27. Which one of the following statements about receptor – enzyme isFALSE?
(1) A receptor enzyme has an extracellular ligand binding domain, a transmembrane domain and an intracellular catalytic (enzyme) domain.
(2) Many types of receptor enzymes are found in animals.
(3) The signal transduction pathways of receptor enzyme involve phosphorylation cascades.
(4) Receptor – enzymes interact directly withintracellular G-proteins.

Introduction

Receptor-enzymes are critical components of cellular signaling, combining both receptor and enzymatic functions into one protein. These proteins typically have an extracellular ligand-binding domain, a transmembrane domain, and an intracellular enzymatic (often kinase) domain. They play pivotal roles in transducing extracellular signals into intracellular phosphorylation cascades that regulate cellular functions.

Key Properties of Receptor-Enzymes

1. Domain Structure: Receptor-enzymes consist of three major domains: a large extracellular ligand-binding domain that recognizes signaling molecules, a single transmembrane alpha helix spanning the membrane, and an intracellular catalytic domain with enzymatic activity, typically kinase activity such as tyrosine kinase.

2. Diversity: There are many types of receptor enzymes in animals, including receptor tyrosine kinases (RTKs), serine/threonine kinase receptors, and receptor guanylyl cyclases.

3. Signal Transduction Pathways: Upon ligand binding, receptor-enzymes often dimerize and auto-phosphorylate, initiating phosphorylation cascades inside the cell that propagate the signal.

4. Interaction with Intracellular Proteins: However, receptor-enzymes do not typically interact directly with intracellular G-proteins. Instead, G-proteins interact primarily with G protein-coupled receptors (GPCRs), which are a distinct receptor class.

Which Statement is FALSE?

Given the typical characteristics:

• (1) Correct: Receptor-enzymes do have extracellular ligand binding, transmembrane, and intracellular catalytic domains.

• (2) Correct: Many receptor enzymes types exist in animals.

• (3) Correct: Phosphorylation cascades are central to their signaling.

• (4) False: They do not interact directly withintracellular G-proteins; that is a characteristic of GPCRs.

Conclusion

The false statement about receptor-enzymes is:

(4) Receptor-enzymes interact directly with intracellular G-proteins.

Understanding these nuances is vital for appreciating how cells decode signals through specific receptor classes.