# Glycosylation Types > N-linked and O-linked glycosylation in MBBS Biochemistry: sequons, dolichol, oligosaccharyltransferase, Golgi processing, ABO antigens and CDG. - Canonical URL: https://prepelephant.com/topics/mbbs/biochemistry/glycosylation-types - Exam / course: MBBS · Subject: Biochemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Glycosylation Types", PrepElephant, https://prepelephant.com/topics/mbbs/biochemistry/glycosylation-types ## Direct answer Asn-X-Ser/Thr is a three-residue password: wherever it appears (X being any amino acid except proline), N-linked glycosylation can attach a prefabricated 14-sugar oligosaccharide — built on a dolichol pyrophosphate carrier in the endoplasmic reticulum membrane — to the amide nitrogen of asparagine, transferred en bloc by oligosaccharyltransferase as the chain emerges from the ribosome. O-linked glycosylation follows the opposite strategy: single sugars are added stepwise in the Golgi onto the hydroxyl oxygen of serine or threonine, beginning with N-acetylgalactosamine, with no strict consensus sequon. Both attachments aid folding, solubility and survival in circulation — the reason erythropoietin and its extra-glycosylated analogue darbepoetin differ in half-life. ## What you must remember - **N-linked mechanics:** sequon Asn-X-Ser/Thr (X not proline), dolichol-pyrophosphate donor, oligosaccharyltransferase, cotranslational transfer in the ER, then glucose trimming drives the calnexin quality-control cycle. - **N-glycan maturation:** high-mannose forms become hybrid or complex types in the Golgi; GlcNAc transferase I commits the branch toward complex glycans, and mannosidase II is the medial-Golgi gatekeeper (swainsonine blocks it, freezing hybrids). - **O-linked mechanics:** Golgi-resident, first sugar GalNAc on Ser/Thr, no sequon requirement; mucins carry hundreds of O-glycans on extended stalks. - **ABO chemistry on the sugar chain:** the H antigen is the base oligosaccharide; the A enzyme transfers N-acetylgalactosamine, the B enzyme galactose, and the O allele encodes a non-functional transferase, leaving H bare. - **Bombay (Oh) phenotype:** described in Mumbai in 1952, these individuals lack the H antigen entirely and carry anti-H, so they can be transfused only with Bombay blood — India's classic immunohaematology viva. - **Congenital disorders of glycosylation:** type Ia (phosphomannomutase 2) is the commonest, with inverted nipples, lipodystrophy and coagulopathy; type Ib (phosphomannose isomerase) uniquely responds to oral mannose. - **Glycoengineering:** extra sialic-acid-bearing chains on darbepoetin extend its half-life roughly threefold over erythropoietin — glycans as a pharmacokinetic dial. - **Distinguish glycation:** HbA1c is non-enzymatic glucose addition, not glycosylation — a one-mark trap. ## Two antigens, one pathway, walked through Trace an A-group person's red-cell membrane. A glycolipid and glycoprotein core chain terminates in fucose — that is H substance, the product of the FUT1 (H) gene. The A transferase, a Golgi enzyme using UDP-N-acetylgalactosamine, caps H with GalNAc; the B version, differing by a handful of amino acids, uses UDP-galactose instead; group O homozygotes possess a frameshifted, inactive enzyme and display naked H. Antibodies develop against absent sugars, hence anti-B in group A. Now compound the lesson with the Bombay phenotype: without H, neither A nor B enzymes have a substrate, so genotypically A or B individuals type as O — until anti-H agglutinates every ordinary donor cell. The takeaway for exams: blood-group serology is applied glycosyltransferase chemistry, and the enzyme list is short enough to recite. ## How the exam frames it Two confusions dominate. Glycosylation versus glycation — enzymatic, ER/Golgi-located, sequon-directed versus non-enzymatic, concentration-driven, Amadori-product forming; examiners increasingly frame this as HbA1c versus normal glycoproteins. Second, site: N-linked begins in the ER (en bloc, dolichol) while O-linked is essentially all Golgi, added sugar by sugar — mixing the two origins costs the whole answer. A favourite follow-up asks why proline cannot occupy the X position: its rigid ring breaks the turn that brings the sequon's hydroxyl alongside the asparagine for transfer. Mentioning oral mannose therapy for CDG-Ib earns the "textbook-specific fact" nod — it is the only CDG with a cheap, effective treatment. ## Frequently asked questions ### What is the sequon required for N-linked glycosylation? Asn-X-Ser/Thr, where X is any amino acid except proline; oligosaccharyltransferase transfers the dolichol-linked oligosaccharide to the asparagine. ### Where do N-linked and O-linked glycosylation begin? N-linked transfer is cotranslational in the endoplasmic reticulum, whereas O-linked addition starts and largely finishes in the Golgi with GalNAc on serine or threonine. ### What distinguishes A and B antigens chemically? A single terminal sugar: N-acetylgalactosamine on H substance in group A, galactose in group B, and no added sugar in group O. ### What is the Bombay phenotype? An H-antigen-deficient (hh) blood group first described in Mumbai in 1952; individuals form anti-H antibodies and can receive only Bombay-group blood. ### Why does darbepoetin outlast erythropoietin? Additional N-glycan chains carrying sialic acid reduce renal clearance, extending the analogue's plasma half-life about threefold.