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גליקנים יכולים להימצא מצורפים לחלבונים כמו גליחלבונים ופרוטגליקנים. באופן כללי, הם נמצאים בחלק החיצוני של תאים. גליקנים מסוג O ו -N נפוצים מאוד באויקרוציטים אבל יכולים להימצא, אולם באופן נדיר יותר, גם בפרוקריוטים.
=== גליקנים מקושרי Nחנקן ===
N-Linked glycans[edit]
Introduction[edit]
N-Linked glycans are attached in the endoplasmic reticulum to the nitrogen (N) in the side chain of asparagine in the sequon. The sequon is an Asn-X-Ser or Asn-X-Thr sequence, where X is any amino acid except proline and the glycan may be composed of N-acetyl galactosamine, galactose, neuraminic acid, N-acetylglucosamine, fucose, mannose, and other monosaccharides.
Assembly[edit]
In eukaryotes, N-linked glycans are derived from a core 14-sugar unit assembled in the cytoplasm and endoplasmic reticulum. First, two N-acdoplasmic reticulum membrane. Five mannose residues are then added to this structure. At this point, the partially finished core glycan is flipped across the endoplasmic reticulum membrane, so that it is now located within the reticular lumen. Assembly then continues within the endoplasmic reticulum, with the addition of four more mannose residues. Finally, three glucose residues are added to this structure. Following full assembly, the glycan is transferred en bloc by the glycosyltransferase oligosaccharyltransferase to a nascent peptide chain, within the reticular lumen. This core structure of N-linked glycans, thus, consists of 14 residues (3 glucose, 9 mannose, and 2 N-acetylglucosamine).
Image: http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=glyco.figgrp.469
Dark squares are N-acetyl-glucosamine; light circles are mannose; dark triangles are glucose.
==== הקדמה ====
גליקנים מקושרי חנקן מצורפים ברשתית תוך-פלזמית לחנקן בשרשרת הצד של asparagine ברצף.
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