Glycogen Metabolism
1) A. Our powerpoints say that glucagon and epinephrine both signal the need for glycogen
breakdown. However, it is also mentioned that glucagon primarily targets the liver cells, where
as epinephrine primarily targets the muscle cells.
2) E. Insulin does in fact stimulate glycogen synthesis by activating glycogen synthase kinase. This
makes sense because activation of the enzyme that helps form glycogen would need to occur,
for glycogen synthesis to proceed.
3) A. Again, remember that epinephrine works in the muscles, so you can eliminate C and E.
Hormonal signals activate g-proteins that initiate glycogen breakdown.
4) D. The phosphorylase can’t remove the glucose unit from the hydroxyl end, because it’s
terminal, and there is nothing to cleave. At the nonreducing side, the phosphorylase can cleave
the alpha-1,4 glycosidic bond, and leave a phosphate group behind. It leaves it on the 1 agroup,
because that’s where it cleaves the bond.
5) E. Because it enters glycolysis as G6P, you save the ATP from the hexokinase step, and since it
saves one, there is a net production of 3 ATP.
6) B. Phosphorylase is the enzyme responsible for cleaving the glycosidic bonds. A phosphatase
cleaves the phosphate bond from a compound, where as the phosphorylase can cleave a
glycosidic bond, and leave a phosphate behind.
7) (Thrown out)
8) C. A special debranching enzyme is needed to cleave the a-1,6 bond when 4 subunits remain.
9) C. You can eliminate A, B, and D because they are the wrong type of enzyme, A and D are
transferases, and B is an oxidoreductase, we’re looking for a “cutter.” We don’t need a special
enzyme for a-1,4 bonds, so that just leaves a-1,6 bonds, or C. Important to note is that you make
plain vanilla glucose, it is not phosphorylated, because you’re not using a phosphorylase
anymore.
10) B. Liver phosphorylase A is inhibited by extra glucose. Glucose binds to the active R state of the
enzyme, and changes it to the deactivated T state, slowing the reaction.
11) D. If you think it through logically, you should be able to eliminate A and C immediately. B is
possible theoretically, but without an enzyme to take a phosphate group off of the GTP, it
probably won’t happen. Leaving you with D and E and viable options. D is a better logical choice
because it has the name of the reactant, and it is a kinase.
12) A. Phosphorylase A is active in generally active in muscle cells. Phosphorylase B gets activated
by hormone-regulated phosphorylation
13) E. The energy charge regulates muscle phosphorylase b. ATP inhibits the stimulation of AMP by
competitive binding. When ATP binds to the active site, it is in the T state, and inhibits the
enzymes functionality.
14) C. 2 ATP is required for the reaction, but the net result will be phosphorylation, and the
glycogen will be broken down because the enzyme is now activated.
15) D. Since phosphorylase is the enzyme, we know we will be cutting a glycosidic bond, and leaving
behind a phosphoryl group on the sugar. We can eliminate A because glycogen is being broken
16)
17)
18)
19)
20)
21)
22)
23)
24)
25)
26)
27)
28)
29)
30)
31)
32)
33)
34)
35)
36)
down, not produced. B and C are odd choices, because we rarely talk about fructose. So logically
speaking you should pick D.
A. Again, think about what your given compound and target compounds are. If you
phosphorylated the inactive phosphorylase b, you will activate it, and make it phosphorylase a.
The enzyme that does this is phosphorylase kinase.
C. PKA partially activates a hormone, and then Ca2+ fully activates it. This fully activated
hormone uses ATP to phosphorylate phosphorylase-b, and activating it into phosphorylase-b.
C. See answer 17.
A. Synthesis of glycogen requires three steps. The first two are preparatory, and the last one is
chain elongating. The first preparatory step is the synthesis of G1P from G6P by
phosphoglucomutase. The second preparatory step is the synthesis of UDP-glucose from G1P by
UDP-glucose phosphorylase. The third step is the synthesis of glycogen from UDP-glucose, and
requires two enzymes: glycogen synthase which grows the chain, and a branching enzyme.
E. Your blood glucose levels will be high after a meal, so you will want to synthesize from
glycogen and store it.
B. See answer 19.
D. See answer 19.
E. Glucose gets phosphorylated by hexokinase, and is made into G6P. G6P gets isomerized into
G1P by phosphoglucomutase.
A. See answer 19.
D. Because glycogen has branches, it needs a specific enzyme to help with those a-1,6 bonds.
E. C is not correct, because it is the opposite. Straight chains are at 1,4 linkages, and branch
points are at 1,6 linkages.
B.
B.
C.
A. Hexokinase has a high glucose affinity, and is inhibited by G6P. It is nonallosteric vs.
[glucose].
C. Glucokinase has a low affinity in general for glucose, so it definitely has a lower affinity than
hexokinase does.
B. The liver is the only thing we talked about that can act as a glucose buffer.
D. Again, never really talked about fructose, not a viable option. If you have a lot of glucose, you
will need to store it, so B is correct. Insulin stimulates glycogen synthesis also (don’t really know
why  it just does).
C.
A.
A.
Both 35 and 36 get phosphorylated, and become deactivated