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

Receptor Tyrosine Kinase is a major component of enzyme-linked receptors. These receptors are involved in the activation of signaling pathways through the phosphorylation of tyrosine residues on specific proteins, leading to cellular responses like growth and differentiation.

Answer 2

Enzyme-linked receptors are transmembrane receptors that carry (or associate with) enzymatic activity on their cytoplasmic side. The question asks which enzyme type dominates this receptor family. Let's assess each option.

1. Receptor Histidine Kinase: Histidine kinases are the sensor component of two-component signalling systems found mainly in bacteria, fungi, and plants. They are not the major enzyme class among mammalian enzyme-linked cell surface receptors.
2. Receptor Threonine Phosphatase: Phosphatases that remove phosphate groups from threonine residues exist, but receptor-linked threonine phosphatases are not a major or common receptor enzyme class.
3. Receptor Serine Phosphatase: Similarly, serine phosphatase activity is not the defining enzymatic feature of the major enzyme-linked receptor family; serine/threonine kinase receptors (such as the TGF-beta receptor family) exist, but as kinases, not phosphatases.
4. Receptor Tyrosine Kinase: Receptor tyrosine kinases (RTKs) make up by far the largest and most studied group of enzyme-linked receptors, including receptors for insulin, epidermal growth factor, and many other growth factors. Ligand binding causes the receptor to autophosphorylate tyrosine residues on its own cytoplasmic tail, which then triggers downstream signalling cascades controlling cell growth, differentiation, and survival.

Among the enzyme classes linked to cell surface receptors, tyrosine kinase activity is both the most common and the best characterised.

Therefore, the correct answer is Receptor Tyrosine Kinase.