Carbohydrate Metabolism Regulation Problem Set

Carbohydrate Metabolism Regulation Problem Set

Question 2: Enzymes Dephosphorylated By Insulin Action

Tutorial to help answer the question

Of the enzymes of carbohydrate metabolism listed below, which are dephosphorylated in liver in response to insulin signaling?

Glycogen synthase
Glycogen phosphorylase
phosphofructokinase-1 (PFK-1)
phosphofructokinase-2 (PFK-2)
pyruvate kinase

Tutorial

Dephosphorylation as a Frequent Endpoint of Insulin Action

Four of the six enzymes named in the above figure are dephosphorylated in response to insulin. These enzymes are denoted by a red -OH, signifying that the phosphate group has been removed from the serine or threonine residue in the enzyme, leaving a free hydroxyl. Recall that enzyme dephosphorylation is often the way in which enzymes are ultimately regulated by insulin (don't worry about the exact mechanism for now - there are too many steps between insulin binding and activation of the protein phosphatase).

In three of the four cases, the enzyme is activated by dephosphorylation. The exception is found in the glycogen synthase/phosphorylase pair, which obviously cannot both be activated at the same time. However, you don't necessarily have to memorize which one is inhibited by insulin - this can be reasoned through by considering which way the reaction should go in times of high blood glucose. You guessed it, toward glucose storage (i.e., glycogen synthesis).

The Biology Project
The University of Arizona
March 16, 2000
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