Mannose structure

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. 

Golgi. However, in lower eukaryotes such as fungi and yeast the high-mannose structures are released from the cell as end products of glycosyla-tion (Meynial-Salles and Combes, 1996). 

By mass spectrometric analysis, Asp-N peptide 19.4 (Figure 3) contained primarily one high mannose structure containing 5 mannose units. Asp-N peptide 20.3 produced mass spectra (Figures 4,5) that are consistent with having high mannose structures with 4, 5, and 8 mannose units. 

L-arabinose. A paper-chromatographic study of the mercaptalation reaction with n-galactose, D-glucose, and D-mannose confirmed that almost complete conversion to the dithioacetal occurred within 5 minutes, but, after several hours, an equilibrium was set up in which two 1-thio-glycosides, probably the anomeric pyranosides, the dithioacetal, and the free aldose were all present the 1-thioglycoside was particularly favored with the D-mannose structure. 3-Amino-3-deoxy-D-mannose reacted (qualitatively) similarly to D-mannose, but more slowly, probably because of the polar influence of the amino group. 

Figure 13 Endo H cleaves only high-mannose structures (n=2-150, x=(Man)l-2, y=H) and hybrid structures (e.g., n=7., x and/or y=NeuAc-Gal-GlcNAc).

The confident assignment of glycan strnctnres depends on the determination of the seqnence and branching strncture. For example, for the released and permethylated UP la glycans whose MALDI-TOF MS profile is shown in Fig. 14A, the assignments of the observed [M-tNa]" peaks to individual high-mannose structures could be made from subsequent ESI MS/MS analyses. For the component with [M + 2Na] + m/z 1209.6 in the ESI mass spectrum (corresponding to [M- -Na]+ m/z 2419.16 in the MALDI-TOF mass spectrum shown in Fig. 14A),... 

Determination of Galactose-to-Galactose and Galactose-to-Mannose Structures in )S-d-Galactofurano- -D-Mannopyranans by C-13 NMR Spectroscopy... 

Figure 1.3 Examples of High Mannose Structures in Glycoproteins. Of the examples shown (M) and (M)g have been found in the membranes of Chinese hamster ovary (CHO) cells and the remainder in ovalbumin. Several other glycoproteins are known to carry closely related structures, in which the arrangement of mannosyl residues is strongly conserved (see Tai et at., 1975 Li and Komfeld, 1979). Monoglucosylated forms of (M)9, (M)g and (M)7 are now known (Mellis and Baenziger, 1983).

Figure 1.5 Phosphorylated High Mannose Structures. These are considered to be peculiar to many, but not all, acid hydrolases of lysosomes. The forms shown have been detected in the murine lymphoma BW 5147.3 (Varki and Kornfeld, 1980a). Glucosylated forms are also known (Gabel and Kornfeld, 1982).

Figure 4.9 Scheme of the Experiments of Harpaz and Schachter (1979) Which Show the Requirement of Mannosidase-II for N-acetylglucosamine at C-3 of the a-Mannose, Linked to C-3 of the p-Mannose. Structures lacking A/-acetylglucosamine were not degraded. The pathway appeared to fork, as shown. 

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