What characterizes enzyme-linked receptors?

Enzyme-linked receptors are characterized by their structure that typically includes a single membrane-spanning region. This is crucial because it allows the receptor to extend across the cell membrane, with one part exposed to the extracellular environment (where the ligand binds) and the other part extending into the cytoplasm (where the enzymatic activity occurs).

The significance of this structure is that when a ligand binds to the extracellular domain, it induces a conformational change in the receptor that activates its enzymatic activity on the cytoplasmic side. This enzymatic activity often involves the phosphorylation of tyrosine residues or the activation of other downstream signaling pathways.

The other statements do not accurately characterize enzyme-linked receptors. They are not always homodimers, as they can exist as monomers, dimers, or even larger oligomeric structures depending on specific signaling requirements. Additionally, all enzyme-linked receptors do possess an extracellular ligand binding domain, which is essential for their function. Finally, while some enzyme-linked receptors can result in changes that lead to the production of secondary messengers, it is not accurate to say they mainly respond to secondary messengers; their main function is the direct activity on the receptor's intracellular domain upon ligand binding.