What is the characteristic of enzyme-linked receptors?

• A. They directly transport ions.
• B. They add phosphate groups to proteins.
• C. They are predominantly found in the nucleus.
• D. They require second messengers to function.

Answer: (B)

Enzyme-linked receptors are known for their ability to catalyze biochemical reactions through the addition of phosphate groups to target proteins, a process known as phosphorylation. This modification can activate or deactivate proteins, thereby influencing cellular signaling pathways and responses. When a ligand binds to the receptor, it induces a conformational change that activates the enzymatic activity of the receptor itself or closely associated proteins, leading to the phosphorylation of specific substrates.

In contrast, receptors that directly transport ions do so via ion channels and not through enzymatic activity. Regarding the nuclear localization, enzyme-linked receptors are primarily found in the plasma membrane, rather than in the nucleus, where other types of receptors, such as steroid hormone receptors, are more prevalent. Lastly, while some signal transduction pathways initiated by enzyme-linked receptors may involve secondary messengers, this is not a defining characteristic of this class of receptors, as many signal transduction events can occur through direct phosphorylation without the need for secondary messengers.