Enzyme-linked receptors are characterized by what?

• A. Multi-pass ion channel receptor
• B. Single membrane-spanning proteins that form dimers upon activation to transduce an extracellular ligand-binding interaction into an intracellular action through the activation of a linked enzymatic domain
• C. Cytosolic receptor that binds lipids
• D. Nuclear receptor that regulates gene expression

Answer: (B)

Enzyme-linked receptors have a single membrane-spanning segment with an extracellular ligand-binding domain and an intracellular catalytic domain. When a ligand binds, these receptors typically dimerize, bringing the catalytic domains together so they become activated, often by autophosphorylation or trans-phosphorylation. This activation turns extracellular binding into an intracellular enzymatic signal, kicking off downstream pathways through phosphorylation events. This is characteristic of receptor tyrosine kinases and related enzymes.

Why the other descriptions don’t fit: a multi-pass ion channel receptor is mainly an ion channel that forms a pore to pass ions, not an activated enzyme; a cytosolic lipid-binding receptor is not anchored as a single-transmembrane protein with an enzymatic domain; a nuclear receptor operates inside the nucleus to regulate gene expression and lacks an intrinsic membrane-embedded enzymatic domain.