosine kinase. These protein kinases mainly phosphorylate the amino acids serine and threonine, which is tyrosine in the case of receptor tyrosine kinase.

Thus,receptor tyrosine kinase does not involve a phosphorylation cascade.

Therefore, the given statement is false.

Explanation of option ‘(D)’

GTP hydrolysis occurs when the ligand binds with the extracellular site of the G-protein coupled receptor that changes conformation and binds with an inactive G-protein that hydrolyses the GTP to activate the G-protein.

Thus, GTP hydrolysis occurs for activation of the G-protein when a ligand binds with the G-protein coupled receptor.

Therefore, the given statement is false.