Binding of extracellular ligands to receptor tyrosine kinases activates the intracellular catalytic domain by

Binding of extracellular ligands to receptor tyrosine kinases activates the intracellular catalytic domain by _

The correct answer and explanation is:

Answer:
Binding of extracellular ligands to receptor tyrosine kinases (RTKs) activates the intracellular catalytic domain by inducing receptor dimerization, which leads to autophosphorylation of specific tyrosine residues on the intracellular domain.

Explanation:

Receptor tyrosine kinases (RTKs) are a major class of cell surface receptors involved in signaling pathways that regulate key cellular processes such as growth, differentiation, metabolism, and survival. They have an extracellular ligand-binding domain, a single transmembrane helix, and an intracellular domain with tyrosine kinase activity.

1. Ligand Binding and Dimerization:
   When an extracellular ligand (such as a growth factor) binds to the RTK’s extracellular domain, it causes two RTK monomers to come together and form a dimer. This process is known as receptor dimerization.
2. Activation of Catalytic Domain:
   Dimerization brings the intracellular kinase domains of the RTKs into close proximity. This proximity allows them to phosphorylate each other on specific tyrosine residues within the intracellular domain—a process called autophosphorylation.
3. Autophosphorylation:
   Autophosphorylation enhances the kinase activity of the receptor itself and creates binding sites for downstream signaling proteins. These proteins recognize and bind to the phosphorylated tyrosines through specialized domains such as SH2 (Src homology 2) or PTB (phosphotyrosine-binding) domains.
4. Downstream Signaling Cascade:
   Once bound, these signaling proteins initiate a cascade of intracellular events leading to altered gene expression, changes in cytoskeletal organization, or other cellular responses depending on the context.
5. Significance:
   This mechanism is highly regulated because inappropriate activation of RTKs can lead to diseases such as cancer. For example, overexpression or mutation of RTKs can result in continuous signaling without ligand binding, driving uncontrolled cell proliferation.

In summary, extracellular ligand binding induces RTK dimerization, which activates the intracellular kinase domain by autophosphorylation on tyrosine residues, thus initiating downstream signaling pathways essential for cell function and response.