Glycogenolysis Enzymes
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Biochemistry › Glycogenolysis Enzymes
Which one of the following statements is correct?
Insulin causes a liver cell to convert its glycogen phosphorylase a to glycogen phosphorylase b.
Glucagon stimulates conversion of muscle glycogen phosphorylase b to muscle glycogen phosphorylase a.
Glucose stabilizes the R-state of liver glycogen phosphorylase a.
5’ AMP binds to muscle glycogen phosphorylase b and inhibits it by an allosteric mechanism.
Explanation
Insulin is released in response to high blood glucose. It causes a signaling cascade that, in addition to other things, stops glycogenolysis. This is done by converting glycogen phosphorylase from it's active "a" form to its inactive "b" configuration. The "R" state is the active state, so the presence of glucose would not trigger the breakdown of glycogen. 5' AMP would not inhibit an inactive form of an enzyme. High AMP would mean a demand for ATP, so it would convert the enzyme to its "a" form.
Which one of the following statements is incorrect?
Breakdown of glycogen in muscle produces mostly glucose, which is released into the blood.
Glycogen provides a way to store energy in tissues that consume large amounts of energy when an organism is active.
Glycogen provides a reservoir of glucose molecules that can be used to replenish the blood with glucose when food is not available.
Both the synthesis and the breakdown of glycogen are regulated.
Explanation
Glycogen is mostly stored in the liver and skeletal muscle. When it is broken down in the liver, the last enzyme, a phosphatase, removes the last phosphate group to release plain glucose into the bloodstream. In the muscle, there is no need to release the glucose, so glycogen is only broken down as far as glucose-6-phosphate. Skeletal muscle cells lack the last phosphatase required to remove the phosphate from carbon 6. This isn't an obstacle, however, because the glucose-6-phosphate is already on to the second stage of glycolysis.
What are some characteristics of glycogen phosphorylase?
I. It is the rate-limiting enzyme of glycogenolysis
II. It breaks alpha 1,4 glycosidic bonds
III. It is activated by epinephrine
IV. It breaks alpha 1,6 glycosidic bonds
I, II, and III
I and IV
II and III
II, III, and IV
I and II
Explanation
Glycogen phosphorylase, the rate-limiting enzyme of glycogenolysis does not breaks alpha 1,6 glycosidic bonds. It releases glucose from glycogen by hydrolyzing alpha 1,4 glycosidic bonds until it reaches a branch point in the glycogen molecule. At this time, another enzyme, a debranching alpha 1,6 glycosidase hydrolyzes the alpha 1,6 glycosidic bonds. Glycogen phosphorylase is under regulation by many hormones, including insulin and glucagon, as well as epinephrine.
Which of the following enzymes is not required to breakdown glycogen into glucose-6-phosphate molecules for further metabolism?
All of these enzymes are necessary in the breakdown of glycogen into glucose-6-phosphate molecules.
Glycogen phosphorylase
Alpha-1,6-glucosidase
Glucosyltransferase
Phosphoglucomutase
Explanation
In order to break down glycogen into individual glucose-6-phosphate units, all of the above enzymes are required. Each plays a specific role in one of the following activities: degradation of glycogen initially, remodeling of the glycogen so that it can be acted upon by the enzymes, and conversion of glucose-1-phosphate to glucose 6-phosphate.
Which enzymes are required for glycogen breakdown?
glycogen phosphorylase, glycogen debranching enzyme, phosphoglutomutase
glycogen synthase, glycogen branching enzyme, UDP-glucose pyrophosphorylase
glycogen phosphorylase, glycogen branching enzyme, phosphoglutomutase
glycogen synthase, glycogen debranching enzyme, UDP-glucose pyrophosphorylase
hexokinase, glycogen synthase, phosphoglucose isomerase
Explanation
Glycogen is first debranched and broken down from its non-reducing end by glycogen phosphorylase to give the product G1P, which is then converted into G6P by phosphoglutomutase. Glycogen synthase, glycogen branching enzyme, and UDP-glucose pyrophosphorylase are required for glycogen synthesis.
Why is glycogen phosphorylase alone not sufficient in in degrading glycogen?
Glycogen phosphorylase can not cleave the alpha-1,6-glycosidic bonds at glycogen branch points
Glycogen phosphorylase can only cleave roughly ten bonds before it runs out of energy
Glycogen phosphorylase can only cleave alpha-1,6-glycosidic bonds, and so when it reaches a branch point it stops
Glycogen phosphorylase can only cleave one glycosidic bond at which point another glycogen phosphorylase enzyme must come cleave the next one
None of these
Explanation
When glycogen phosphorylase reaches a branching point in glycogen, the bonds switch from being alpha-1,4-glycosidic bonds to alpha-1,6-glycosidic bonds. It is unable to cleave these bonds, and so other enzymes (a transferase and a glucosidase) must come into play.