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 Missing in annotation
Dolichyl-diphosphooligosaccharide-protein glycotransferase (STT) 2.4.1.119 CMU_007850
Dolichyl-diphosphooligosaccharide-protein glycotransferase (STT) 2.4.1.119 CMU_013170
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) CMU_016380
alpha-1,2-mannosyltransferase (ALG11) 2.4.1.131 Missing in annotation
Mannosyltransferase (ALG2) 2.4.1.132; 2.4.1.257 CMU_034900
N-acetylglucosaminyldiphospho dolichol N-acetylglucosaminyltransferase (ALG14) 2.4.1.141 CMU_005550
Chitobiosyldiphosphodolichol beta-mannosyltransferase (ALG1) 2.4.1.142 CMU_033850

Dol-P-Glc:Glc1-Man9-GlcNAc2-PP -Dol alpha-1,3-glucosyltransferase (ALG8)

2.4.1.265 Missing in annotation
Dol-P-Glc:Man9-GlcNAc2-PP-Dol alpha-1,3-glucosyltransferase (ALG6)

 

2.4.1.267 Missing in annotation
Dolichyl-phosphate beta-D-mannosyltransferase (DPM1) 2.4.1.83 CMU_039780
Farnesyltransferase 2.5.1.29 CMU_028740
Dolichol kinase 2.7.1.108 CMU_013270
UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosaminephospho transferase (ALG7) 2.7.8.15 CMU_039560
Glucan 1,3-alpha-glucosidase (GANAB) 3.2.1.84 CMU_043240
Dolichyldiphosphatase 3.6.1.43 CMU_011730

 

Open in a new window

 

 

Sources and fates of metabolites

 

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