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.

 

Of the enzymes required for the incorporation of 9 mannose, 2 N-acetylglucosamine and 3 glucose residues, the enzymes for the incorporation of second N-acetylglucosamine and third glucose residues are missing in Cryptosporidium parvum and Cryptosporidium hominis, whereas the enzymes for the incorporation of all three glucose residues and the second N-acetylglucosamine residues are missing in Cryptosporidium muris genome. The comparison to the N-glycan biosynthesis pathway of Toxoplasma gondii showed that the last two enzymes in T. gondii pathway catalysing the removal of 4 mannose residues and the addition of third N-acetylglucosamine residue are absent in Cryptosporidium species.

 

Enzyme EC Number Gene id
Dolichyl-phosphate beta-glucosyltransferase (ALG5) 2.4.1.117 cgd5_2590
Dolichyl-diphosphooligosaccharide-protein glycotransferase (STT) 2.4.1.119 cgd2_1650
Dolichyl-diphosphooligosaccharide-protein glycotransferase (STT) 2.4.1.119 cgd6_2040
Mannosyltransferase (ALG3/9/12) 2.4.1.130 (Entry transferred to 2.4.1.258, 2.4.1.259, 2.4.1.260 & 2.4.1.261) cgd3_3590
alpha-1,2-mannosyltransferase (ALG11) 2.4.1.131 cgd4_2990
Mannosyltransferase (ALG2) 2.4.1.132; 2.4.1.257 cgd1_230
N-acetylglucosaminyldiphospho dolichol N-acetylglucosaminyltransferase (ALG14) 2.4.1.141 Missing in annotation
Chitobiosyldiphosphodolichol beta-mannosyltransferase (ALG1) 2.4.1.142 cgd7_1810
Dol-P-Glc:Glc1-Man9-GlcNAc2-PP -Dol alpha-1,3-glucosyltransferase (ALG8) 2.4.1.265 cgd1_2100
Dol-P-Glc:Man9-GlcNAc2-PP-Dol alpha-1,3-glucosyltransferase (ALG6) 2.4.1.267 cgd4_3120
Dolichyl-phosphate beta-D-mannosyltransferase (DPM1) 2.4.1.83 cgd5_2040
Farnesyltransferase 2.5.1.29 cgd1_1980
Dolichol kinase 2.7.1.108 cgd2_1560
UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosaminephospho transferase (ALG7) 2.7.8.15 cgd5_2240
Glucan 1,3-alpha-glucosidase (GANAB) 3.2.1.84 cgd3_1580
Glucan 1,3-alpha-glucosidase (GANAB) 3.2.1.84 cgd8_1420
Dolichyldiphosphatase 3.6.1.43 cgd8_5200

 

Open in a new window

 

 

Sources and fates of metabolites

 

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