G-proteins activation statement.

• A. They become activated by receptor binding and exchange GDP for GTP
• B. They are always active and do not require receptors
• C. They do not interact with receptors
• D. They are cytosolic enzymes that degrade second messengers

Answer: (A)

G proteins are activated through receptor binding that promotes GDP-to-GTP exchange on the alpha subunit. When a ligand binds a GPCR, the receptor changes shape and acts as a guanine nucleotide exchange factor, causing GDP to dissociate from the Gα subunit. GTP then binds because it’s abundant in the cell, turning Gα into the active Gα-GTP form. This active subunit, together with the freed Gβγ dimer, goes on to regulate downstream effectors such as adenylyl cyclase or phospholipase C, propagating the signal. The signal is terminated when Gα hydrolyzes GTP back to GDP, returning to the inactive heterotrimer. Statements claiming G proteins are always active or do not interact with receptors miss this regulated switch, and descriptions of cytosolic enzymes that degrade second messengers refer to other proteins, not G proteins.