Endoplasmic reticulum glycoprotein synthesis

During the transfer of the nascent peptide into the lumen of the cistern ae of the endoplasmic reticulum, the leader sequence is cleaved off. The glycoproteins remain anchored into the membrane by either their N- or carboxyl-terminus. The attachment of the oligosaccharide occurs concomitantly with the synthesis of the protein. As discussed in Section 11,2, this is a high-mannose structure that can be modified during transport on intracellular membranes, and then yields a complex type. 

The biosynthesis in yeast of two enzymes that are D-mannoproteins has been studied. A membrane-associated isozyme of invertase (EC 3.2.1.26) has been shown to be a precursor of the external invertase.190 Its molecular weight, as determined by SDS-poly(acrylamide) gel electrophoresis, is 50,000, that is, smaller than that of the external invertase, and it correlates well with the presence of only the inner-core sugars of the final form. It is strictly bound to membranes, possibly those of the endoplasmic reticulum, and it can be completely split191 by endo-/3-N-acetylglucosaminidase H (EC 3.2.1.30). The addition of tunicamycin, which specifically inhibits formation of d-GIcNAc-PP-DoI, inhibits synthesis of external invertase, as well as further formation of the membrane-associated form, which completely disappears after addition of the antibiotic.190 In these aspects, the synthesis of this extracellular enzyme follows the pathway for secreted glycoproteins in animal systems. 

The reactions of phase 2 relate to the attachment of the bridge-carbohydrate residues to the polypeptide chain. There is evidence showing that this addition occurs while the polypeptide chain is still attached to, or perhaps still being synthesized on, the ribosomes.101-103 Thus, 14C-labeled 2-amino-2-deoxy-D-glucose, injected into the circulatory system of the rat, was incorporated into protein in the ribosomes of the rough endoplasmic-reticulum of the liver. Administration of puromycin caused release of the 14C-labeled glycoprotein, which could be isolated by acid-precipitation methods. Examination of the radioactivity data revealed that the subcellular structures most actively involved in glycoprotein synthesis were the ribosomes bound to the membrane, and not free polysomes. 

Famesyl pyrophosphate is used for the synthesis of ubiquinone, dolichol, and squaJenc, Ubiquinone is a cofactor in the respiratory chain of the mitochondrion, Dolichol phosphate serves as a biochemical "handle," and is used to hold the cote oligosaccharide, and to facilitate its transfer to newly made proteins in the endoplasmic reticulum, to form glycoproteins. Squalene is the product of condensation of two FFF molecules. 

---