A 16-year-old type 1 diabetic female checks her finger stick blood glucose before a meal. She injects herself subcutaneously with exogenous insulin and then starts to eat. As Insulin is absorbed in to her blood, it binds to insulin receptors that activate:
A. Tyrosine kinase
B. Adenylate cyclase
C. Cyclic AMP
D. Protein kinase C
E. Phospholipase C
The correct answer is- A- Tyrosine kinase.
Insulin binding to its receptor stimulates intrinsic tyrosine kinase activity (figure -1) leading to receptor auto phosphorylation and the recruitment of intracellular signaling molecules, such as insulin receptor substrates (IRS). IRS and other adaptor proteins initiate a complex cascade of phosphorylation and dephosphorylation reactions, resulting in the widespread metabolic and mitogenic effects of insulin.
Figure -1- showing the structure of Insulin receptor. The receptor is composed of two extracellular α-subunits that are each linked to a ß-subunit and to each other by disulfide bonds.
As an example, activation of the phosphatidylinositol-3′-kinase (PI-3-kinase) pathway stimulates translocation of glucose transporters (e.g., GLUT4) to the cell surface, an event that is crucial for glucose uptake by skeletal muscle and fat (Figure-2), induce glycogen synthesis, protein synthesis, and lipogenesis. Activation of other insulin receptor signaling pathway (MAP kinase) induces cellular proliferation and growth, and regulation of expression of various genes in insulin-responsive cells.
Figure-2- showing the mechanism of action of Insulin
As regards other options-
Adenylate cyclase is an enzyme that catalyzes the conversion of ATP to c- AMP. Hormones such as glucagon and epinephrine activate Adenylate cyclase , the released c AMP acts as a second messenger for their action.
Oxytocin and Thyrotropin- releasing hormone (TRH) activate protein kinase C.
Phospholipase C cleaves PIP2 into Diacyl glycerol and Inositol triphosphate. DAG further activated protein kinase C(figure-3).
Figure-3- Mechanism of action of phospholipase C
For further details of insulin action follow the link-