The structure and function of glycogen
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The structure and function of glycogen
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Glycogen is the major storage form of glucose in the human body. Figure 1 depicts a four unit glycogen polysaccharide.
Figure 1: Glycogen tetramer (C_24 H_42 O_21)
This molecule is a good representation of the structural aspects of glycogen, because it clearly depicts the two different types of glycosidic linkages that confer the distinctive branching pattern of glycogen. The reactions creating these linkages during glycogen synthesis are catalyzed by glycogen synthase and glycogen branching enzyme. The reactions that cleave these linkages during the breakdown of glycogen are catalyzed by glycogen phosphorylase and glycogen debranching enzyme. For instance, glycogen phosphorylase catalyzes the cleavage of the glycosidic linkage between the anomeric carbon of one glucose subunit and the fourth position carbon of the adjacent glucose subunit.
The liver, kidney, and muscle contain ample supplies of these and other enzymes involved in the synthesis and breakdown of glycogen. Mutations in the genes for the above mentioned enzymes can result in a variety of metabolic diseases that fall under the umbrella term of glycogen storage disease (GSD). The following table lists five main types of GSD.
Table 1. Five types of glycogen storage disease (GSD). Hypoglycemia refers to low levels of circulating blood glucose. Hepatomegaly refers to an enlarged liver. Cirrhosis is a serious condition of the liver resulting in the accumulation of scar tissue.
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[question] => How many anomeric carbons are contained in the molecule depicted in figure 1?
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[description] => Reason for the Correct Answer:
An anomeric carbon is the stereocenter created at the carbonyl carbon of a carbohydrate when the molecule goes from its straight chain form to its cyclical form.
The molecule depicted in figure 1 has 4 such stereocenters.
The molecule depicted in figure 1 has 4 anomeric carbons.
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[each_answer] => A.4
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[each_answer] => C.8
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[question] => If the glycogen polymer in figure 1 were reacted with glycogen phosphorylase and excess phospate anion, how many molecules of glucose-1-phosphate would be produced?
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[description] => Reason for the Correct Answer:
Glycogen phosphorylase catalyzes the hydrolysis of the terminal alpha (1\arrow 4) linkage of glycogen. There are two such linkages in the molecule depicted in figure 1.
Enzymes are regenerated in any enzymatic reaction.
Glycogen has two types of glycosidic linkage, and requires a specific enzyme to catalyze the hydrolysis of each linkage.
Two molecules of glucose-1-phosphate would be produced.
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[each_answer] => A.Two
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[question] => Using figure 1 as a reference, glycogen contains which of the following types of glycosidic linkages?
I alpha (1 to 4)
II alpha (1 to 6)
III alpha (1 to 1)
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[answer] => 2
[description] => Reason for the Correct Answer:
The anomeric carbon in the cyclic form of glucose is numbered 1.
The methyl group carbon on the fifth carbon in the glucose ring is numbered 6.
Glycogen contains alpha (1\arrow 4) and alpha (1\arrow 6) linkages.
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[each_answer] => A.I and III only
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[each_answer] => B.I and II only
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[each_answer] => C.II and III only
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[each_answer] => D.I only
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[question] => Storing glucose as a large branched polymer likely arose as an evolutionary adaptation that conferred a selective advantage over organisms that stored glucose in straight-chain form, or as individual phosphorylated glucose molecules. Which of the following does not represent a possible explanation for this selective advantage?
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[description] => Reason for the Correct Answer:
A large unbranched polymer of glucose molecules only has two terminal ends, while a large branched polymer of glucose molecules will have many terminal ends. Glycogen phosphorylase works on terminal ends.
Osmotic pressure increases with molarity.
Chylomicrons are lipoproteins and have nothing to do with glucose transport.
Large branched polymers of glucose are not more easily transportable in the blood, and do not represent a possible explanation for the selective advantage of storing glucose as glycogen.
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[each_answer] => A.Large branched polymers of glucose can offer many simultaneous sites for hydrolysis of glycosidic linkages.
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[each_answer] => B.Large branched polymers of glucose take up less volume, and precipitate less easily, than large unbranched polymers.
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[each_answer] => C.Large branched polymers of glucose do not increase the osmotic pressure in cells to the extent that individual glucose molecules would.
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[each_answer] => D.Large branched polymers of glucose are able to form chylomicrons more easily, resulting in increased ease of transport in the blood.
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[question] => The symptoms of GSD type 1 (not shown in Table 1), which results from a deficiency in glucose-6-phosphatase, a liver enzyme that catalyzes the conversion of glucose-6-phosphate to glucose, would most closely resemble which disease from table 1?
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GSD type 0 and GSD type 4 have to do with the synthesis of glycogen.
GSD type 5 results from a deficiency in a muscle enzyme, not a liver enzyme.
Both GSD type 6 and GSD type 1 affect the pathway by which liver glycogen is converted to glucose for transport out of the liver and throughout the body.
The symptoms of GSD type 1 most closely resemble GSD type 6.
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[each_answer] => A.GSD type 6
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[each_answer] => B.GSD type 0
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[each_answer] => C.GSD type 5
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[each_answer] => D.GSD type 4
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Figure 1: Glycogen tetramer (C_24 H_42 O_21)
This molecule is a good representation of the structural aspects of glycogen, because it clearly depicts the two different types of glycosidic linkages that confer the distinctive branching pattern of glycogen. The reactions creating these linkages during glycogen synthesis are catalyzed by glycogen synthase and glycogen branching enzyme. The reactions that cleave these linkages during the breakdown of glycogen are catalyzed by glycogen phosphorylase and glycogen debranching enzyme. For instance, glycogen phosphorylase catalyzes the cleavage of the glycosidic linkage between the anomeric carbon of one glucose subunit and the fourth position carbon of the adjacent glucose subunit.
The liver, kidney, and muscle contain ample supplies of these and other enzymes involved in the synthesis and breakdown of glycogen. Mutations in the genes for the above mentioned enzymes can result in a variety of metabolic diseases that fall under the umbrella term of glycogen storage disease (GSD). The following table lists five main types of GSD.
