which of the following statements concerning gpcr (7tm) is true? a) heterotrimeric g-protein binding occurs only at the c-terminal tail of the receptor. b) upon hormone binding, there is activation of an intrinsic tyrosine kinase that is associated with the n-terminus of the receptor. c) their activation is terminated by alpha-subunit catalyzed hydrolysis of gtp to gdp. d) their activation involves phosphorylation of gdp to form gtp on the g-protein alpha subunit. e) upon hormone binding, the membrane-spanning subunits of a gpcr form an ion channel.
