Question

Which one of the following enzymes comprises a major part of enzyme-linked receptors:

• A. Receptor Histidine Kinase
• B. Receptor Threonine Phosphatase
• C. Receptor Serine Phosphatase
• D. Receptor Tyrosine Kinase

Hint:

Receptor Tyrosine Kinases (RTKs) play a key role in cellular communication and are essential for many important signaling pathways, particularly in regulating cell growth and metabolism.

Answer 1

The Correct Option is D

Enzyme-linked receptors work by having catalytic activity built into (or tightly coupled to) their cytoplasmic domain, and different receptors use different enzyme types. Comparing how common each type is among known receptors settles this question.

1. Receptor Histidine Kinase: This enzyme type is central to bacterial and plant two-component signalling systems, where a sensor histidine kinase phosphorylates a response regulator. It is not a major feature of the human enzyme-linked receptor superfamily.
2. Receptor Threonine Phosphatase: Phosphatases acting on threonine are known enzymes in cell signalling, but receptor-associated threonine phosphatase activity is rare, not a defining receptor class.
3. Receptor Serine Phosphatase: Serine-specific phosphatase activity linked directly to a receptor is likewise uncommon; serine/threonine receptors that exist (like the TGF-beta family) work as kinases, adding phosphate rather than removing it.
4. Receptor Tyrosine Kinase: This is the largest receptor enzyme family by number of members. On ligand binding, the receptor dimerises and each monomer phosphorylates tyrosine residues on its partner, switching on pathways like RAS-MAPK and PI3K-AKT that drive cell proliferation and survival.

Because tyrosine kinase activity accounts for the majority of enzyme-linked receptors, the correct answer is Receptor Tyrosine Kinase.