Carbohydrates in Chemistry and Biology || Lysosomal Degradation of Glycoproteins

✍ Scribed by Ernst, Beat; Hart, Gerald W.; Sina, Pierre

• Publisher: Wiley-VCH Verlag GmbH
• Year: 2000
• Tongue: German
• Weight: 787 KB
• Edition: 1
• Category: Article
• ISBN: 3527295119
• DOI: 10.1002/9783527618255.ch67

✦ Synopsis

Glycoproteins, like all other biological macromolecules, undergo catabolic turnover, a process that helps to regulate their concentration in order to meet changing physiological demands for their function. The lysosome is the only organelle that contains a complete set of glycosidases, proteases and glycosylasparaginase necessary for complete breakdown of Asn-linked glycoproteins to their constituent sugars and amino acids. These monomeric products are essential for being able to be transported out of the lysosomes. They also are the only final products that can be metabolically reused for production of free energy or resynthesis into polymers or other important metabolites. The lysosomal hydrolases involved in Asn-linked glycoprotein degradation are organized in a two-part, bidirectional metabolic pathway. Sugars are removed sequentially from the non-reducing ends of the carbohydrate chains by exoglycosidases, while degradation of the protein and protein-tocarbohydrate region occurs as an ordered series of reactions delimited by unique substrate specificities of the enzymes involved. Besides general turnover of glycoproteins in the lysosomes, there are now examples of final digestive products that are essential for the overall metabolic well-being of the organism. In humans, for example, the lysosomal degradative process includes the production of antigens and the formation of thyroid hormone. In addition, lysosomes take part in a degradative pathway for polymannose oligosaccharides that appears to be important in the quality control of N-linked glycoprotein biosynthesis. In this chapter these various aspects of N-linked glycoprotein catabolism by lysosomes will be discussed.