N-Glycan biosynthesis

The term ‘Glycan’ refers to oligosaccharides or polysaccharides. They also refer to glycoconjugates such as glycolipids and glycoproteins. Two different types of glycans attached to proteins are N-glycans and O-glycans. N-glycans, more precisely N-linked glycans are attached to the Asp residue of the tripeptide sequence of Asp-X-Ser/Thr (X is any amino acid except Pro) of proteins. In most eukaryotes, the N-glycan anchors are derived from a 14-sugar oligosaccharide containing 9 mannose, 3 glucose and 2 N-acetylglucosamine molecules. This process begins with the addition of 2 N-acetylglucosamine to dolichyl-phosphate in the cytoplasmic side of endoplasmic reticulum membrane. After the addition of 2 N-acetylglucosamine and 5 mannose residues, the remaining process continues in the internal side of ER. Then the enzyme oligosaccharide-protein glycosyltransferase links this glycan anchor to the above mentioned Asp residue of polypeptides in the lumen of ER. The glycan anchor can then be processed (by trimming and extension of sugar residues) to produce various types of N-glycans. This process takes place in golgi apparatus. The main processing includes removal of glucose and mannose residues and addition of N-acetylglucosamine, N-acetylneuraminic acid and fucose residues. The attachment of the glycan anchors is important for the proper folding of many proteins.

 

This pathway was not reconstructed for Plasmodium falciparum in MPMP. This pathway is reconstructed with KEGG reference pathway of N-glycan biosynthesis as template. The analysis of Toxoplasma gondii gene models showed that it has all the enzymes necessary for the formation of 14-sugar core anchor from dolichyl-phosphate. The enzyme which catalyses N-linked glycosylation (oligosaccharide-protein glycosyltransferase) is also present. In addition they also possess the enzymes for the trimming of 3 glucose residues and this forms high-mannose oligosaccharide N-glycans. Except MAN1, all the enzymes required for the removal of 4 mannose residues and addition one more N-acetylglucosamine sugar (forms complex oligosaccharide N-glycans) is also present in the gene models. The two enzymes which are present in P. falciparum (dolichol metabolism pathway) and involved in conversion of dolichol phosphate from dolichol (dolichol kinase) and dolichol diphosphate (dolichyldiphosphatase) were absent in the release 7 T. gondii gene models. The gene for dolichol kinase was recently identified in the newer release 8 gene models.

 

Enzyme EC Number Gene id Protein localisation Localisation data source
alpha-1,3-mannosyl-glycoprotein 2-beta-N-acetylglucosaminyl transferase (MGAT1) 2.4.1.101 TGME49_266310    
Dolichyl-phosphate beta-glucosyltransferase (ALG5) 2.4.1.117 TGME49_216540    

Dolichyl-diphosphooligosaccharide-protein glycotransferase

(STT)

2.4.1.119 TGME49_203970    

Dolichyl-diphosphooligosaccharide-protein glycotransferase

(STT)

2.4.1.119 TGME49_231430 Plasma membrane Go annotation; Previous publication

Mannosyltransferase

(Dol-P-Man:Man5-GlcNAc2-PP-Dol alpha-1,3-mannosyltransferase/Dol-P-Man:Man6-GlcNAc2-PP-Dol alpha-1,2-mannosyltransferase/Dol-P-Man:Man7-GlcNAc2-PP-Dol alpha-1,6-mannosyltransferase/Dol-P-Man:Man8-GlcNAc2-PP-Dol alpha-1,2-mannosyltransferase

(ALG3/9/12)

2.4.1.130; Entry changed to: 2.4.1.258, 2.4.1.259, 2.4.1.260 & 2.4.1.261 TGME49_321660 Endoplasmic reticulum membrane; Plasma membrane GO annotation; Previous publication

alpha-1,2-mannosyltransferase (ALG11)

2.4.1.131 TGME49_246982 Plasma membrane Previous publication

Mannosyltransferase

(GDP-Man:Man1-GlcNAc2-PP-Dol alpha-1,3-mannosyltransferase/ GDP-Man:Man2-GlcNAc2-PP-Dol alpha-1,6-mannosyltransferase)

(ALG2)

2.4.1.132;

2.4.1.257

TGME49_227790    
N-acetylglucosaminyldiphospho dolichol N-acetylglucosaminyltransferase (ALG14) 2.4.1.141 TGME49_268340    

Chitobiosyldiphosphodolichol beta-mannosyltransferase

(ALG1)

2.4.1.142 TGME49_230590 Extracellular? Previous publication

Dol-P-Glc:Glc2-Man9-GlcNAc2-PP -Dol alpha-1,2-glucosyltransferase

(ALG10)

2.4.1.256 TGME49_263690    

Dol-P-Glc:Glc1-Man9-GlcNAc2-PP -Dol alpha-1,3-glucosyltransferase

(ALG8)

2.4.1.265 TGME49_314730 Endoplasmic reticulum membrane; plasma membrane GO annotation; Previous publication

Dol-P-Glc:Man9-GlcNAc2-PP-Dol  alpha-1,3-glucosyltransferase

(ALG6)

2.4.1.267 TGME49_262030    

Dolichyl-phosphate beta-D-mannosyltransferase

(DPM1)

2.4.1.83 TGME49_277970 Endoplasmic reticulum Previous publication
Farnesyltransferase 2.5.1.29 TGME49_200370    
Dolichol kinase 2.7.1.108 TGME49_271170 (release 8 gene model)    
UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosaminephospho transferase (ALG7) 2.7.8.15 TGME49_244520    
Mannosyl-oligosaccharide glucosidase (GCS1) 3.2.1.106 TGME49_242020 Mitochondrion Previous publication
Mannosyl-oligosaccharide 1,2-alpha-mannosidase (MAN1) 3.2.1.113 Missing in annotation    
Glucan 1,3-alpha-glucosidase (GANAB) 3.2.1.84 TGME49_253030 Plasma membrane Previous publication
Dolichyldiphosphatase 3.6.1.43 Missing in annotation    

 

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Sources and fates of metabolites

 

Substrate Source pathways Product Fate pathways
Dolichol Host    
Farnesyl-PP Terpenoid metabolism    
Isopentenyl-PP Isoprenoids metabolism    
UDP-Glucose Starch and galactose metabolism Glucose Glycolysis, Starch and galactose metabolism
UDP-N-acetyl-glucosamine Aminosugars metabolism    
GDP-D-mannose Mannose and fructose metabolism