Question

Which is not involved in the regulation of glycogen phosphorylase?

A) Allosteric inhibition of phosphorylase by ATP

B) Indirect, positive regulation by glucagon

C) Phosphorylation of phosphorylase by protein kinase A (PKA)

D) Direct and indirect regulation by protein phosphatase 1 (PP1)

E) Indirect, negative regulation by protein phosphatase 2B (PP2B)

Answer 1

Glycogen phosphorylase is an enzyme that catalyzes the rate limiting step in glycogenolysis, the reaction that allows the releasing of 1 molecule of glucose 1 P from the extreme of glycogen molecule where glucose units are linked by alfa 1,4-glycosidic bonds

This enzyme can be regulated by 2 ways:

- reversible phosphorylation ( active state is when the enzyme is phosohorilated)

- allosteric reversible effect ,.It has a regulatory site where activators or inhibitors can bind, introducing a conformational change in the molecule and modifying its affinity for substrate

It is known that AMP is an activator of this enzyme,but ATP has an inhibitory effect( competitive inhibitor)

Glucagon and epinephrine, hormones segregated during fasting in liver, estimulates the glycogenolisis via AMP cyclic receptor cascades, so glycogen phosphrylase is finally phosphorylated ( active state) via PKA

In feeding state, there is no need to obtain glucose via glycogenolysis , so if insulin levels rise ( what happens during feeding ) , insulin binds to a receptor that activates (via akt pathway ), a protein phosphatase 1 (PP1) which dephosphorylase glycogen phosphorylase adquiring an inactive state.

PP2B is another kind of phosphatase that doesn t takes part of all this metabolic pathways.

Answer E is the incorrect sentence