A ION-CHANNEL-LINKED RECEPTORS Figure 15-15 Three classes of cell-surface receptors. (A) lon-channel linked receptors. (B)G-protein-linked plasma membrane receptors, and(c)enzyme-linked signal molecule receptors. Although many enzyme-linked receptors have intrinsic enzyme activity, as shown on the left, many others rely on associated enzymes, as shown on the 8 G-PROTEIN-LINKED RECEPTORS signal molecule G protein activated activated G protein enzyme O ENZYME-LINKED RECEPTORS ignal molecule signal molecule nform of a dime inactive catalytic active catalytic activated domain enzyme