Step-by-step degradation. What are the three steps in glycogen degradation and what enzymes are required?

Match ‘em. Match each term with its description.

Choice is good. Glycogen is not as reduced as fatty acids are and consequently not as energy rich. Why do animals store any energy as glycogen? Why not convert all excess fuel into fatty acids?

If a little is good, a lot is better. α-Amylose is an unbranched glucose polymer. Why would this polymer not be as effective a storage form of glucose as glycogen?

Telltale products. A sample of glycogen from a patient with liver disease is incubated with orthophosphate, phosphorylase, the transferase, and the debranching enzyme (α-1,6-glucosidase). The ratio of glucose 1-phosphate to glucose formed in this mixture is 100. What is the most likely enzymatic deficiency in this patient?

Dare to be different. Compare the allosteric regulation of phosphorylase in the liver and in muscle, and explain the significance of the difference.

A thumb on the balance. The reaction catalyzed by phosphorylase is readily reversible in vitro. At pH 6.8, the equilibrium ratio of orthophosphate to glucose 1-phosphate is 3.6. The value of ΔG°′ for this reaction is small because a glycosidic bond is replaced by a phosphoryl ester bond that has a nearly equal transfer potential. However, phosphorolysis proceeds far in the direction of glycogen breakdown in vivo. Suggest one means by which the reaction can be made irreversible in vivo.

Excessive storage. Suggest an explanation for the fact that the amount of glycogen in type I glycogen-storage disease (von Gierke disease) is increased.

Recouping an essential phosphoryl. The phosphoryl group on phosphoglucomutase is slowly lost by hydrolysis. Propose a mechanism that utilizes a known catalytic intermediate for restoring this essential phosphoryl group. How might this phosphoryl donor be formed?

Not all absences are equal. Hers disease results from an absence of liver glycogen phosphorylase and may result in serious illness. In McArdle disease, muscle glycogen phosphorylase is absent. Although exercise is difficult for patients suffering from McArdle disease, the disease is rarely life threatening. Account for the different manifestations of the absence of glycogen phosphorylase in the two tissues. What does the existence of these two different diseases indicate about the genetic nature of the phosphorylase?

Hydrophobia. Why is water excluded from the active site of phosphorylase? Predict the effect of a mutation that allows water molecules to enter.

Removing all traces. In human liver extracts, the catalytic activity of glycogenin was detectable only after treatment with α-amylase, an enzyme the hydrolyzes α-1,4-bonds in starch and glycogen. Why was α-amylase necessary to reveal the glycogenin activity?

Two in one. A single polypeptide chain houses the transferase and debranching enzyme. Cite a potential advantage of this arrangement.

How did they do that? A strain of mice has been developed that lack the enzyme phosphorylase kinase. Yet, after strenuous exercise, the glycogen stores of a mouse of this strain are depleted. Explain how this depletion is possible.

An appropriate inhibitor. What is the rationale for the inhibition of muscle glycogen phosphorylase by glucose 6-phosphate when glucose 1-phosphate is the product of the phosphorylase reaction?

Passing along the information. Outline the signal-transduction cascade for glycogen degradation in muscle.

Slammin’ on the breaks. There must be a way to shut down glycogen breakdown quickly to prevent the wasteful depletion of glycogen after energy needs have been met. What mechanisms are employed to turn off glycogen breakdown?

Diametrically opposed. Phosphorylation has opposite effects on glycogen synthesis and breakdown. What is the advantage of its having opposing effects?

Feeling depleted. Glycogen depletion resulting from intense, extensive exercise can lead to exhaustion and the inability to continue exercising. Some people also experience dizziness, an inability to concentrate, and a loss of muscle control. Account for these symptoms.

Everyone had a job to do. What accounts for the fact that liver phosphorylase is a glucose sensor, whereas muscle phosphorylase is not?

Yin and Yang. Match the terms on the left with the descriptions on the right.

Team effort. What enzymes are required for the synthesis of a glycogen particle starting from glucose 6-phosphate?

Force it forward. The following reaction accounts for the synthesis of UDP-glucose. This reaction is readily reversible. How is it made irreversible in vivo?

If you insist. Why does activation of the phosphorylated b form of glycogen synthase by high concentrations of glucose 6-phosphate make good biochemical sense?

An ATP saved is an ATP earned. The complete oxidation of glucose 6-phosphate derived from free glucose yields 30 molecules ATP, whereas the complete oxidation of glucose 6-phosphate derived from glycogen yields 31 molecules of ATP. Account for this difference.

Dual roles. Phosphoglucomutase is crucial for glycogen breakdown as well as for glycogen synthesis. Explain the role of this enzyme in each of the two processes.

Working at cross-purposes. Write a balanced equation showing the effect of simultaneous activation of glycogen phosphorylase and glycogen synthase. Include the reactions catalyzed by phosphoglucomutase and UDP-glucose pyrophosphorylase.

Achieving immortality. Glycogen synthase requires a primer. A primer was formerly thought to be provided when the existing glycogen granules are divided between the daughter cells produced by cell division. In other words, parts of the original glycogen molecule were simply passed from generation to generation. Would this strategy have been successful in passing glycogen stores from generation to generation? How are new glycogen molecules now known to be synthesized?

Synthesis signal. How does insulin stimulate glycogen synthesis?

Family resemblance. Enzymes of the α-amylase family (problem 12) catalyze a reaction by forming a covalent intermediate to a conserved aspartate residue. Propose mechanisms for the two enzymes catalyzing steps in glycogen debranching on the basis of their potential membership in the α-amylase family.

Double activation. What pathway in addition to the cAMP-induced signal transduction pathway is used in the liver to maximize glycogen breakdown?

Carbohydrate conversion. Write a balanced equation for the formation of glycogen from galactose.

Working together. What enzymes are required for the liver to release glucose into the blood when an organism is asleep and fasting?

A shattering experience. Crystals of phosphorylase a grown in the presence of glucose shatter when a substrate such as glucose 1-phosphate is added. Why?

I know I’ve seen that face before. UDP-glucose is the activated form of glucose used in glycogen synthesis. However, we have previously met other similar activated forms of carbohydrate in our consideration of metabolism. Where else have we seen UDP-carbohydrate?

Same symptoms, different cause. Suggest another mutation in glucose metabolism that causes symptoms similar to those of von Gierke disease.

An authentic replica. Experiments were performed in which serine (S) 14 of glycogen phosphorylase was replaced by glutamate (E). The V_max of the mutant enzyme was then compared with the wild type phosphorylase in both the a and the b form.

Glycogen isolation 1. The liver is a major storage site for glycogen. Purified from two samples of human liver, glycogen was either treated or not treated with α-amylase and subsequently analyzed by SDS-PAGE and western blotting with the use of antibodies to glycogenin. The results are presented in the adjoining illustration on the next page.

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Glycogen isolation 2. The gene for glycogenin was transfected into a cell line that normally stores only small amounts of glycogen. The cells were then manipulated according to the following protocol, and glycogen was isolated and analyzed by SDS-PAGE and western blotting by using an antibody to glycogenin with and without α-amylase treatment. The results are presented in the adjoining illustration.

The protocol: Cells cultured in growth medium and 25 mM glucose (lane 1) were switched to medium containing no glucose for 24 hours (lane 2). Glucose-starved cells were refed with medium containing 25 mM glucose for 1 hour (lane 3) or 3 hours (lane 4). Samples (12 μg of protein) were either treated or not treated with α-amylase, as indicated, before being loaded on the gel.