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

The insulin receptor sits in the plasma membrane and, unlike steroid receptors (intracellular, bind lipophilic hormones), it binds a peptide hormone at the cell surface. Unlike serpentine receptors, it does not signal through G-proteins, and unlike adhesion receptors, its job is not physical attachment between cells.

Instead, insulin binding directly activates the receptor's own intrinsic tyrosine kinase activity in its cytoplasmic domain, which autophosphorylates and phosphorylates downstream targets.

This built-in enzymatic activity makes the insulin receptor a receptor enzyme.

Answer 2

Looking at the downstream signalling cascade triggered by each receptor type helps confirm the classification.

1. Steroid receptors: Once bound to their hormone, these act directly as transcription factors in the nucleus; insulin signalling instead begins at the cell surface with a phosphorylation cascade, not direct gene binding.
2. Serpentine receptors: These trigger second-messenger cascades via G-proteins; insulin receptor signalling instead proceeds through phosphotyrosine-based recruitment of adaptor proteins, a hallmark of enzyme-linked receptors.
3. Adhesion receptors: These primarily transmit mechanical or structural signals related to cell attachment, unrelated to the metabolic phosphorylation cascade triggered by insulin.
4. Receptor enzyme: Insulin binding triggers autophosphorylation of the receptor's own tyrosine kinase domain, which then phosphorylates substrates such as IRS proteins -- a signalling pattern characteristic specifically of enzyme-linked receptors.

The phosphorylation-cascade signature of insulin signalling matches the receptor enzyme category rather than any of the other receptor classes.

So, the correct answer is receptor enzyme.