Why are receptors like insulin described as enzyme-linked?

Receptors like insulin are described as enzyme-linked primarily because they induce cellular changes following the binding of their ligand, which in this case is insulin. When insulin binds to its receptor, it activates intrinsic tyrosine kinase activity associated with the receptor. This leads to autophosphorylation of the receptor and subsequent phosphorylation of various downstream signaling proteins. This phosphorylation cascade initiates various cellular responses, such as glucose uptake and metabolism, cellular growth, and differentiation.

This mechanism highlights how the receptor functions as both a binding site for the hormone and as an initiator of internal cellular processes, aligning well with the definition of enzyme-linked receptors. The direct action of the receptor influences vital metabolic pathways and cell behavior in response to hormonal signals, emphasizing the importance of this type of receptor in regulating homeostasis and metabolic functions.