Glycoproteins catabolism

Based on its primary sequence, mannosidase II is a member of the class II mannosidase family (CAZy glycohydrolase family 38, which includes also the lysosomal mannosidase involved in glycoprotein catabolism). Like the lysosomal enzyme, mannosidase II is inhibited by swain-sonine [62], thus this reagent can be used to block the formation of complex A-glycans. The... 

Glycoprotein Catabolism multiple routes for glycoprotein breakdown... 

Several mannosidases have been identified that are involved in glycoprotein biosynthetic reactions and in glycoprotein catabolism. Comparisons of biochemical characteristics and sequence similarities have allowed the exo-mannosidases to be divided into two classes. Within each of the classes are subgroups that have been... 

Table 4. Lysosomal disorders involving glycoprotein catabolism and other enzymes.

Ashwell, G., and Morell, A. G., 1971, Galactose A cryptic determinant of glycoprotein catabolism, in Glycoproteins of Blood Cells and Plasma (G. A. Jamieson and T. J. Greenwalt, eds.), pp. 173-189, Lippincott, Philadelphia. 

Transferrin (Tf) is a Pj-globulin with a molecular mass of approximately 76 kDa. it is a glycoprotein and is synthesized in the liver. About 20 polymorphic forms of transferrin have been found, it plays a central role in the body s metabolism of iron because it transports iron (2 mol of Fe + per mole of Tf) in the circulation to sites where iron is required, eg, from the gut to the bone marrow and other organs. Approximately 200 billion red blood cells (about 20 mL) are catabolized per day, releasing about 25 mg of iron into the body—most of which will be transported by transferrin. 

Although a-D-mannosidase appears to be under endocrine control,28,28 its natural function remains obscure. It is probable that the enzyme is important in the catabolism of D-mannose-containing glycoproteins,12,16,88 and, therefore, that the important trace element zinc could be indirectly implicated in this process. It is well known that zinc deficiency has an adverse effect on fertility in the male,89,90 and it could be argued that the changes observed in a-D-mannosidase activity in uterus and epididymis indicate that this enzyme is important in reproduction. It is scarcely justifiable to ascribe a cause-and-effect relationship to the correlations observed between the zinc content and the a-D-mannosidase activity in epididymis. All that can be said with any certainty is that, under certain circumstances, the two variables respond together to a common stimulus. 

Interestingly, all of the enzymes that comprise the sphingolipid-catabolic pathway are glycoproteins and may be found in different places within the organelle. For example, glucocerebrosidase is firmly associated with the lysosomal membrane, whereas others, like hexoseaminidase A, exist largely in soluble form in the lysosomal matrix. A common property of the lysosomal hydrolases, however, is their expression of maximum activity at a relatively acidic pH (i.e., pH 4.0-5.5), hence the term acid hydrolase. This is not unexpected because ATP-driven proton pumps sustain an acidic milieu (pH 5.2) within the lysosome. 

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