Lysosomal Degradation of Glycoproteins

LYSOSOMAL DEGRADATION OF GLYCOPROTEINS

Degradation of glycoproteins is similar to that of the GAGs. The lysosomal acid hydrolases are each generally specific for the removal of one component of the glycoprotein. They are primarily exoenzymes that remove their respective groups in the reverse order of their incorporation (“last on, first off”). If any one degradative enzyme is missing, degradation by the other exoenzymes cannot continue. A group of very rare, autosomal recessive diseases called the glycoprotein storage diseases (oligosaccharidoses), caused by a deficiency of any one of the degradative enzymes, results in accumulation of partially degraded structures in the lysosomes. For example, α-mannosidosis type 1 is a progressive, fatal deficiency of the enzyme, α-mannosidase. Presentation is similar to Hurler syndrome, but immune deficiency is also seen. Mannose-rich oligosaccharide fragments appear in the urine. Diagnosis is by enzyme assay.