Final answer:
The tyrosine kinase receptor is the simplest type of enzyme-linked receptor.
Step-by-step explanation:
The simplest type of enzyme-linked receptor is the tyrosine kinase receptor. This receptor is an enzyme that transfers phosphate groups to tyrosine molecules. When signaling molecules bind to the extracellular domain of two nearby tyrosine kinase receptors, the receptors bond together and phosphates are added to tyrosine residues on the intracellular domain. The phosphorylated residues then transmit the signal to the next messenger within the cytoplasm.