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Chapter 2: Biological Building Blocks: Carbohydrates

The parents of a 7-month-old infant bring their son to the pediatrician because they have noticed that their child has a dull response to outside stimuli. In addition, they note that their child exhibits an exaggerated startle response (sudden extension of arms and legs) to sharp sounds and that he seems to be losing previously acquired motor and mental skills. His pediatrician makes a diagnosis of Tay-Sachs disease. Elevated intracellular levels of which of the following carbohydrate-containing compounds would provide useful information for confirmation of this diagnosis?

A. abnormal glycogen

B. aldohexoses

C. aldoketoses

D. gangliosides

E. glycosaminoglycans

Answer D: In the genetic disorder known as Tay-Sachs disease, ganglioside GM2 is not catabolized. As a consequence, the ganglioside concentration is elevated many times higher than normal. The functionally absent lysosomal enzyme is β-N-acetyl hexosaminidase (more commonly called hexosaminidase A). The elevated GM2 results in irreversible brain damage to infants, who usually die before the age of 3 years. Under normal conditions, this enzyme cleaves N-acetylgalactosamine from the oligosaccharide chain of this complex sphingolipid, allowing further catabolism to occur.

The parents of an 18-month-old girl are alarmed at the rapid deterioration in her motor skills and the appearance of an engorged belly. They note that when their daughter tries to follow the movement of an object she thrusts her head in the direction of movement as if she cannot move her eyes from side-to-side. Physical and laboratory examination reveals that the child is suffering from Gaucher disease. Which of the following carbohydrate-containing compounds would be expected to be elevated in macrophages from this patient?

A. abnormal glycogen

B. cerebrosides

C. glycosaminoglycans

D. GM2 ganglioside

E. neuraminic acid

Answer B: Gaucher disease is characterized by the lysosomal accumulation of glucosylceramide (glucocerebroside) which is a normal intermediate in the catabolism of globosides and gangliosides. Gaucher disease results from defects in the gene encoding the lysosomal hydrolase: acid β-glucosidase, also called glucocerebrosidase. The hallmark feature of Gaucher disease is the presence of lipid-engorged cells of the monocyte/macrophage lineage with a characteristic appearance in a variety of tissues.

The aldehyde and ketone moieties of the carbohydrates with 5 and 6 carbons will spontaneously react with alcohol groups present in neighboring carbons to produce ring structures. The rings can open and close allowing for different ...

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