74. Which of the followings is NOT true of G protein-coupled receptors (GPCRs)?
(A) GPCRs are cell-surface transmembrane receptors that work with the help of monomer G proteins.
(B) GPCRs have similar structure in which a single polypeptide has seven transmembrane helices.
(C) Epinephrine can target the same type of GPCR in liver cell and skeletal muscle blood vessel.
(D) G protein systems are involved in cholera and pertussis diseases.
(E) G protein functions as a molecular switch that is either on or off depending on GTP or GDP is attached.