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Chapter 38: Glycoproteins

I-cell disease (also identified as mucolipidosis type II) is characterized by the presence of inclusion bodies in fibroblasts (hence the derivation of the term I-cell), severe psychomotor retardation, corneal clouding, and dysostosis multiplex. These symptoms arise from a defect in the targeting of lysosomal enzymes due to an inability to carry out which of the following processes?

A. produce mannose-6-phosphate modifications in lysosomal enzymes

B. recycle the lysosomal receptor for mannose 6-phosphate present on lysosomal enzymes

C. remove mannose 6-phosphates from lysosomal enzymes prior to their transport to the lysosomes

D. synthesize the mannose 6-phosphate receptor found in lysosomes

E. transport mannose-6-phosphate receptors to lysosomes

Answer A: Enzymes that are destined for the lysosomes (lysosomal enzymes) are directed there by a specific carbohydrate modification. During transit through the Golgi apparatus, a residue of GlcNAc-1-phosphate (GlcNAc-1-P) is added to the carbon-6 hydroxyl group of one or more specific mannose residues that have been added to these enzymes. A specific Man-6-P receptor (MPR) is present in the membranes of the Golgi apparatus. Binding of Man-6-P to this receptor targets proteins to the lysosomes.

You are treating an 8-month-old female infant brought in by her parents because of seizures. Physical examination indicates microcephaly, microphthalmia with retinal detachment. and severe mental retardation. You suspect these symptoms are associated with a specific gene defect and order a test for the presence of mutations in α-dystroglycan. Test results are positive. A defect in which of the following, related to α-dystroglycan processing, is most likely in this patient?

A. N-glycosylation

B. O-GlcNAcylation

C. O-glycosylation mucin type

D. O-mannosylation

Answer D: O-mannosylation has been identified on α-dystroglycan (α-DG) from nerve and muscle, chondroitin sulfate proteoglycans and total glycopeptides from brain tissue. Clinically, O-mannosylation of α-DG is the most significant as evidenced by the constellation of symptoms that result from defects in the processes of O-mannosylation. Defective O-mannosylation is associated with a group of autosomal recessive muscular dystrophies termed congenital muscular dystrophies (CMDs). In addition to muscle dysfunction, these CMDs are also associated with variable brain and ocular abnormalities.

You are studying the responses of adipocytes in culture to insulin following addition of a test compound. In the absence of the compound, addition of insulin to your culture system results in increased glucose uptake. However, following addition of the test compound, the effects of insulin on glucose ...

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