Chapter 362

Carbohydrate metabolism plays a vital role in cellular function by providing the energy required for most metabolic processes. The relevant biochemical pathways involved in the metabolism of these carbohydrates are shown in Fig. 362-1. Glucose is the principle substrate of energy metabolism in humans. Metabolism of glucose generates ATP through glycolysis and mitochondrial oxidative phosphorylation. The body obtains glucose through the ingestion of polysaccharides, primarily starch, and disaccharides including lactose, maltose, and sucrose. Galactose and fructose are two other monosaccharides that serve as sources of fuel for cellular metabolism; however, their role as fuel sources is much less significant than that of glucose. Galactose is derived from lactose (galactose + glucose), which is found in milk products, and is an important component for certain glycolipids, glycoproteins, and glycosaminoglycans. Fructose is found in fruits, vegetables, and honey. Sucrose (fructose + glucose) is another dietary source of fructose and is a commonly used sweetener.

###### Figure 362-1

Metabolic pathways related to glycogen storage diseases and galactose and fructose disorders. Nonstandard abbreviations are as follows: GSa, active glycogen synthase; GSb, inactive glycogen synthase; Pa, active phosphorylase; Pb, inactive phosphorylase; PaP, phosphorylase a phosphatase; PbKa, active phosphorylase b kinase; PbKb, inactive phosphorylase b kinase; G, glycogenin, the primer protein for glycogen synthesis. [Modified from AR Beaudet, in KJ Isselbacher et al (eds): Harrison's Principles of Internal Medicine, 13th ed., New York, McGraw-Hill, 1994, p 1855.]

Glycogen, the storage form of glucose in animal cells, is composed of glucose residues joined in straight chains by α1-4 linkages and branched at intervals of 4–10 residues by α1-6 linkages. Glycogen forms a treelike molecule and can have a molecular weight of many millions. Glycogen may aggregate to form structures recognizable by electron microscopy. With the exception of glycogen storage disease (GSD) type 0, defects in glycogen metabolism typically cause an accumulation of glycogen in the tissues, hence the name glycogen storage diseases. The stored glycogen could be normal or abnormal in structure in the various disorders. The defects in gluconeogenesis or glycolytic pathways, including galactose and fructose metabolism, do not usually result in glycogen accumulation.

Clinical manifestations of the various disorders of carbohydrate metabolism differ markedly. The symptoms range from harmless to lethal. Unlike disorders of lipid metabolism, mucopolysaccharidoses, or other storage diseases, dietary therapy has been effective in many of the carbohydrate disorders. All of the genes responsible for the inherited defects of carbohydrate metabolism have been cloned, and mutations have been identified. Advances in our understanding of the molecular basis of these diseases are being used to improve diagnosis and management. Some of these disorders are candidates for enzyme replacement, substrate reduction, and early trials of gene therapy.

Historically, the glycogen storage diseases were categorized ...

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