Question

Insulin receptor is an example of

• A. Steroid receptors
• B. Serpentine receptors
• C. Adhesion receptors
• D. Receptor enzyme

Hint:

Receptor enzymes, like the insulin receptor, combine ligand binding with enzymatic activity, allowing them to regulate cellular processes through phosphorylation.

Answer 1

The Correct Option is D

Step 1: Understanding the insulin receptor.
The insulin receptor is a receptor tyrosine kinase, which is a type of receptor enzyme. It acts by binding insulin, causing a conformational change that activates its intrinsic kinase activity, leading to the phosphorylation of downstream targets.
Step 2: Analyzing the options.
(1) Steroid receptors: This is incorrect. Steroid receptors are intracellular and bind steroid hormones, but insulin receptors are membrane-bound and function through a different mechanism.
(2) Serpentine receptors: This is incorrect. Serpentine receptors (G-protein coupled receptors) are involved in other signaling pathways, not insulin signaling.
(3) Adhesion receptors: This is incorrect. Adhesion receptors mediate cell-cell and cell-matrix interactions, not insulin signaling.
(4) Receptor enzyme: Correct — The insulin receptor is a receptor enzyme because it has intrinsic enzyme activity (tyrosine kinase) that is activated upon insulin binding.
Step 3: Conclusion.
The correct answer is (4) Receptor enzyme, as the insulin receptor has enzymatic activity that is activated by insulin binding.

Answer 2

Classifying a receptor comes down to where it sits in the cell and how it transmits its signal once its ligand binds. Testing each option against the insulin receptor's known behaviour:

1. Steroid receptors: Steroid receptors are located inside the cell and bind small lipophilic hormones that diffuse across the plasma membrane; insulin is a peptide hormone that cannot cross the membrane, so the insulin receptor cannot belong to this class.
2. Serpentine receptors: These are G-protein-coupled receptors that snake across the membrane seven times and signal through intracellular G-proteins; the insulin receptor has a completely different single-pass transmembrane architecture and does not use G-protein coupling.
3. Adhesion receptors: These mediate physical attachment between cells or between a cell and the extracellular matrix; the insulin receptor's role is hormone signalling, not physical adhesion.
4. Receptor enzyme: The insulin receptor is a transmembrane protein whose cytoplasmic tail has intrinsic tyrosine kinase activity -- when insulin binds the extracellular domain, the receptor autophosphorylates and then phosphorylates downstream substrates, directly carrying out an enzymatic reaction as part of signal transduction.

Since insulin's binding directly switches on an enzymatic activity built into the receptor itself, rather than working through G-proteins, intracellular diffusion, or physical adhesion, it belongs to the receptor enzyme class.

Therefore, the correct answer is receptor enzyme.