Glycosylphosphatidylinositol (GPI) anchor biosynthesis

Glycosylphosphatidylinositol anchors are glycolipids composed of phosphatidylinositol and the carbohydrate residues, glucosamine and mannose. The carbohydrates are glycosidically attached to the inositol groups. These can be attached to the C-terminal of proteins via post-translational modifications. The fatty acid chains within the phosphatidylinositol residues are hydrophobic and therefore anchor proteins to membranes. The carbohydrate moieties, glucosamine and mannose are provided by the substrates UDP-N-acetyl-glucosamine and dolichyl-phosphate D-mannose from the aminosugars metabolism and N-glycan biosynthesis pathways respectively. The synthesis of GPI anchors start at the outside of the ER membrane and the transfer of phosphoethanolamine group takes place inside of ER membrane. The proteins which are transferred to GPI anchor have a signal peptide (cleavage site, spacer and hydrophobic stretch) near the C-terminus. As they have a stretch of hydrophobic amino acids, they are attached to the ER membrane. GPI anchor transamidase cleaves the appropriate peptide bond and transfers GPI anchor to the protein. These GPI-anchored proteins process through the secretory pathway and ends up in the outer side of plasma membrane.

 

Enzyme EC Number Gene id
Manosyltransferase - PIG M 2.4.1.- cgd1_3720
Manosyltransferase - PIG V 2.4.1.- Cgd6_5100
Dolichyl-phosphate-mannose-glycolipid a-mannosyltransferase 2.4.1.130 cgd3_3590
Phosphatidylinositol N-acetylglucosaminyltransferase 2.4.1.198 cgd1_3380
Phosphatidylinositol N-acetylglucosaminyltransferase - PIG-P 2.4.1.198 cgd2_840
Phosphatidylinositol N-acetylglucosaminyltransferase - PIG-A 2.4.1.198 cgd4_2100
Phosphatidylinositol N-acetylglucosaminyltransferase 2.4.1.198 cgd5_1850
Phosphatidylinositol N-acetylglucosaminyltransferase - PIG-A 2.4.1.198 cgd5_3140
N-acetylglucosaminylphosphatidylinositol deacetylase 3.5.1.89 cgd7_700
GPI anchor transamidase - PIG-U none cgd1_220
GPI anchor transamidase - PIG-T none cgd6_3240
PIG-O none cgd8_2260
PIG-W none Missing in annotation

 

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Sources and fates of metabolites

 

Substrate Source pathways Product Fate pathways
Phosphatidyl-1D-myo-inositol Inositol phosphate metabolism    
UDP-N-acetyl-glucosamine Aminosugars metabolism Acetate Pyruvate metabolism
Palmitoyl-CoA Fatty acid elongation in the ER    
Dolichyl-P-mannose N-glycan biosynthesis Dolichyl-P N-glycan biosynthesis
Phosphatidylethanolamine Phosphatidylethanolamine and phosphatidylserine metabolism