G-protein-coupled receptors __________, whereas receptor tyrosine kinases __________.

• A. act by phosphorylating a protein; open an ion channel when bound to a signal molecule
• B. are transmembrane proteins; are found only on the cytoplasmic side of the plasma membrane
• C. form a dimer when activated; catalyze the transfer of a phosphate group from ATP to an amino acid
• D. phosphorylate the amino acid guanine; phosphorylate the amino acid threonine
• E. are not enzymes; have enzymatic function

Answer: (E)

Verified step by step guidance

1. Understand the role of G-protein-coupled receptors (GPCRs) in cell signaling. GPCRs are a large family of cell surface receptors that respond to various external signals and activate internal signal transduction pathways.
2. Recognize that GPCRs are not enzymes themselves. Instead, they activate G-proteins by facilitating the exchange of GDP for GTP on the G-protein, which then goes on to activate other proteins in the signaling pathway.
3. Learn about receptor tyrosine kinases (RTKs), which are another class of cell surface receptors. RTKs have intrinsic enzymatic activity, meaning they can catalyze reactions themselves.
4. RTKs become activated upon binding to a signaling molecule, which often leads to dimerization (pairing of two receptor molecules). This activation allows them to transfer a phosphate group from ATP to specific tyrosine residues on target proteins, a process known as phosphorylation.
5. Compare the two: GPCRs do not have enzymatic activity and rely on G-proteins to propagate the signal, whereas RTKs have enzymatic function and directly phosphorylate proteins to transmit the signal.