Glycogen is a branched polysaccharide of glucose , a homoglucan. It contains only two types of glycosidic linkages, chains of α1→4-linked glucose residues with α1→6 branches spaced about every 4-6 residues along the α1→4 chain (Fig. 12.2). Glycogen is closely related to starch, the storage polysaccharide of plants, but starch consists of a mixture of amylose and amylopectin. The amylose component contains only linear α1→4 chains; the amylopectin component is more glycogen-like in structure but with fewer α1→6 branches, about one per 12 α1→4-linked glucose residues. The gross structure of glycogen is dendritic in nature, expanding from a core sequence bound to a tyrosine residue in the protein glycogenin and developing into a final structure resembling a head of cauliflower. The many glucose molecules on the surface of the glycogen molecule provide ready access for enzymes involved in rapid release of glucose from the glycogen polymer.
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| page 157 |  | | page 158 |
| Figure 12.1 Sources of blood glucose during a normal day. Between meals, blood glucose is derived primarily from hepatic glycogen. Depending on the frequency of snacking, glycogenolysis and gluconeogenesis may be more or less active during the day. Late in the night or in early morning, following depletion of a major fraction of hepatic glycogen, gluconeogenesis becomes the primary source of blood glucose. |
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Table 12-1.
Tissue distribution of carbohydrate energy reserves (70 kg adult). |
| Body_ID: None |
| Glucose and glycogen stores in the body (70 kg adult) |
| Body_ID: T012001.50 |
| Tissue | Type | Amount | % of tissue mass | Calories |
| Body_ID: T012001.100 |
| liver | glycogen | 75 g | 3-5% | 300 |
| Body_ID: T012001.150 |
| muscle | glycogen | 250 g | 0.5-1.0% | 1000 |
| Body_ID: T012001.200 |
| blood and extracellular fluid | glucose | 10 g | - | 40 |
| Body_ID: T012001.250 |
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