Acetylcholine acts at a G-protein-linked receptor on heart muscle to make the heart beat more slowly by the effect of the G protein on a K+ channel. Which one or more of the following would enhance this effect of acetylcholine? Explain your reasoning to all answers.
(a) A high concentration of a non-hydrolyzable analog of GTP.
(b) Modification of the acetylcholine-receptor-linked G protein α subunit by cholera toxin.
(c) Mutations in the acetylcholine receptor that weaken the interaction between the receptor and acetylcholine.
(d) Mutations in the acetylcholine receptor that weaken the interaction between the receptor and the G protein.
(e) Mutations in the G protein β subunit that weaken the interaction between βγ and the K+ channel.