| Description | The E. coli O121 antigen monomer is a linear tetrasaccharide with the structure [->3)bDQui4NGlyAc(1->4)aDGalNAcAN(1->4)aDGalNAcA(1->3)aDGlcNAc(1->].
The 4-aminoquinovose sugar is also known as viosamine and the NGlyAc is an acetylglycylamido group; GalNAcA is N-acetylgalturonic acid, and GalNAcAN is N-acetylgalacturonamide. Additionally, the DGalNAcAN is reported to be 60% O-acetylated at the 3-position. Three glycosyltransferases are present in the sequenced O121(Ec) cluster, WbqEHI. WbqI shares 47% identity with the WfaH enzyme from the O138(Ec) antigen cluster, and O138 shares the [DGlcNAc-a-1,3-DGalNAcA] reaction. WbqE shares 37% identity with WfbE from the O123(Ec) antigen cluster, and O123 also includes a 4-substituted quinovose sugar in the first position of the monomer, although the incorporated amino acid is alanine instead of glycine. By process of elimination, the WbqH enzyme is assigned to the middle linkage. The N-acetylgalacturonamide is likely derived from N-acetylgalturonic acid by the action of the asparagine synthase homolog WbqG. The O-acetylation is likely carried out by WbqF. |
