Insulin

Insulin is a peptide hormone that functions in lowering blood glucose levels. Insulin has several activities that accomplish this goal, summarized below:

> 1. Insulin inhibits transcription of the enzyme phosphoenolpyruvate carboxykinase (PEPCK). PEPCK is a key enzyme in gluconeogenesis and transcription is the primary means of regulating it. By inhibiting PEPCK transcription, insulin can depress glucose production tremendously. (Conversely, the hormone glucagon, which increases blood glucose levels, stimulates PEPCK transcription.)

> 2. Insulin stimulates translocation of the glucose transporter protein from cytosol to the cell surface. Glucose transport protein carries out the facilitated transport of glucose.

> 3. Insulin stimulates phosphatase activity which removes phosphates from molecules activated by the kinase cascade. Thus, insulin opposes the effects of glucagon and epinephrine.

Insulin also stimulates fatty acid biosynthesis (see here) (Figure 18.34) as follows:

> 1. Insulin favors entry of glucose into cells, which, in turn, favors production of NADPH via entry of glucose-6-phosphate into the pentose phosphate pathway.

> 2. Insulin activates the pyruvate dehydrogenase complex, which favors production of acetyl-CoA.

> 3. Insulin tends to reverse the effects of the kinase cascade, and stimulates dephosphorylation of acetyl-CoA carboxylase, which favors polymerization of the enzyme in an active form.

See also: Action of Insulin (from Chapter 23), Gluconeogenesis, Control of Fatty Acid Synthesis, Hormonal Regulation of Fuel Metabolism, Figure 5.21, Table 23.2