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Course: MCAT > Unit 2

Lesson 1: Foundation 1: Biomolecules

Metabolism: The structure and function of glycogen


Glycogen is the major storage form of glucose in the human body. Figure 1 depicts a four unit glycogen polysaccharide.
Figure 1 Glycogen tetramer C24 H42 O21
Two different types of glycosidic linkages confer the distinctive branching pattern of glycogen. The reactions creating these linkages during glycogen synthesis are catalyzed by glycogen synthase and glycogen branching enzyme, whereas the reactions that cleave these linkages are catalyzed by glycogen phosphorylase and glycogen debranching enzyme. Glycogen phosphorylase specifically catalyzes the cleavage of glycosidic linkage between the anomeric carbon of one glucose subunit and the fourth position carbon of the adjacent glucose subunit.
Mutations in genes coding for these enzymes can result in a variety of metabolic diseases that fall under the umbrella term of glycogen storage disease (GSD). Table 1 shows five types of GSDs caused by mutations in different enzymes. Only Type 4 regularly progresses to cirrhosis, a serious condition resulting from the accumulation of scar tissue in the liver, which results in failure to thrive and often in early death even with dietary interventions.
Table 1 Five types of glycogen storage disease (GSD) and their association with hypoglycemia (low levels of circulating blood glucose) and hepatomegaly (enlarged liver)
TypeEnzyme deficiencyHypoglycemia?Hepatomegaly?Prognosis
GSD type 0Glycogen synthaseYesNoSome long-term concerns, but not life threatening
GSD type 3Glycogen debranching enzymeYesYesFailure to thrive; Symptoms usually regress with age and changes to diet
GSD type 4Glycogen branching enzymeNoYes, progressing to cirrhosisFailure to thrive; Liver and/or heart failure by 5 years of age
GSD type 5muscle glycogen phosphorylaseNoNoMuscle weakness and activity-induced cramps
GSD type 6liver glycogen phosphorylaseYesYesMild symptoms, not life threatening
How many anomeric carbons are contained in the glycogen tetramer depicted in Figure 1?
Choose 1 answer: