Question

29) Insulin is a water-soluble hormone transported in the blood. Which describes a likely way in which it exerts its actions?

A) crosses the membrane of cells and binds to transcription factors

B) crosses the membranes of cells and bind to DNA

C) binds to G-protein coupled receptor proteins in the plasma membrane and generates second messengers.

D) binds to G-protein coupled receptors which bind to DNA

E) binds to the plasma membrane and is endocytosed into the cell to exert its primary actions.

Answer 1

29. The right answer is option C - binds to G-protein coupled receptor proteins in the plasma membrane and generates second messengers.

Explanation :

• First of all, when it comes to the entry of the hormone in the target cell, the basic difference between fat soluble and water soluble hormones is that fat soluble hormones can pass through the plasma membrane of the target cell and bind to the receptor protein but a water soluble hormone cannot do this. Water soluble hormones bind to cell surface receptors and transduce the signals mostly via other secondary messengers.
• This is because the plasma membrane is made of lipids and hence the fat soluble hormones (which also have lipid content) can easily diffuse through the plasma membrane. Therefore insulin being a water soluble hormone eliminates the options A and B easily.
• Now, if we study the insulin signal transduction pathway, we will see that it binds to GPCRs and then transduces the signal via secondary messengers like PI3K and MAPKs. Therefore option C is correct.
• Options D and E are incorrect as in these two options there is no role of second messengers and we know that insulin signalling pathway uses second messengers.