Question

1. Glycogen synthase is active when _______________.

A. phosphorylated

B. Glycogen synthase activity is not affected by phosphorylation.

C. dephosphorylated

2. The acetyl-coA carboxylase is activated by:

A. glucagon and epinephrine

B. polymerization

C. depolymerization

D. Palmitoyl-coA.

Answer 1

1) The answer is

C. dephosphorylated

Glycogen is synthesised when there is excess of glucose available in blood by glycogen synthase. and Glycogen breakdown is active at low glucose level which is catalysed by Glycogen phosphorylase enzyme. Both the enzymes are not active on same time and are regulated by cAMP dependent protein kinase.

Phosphorylation of glycogen synthase and Glycogen phosphorylase inactivates and activates the enzymes respectively.

So dephosphorylation makes glycogen synthase inactive and glycogen phosphorylase active.

2) The answer is B. Polymerisation.

The major function of ACC is to provide malonyl CoA to Fatty acid synthesis.

Acetyl-CoA carboxylase (ACC) -  biotin-dependent enzyme.

catalyzes the irreversible carboxylation of acetyl-CoA to produce malonyl-CoA .

It is a multi enzyme complex which has Biotin carboxylase (BC) activity, biotin carboxyl carrier protein (BCCP), and carboxyl transferase (CT) activity are each contained on a different subunit. So polymerization of these three subunits are needed for the activation of ACC.

When these subunits are deploymerized state, the enzyme activity is not possible as ACC is a multienzyme Complex.

Palmitoyl CO A is the end product of fatty acid synthesis and this inhibit ACC by feed back mechanism at high concentrations.