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Related Experiment Videos

Mammalian alpha-mannosidases--multiple forms but a common purpose?

P F Daniel1, B Winchester, C D Warren

  • 1Department of Biomedical Sciences, Shriver Center for Mental Retardation, Waltham, MA 02254.

Glycobiology
|October 1, 1994
PubMed
Summary

This review reclassifies alpha-mannosidases, proposing endoplasmic reticulum (ER) enzymes have key catabolic roles in degrading dolichol intermediates and defective glycoproteins, explaining disease phenotypes.

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Area of Science:

  • Biochemistry
  • Glycobiology
  • Enzymology

Background:

  • Alpha-mannosidases were traditionally viewed as processing newly formed N-glycans or degrading mature glycoproteins.
  • This perspective is challenged by new evidence suggesting significant catabolic functions for ER-resident alpha-mannosidases.

Purpose of the Study:

  • To propose a revised classification and functional understanding of alpha-mannosidases, particularly focusing on ER-localized enzymes.
  • To elucidate the roles of specific ER alpha-mannosidases in the degradation of dolichol intermediates and malfolded glycoproteins.

Main Methods:

  • Review of existing literature and recent experimental evidence.
  • Analysis of oligosaccharide excretion patterns in animal models and human patients with alpha-mannosidosis and beta-mannosidosis.

Related Experiment Videos

  • Inhibitor studies using pyranose and furanose analogues to classify alpha-mannosidases.
  • Comparison with protein sequence homology-based classifications.

    • Main Results:

    • Two ER alpha-mannosidases, previously assigned processing roles, are now proposed to have critical catabolic activities.
    • ER/cytosolic mannosidase is implicated in degrading dolichol intermediates and explains specific oligosaccharide excretion in cats and cattle.
    • Soluble Man9-mannosidase degrades glycans on malfolded proteins retained in the ER, and its action, along with lysosomal enzymes, explains patient excretion patterns.
    • Alpha-mannosidases can be classified into Class 1 (e.g., Golgi mannosidase I) and Class 2 (e.g., lysosomal alpha-mannosidase) based on linkage specificity and inhibitor studies.

    Conclusions:

    • The proposed dual roles of ER alpha-mannosidases in catabolism provide a unified explanation for observed phenotypes in lysosomal storage diseases.
    • The classification of alpha-mannosidases into two classes based on linkage specificity and sequence homology suggests an evolutionary divergence from two primordial genes.
    • Further research into the specific functions and regulation of these enzymes can deepen our understanding of glycoprotein quality control and lysosomal storage diseases.