Carbohydrate synthesis and degradation play a vital role in cellular
function by providing the energy required for most metabolic processes.
The carbohydrates this chapter covers include three monosaccharides—glucose,
galactose, and fructose. We also discuss the polysaccharide glycogen.
The relevant biochemical pathways of these carbohydrates are shown
in Figure 154-1.
Glucose is the principal substrate of energy metabolism in humans.
Metabolism of glucose generates adenosine triphosphate (ATP) via
glycolysis (conversion of glucose or glycogen to pyruvate) or oxidative
phosphorylation in the mitochondria (conversion of pyruvate to carbon
dioxide and water), or both. A continuous source of glucose from
dietary intake, gluconeogenesis (glucose made de novo from amino
acids, primarily alanine), and degradation of glycogen maintains
normal blood glucose levels. We obtain glucose in our diet by ingesting
polysaccharides, primarily starch, and disaccharides, including
lactose maltose and sucrose.
Galactose and fructose are two other monosaccharides that can
provide fuel for cellular metabolism; however, their role is much less
significant than that of glucose. Galactose is derived from lactose
(galactose + glucose), which is found primarily in milk
and milk products. If necessary, galactose can be incorporated into
glycogen via galactose-1-phosphate and glucose 1-phosphate, becoming
a source of glucose. Galactose is also an important component for
certain glycolipids, glycoproteins, and glycosaminoglycans. The
two dietary sources of fructose are sucrose (fructose + glucose),
a commonly used sweetener, and fructose, which is found in fruits,
vegetables, and honey.
The defects in glycogen metabolism typically cause an accumulation
of glycogen in the tissues, hence the name glycogen storage disease
(GSD). The defects in gluconeogenesis or in the glycolytic pathway,
including galactose and fructose metabolism, do not usually result
in an accumulation of glycogen in the tissues.
Clinical manifestations of the various disorders of carbohydrate
metabolism differ markedly. The symptoms range from relatively benign
Metabolic pathways related to glycogen storage diseases
and to 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. Glycogen storage diseases. In: Isselbacher KJ, et al, eds. Harrison’s
Principles of Internal Medicine. 13th ed. New York: McGraw-Hill; 1994.)