How do enzyme-linked receptors modify proteins?

• A. Through degradation by proteases
• B. By altering receptor conformation without phosphorylation
• C. Through the addition or removal of phosphate groups to or from amino acid residues via the receptors' linked enzymatic domain
• D. By transporting proteins across the membrane

Answer: (C)

Enzyme-linked receptors modify proteins by phosphorylation. When the receptor binds its ligand, it activates an enzymatic domain that adds phosphate groups to specific amino acids on target proteins (or recruits a kinase that does this). This phosphorylation changes the target’s shape and charge, which can turn its activity on or off, alter how it interacts with other proteins, or change where it localizes in the cell. The added phosphate also creates docking sites for downstream signaling proteins, propagating signals through pathways like MAPK or PI3K-Akt. Dephosphorylation by phosphatases can reset the signal, and other processes like proteolysis or membrane transport aren’t how these receptors convey signals.