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

Answer 1

The Correct Option is D

The correct option is (D): Receptor Tyrosine Kinase.

Answer 2

Enzyme-linked receptors are a class of cell-surface receptors that, once activated by a ligand, work directly as an enzyme or activate an enzyme bound to them, most commonly by adding a phosphate group to specific amino acid residues on target proteins. Let's check each option against what is actually seen in the largest family of these receptors.

1. Receptor Histidine Kinase: Histidine kinases phosphorylate a histidine residue instead of a hydroxyl-bearing amino acid, and this two-component signaling system is common in bacteria, fungi and plants. It is not the type that dominates enzyme-linked receptors in humans.
2. Receptor Threonine Phosphatase: A phosphatase removes a phosphate group rather than adding one, so it works opposite to the signal-triggering step that defines most enzyme-linked receptors. Threonine-specific phosphatase activity is not the major receptor class either.
3. Receptor Serine Phosphatase: Like the threonine phosphatase option, this describes a phosphate-removing enzyme acting on serine residues. Since receptor activation in this family works by adding phosphate groups, not stripping them, this option does not fit.
4. Receptor Tyrosine Kinase: This is by far the largest and best-studied group of enzyme-linked receptors in the human body. Ligand binding, for example insulin or growth factor binding, causes the receptor to dimerize and autophosphorylate tyrosine residues on its own cytoplasmic tail, which then recruits downstream signaling proteins. This mechanism accounts for the bulk of enzyme-linked receptor signaling in mammalian cells.

Since receptor tyrosine kinases make up the majority of enzyme-linked receptors and drive signaling through tyrosine autophosphorylation, this is the option that fits the question.

The correct answer is Receptor Tyrosine Kinase.