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# Carbohydrates: The structure and function of glycogen

## Problem

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, start subscript, 24, end subscript H, start subscript, 42, end subscript O, start subscript, 21, end subscript.
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.
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 molecule depicted in figure 1?