A branched N-linked glycan at atomic resolution in the 1.12 A structure of rhamnogalacturonan acetylesterase

The crystal structure of the glycoprotein rhamnogalacturonan acetylesterase from Aspergillus aculeatus has been refined to a resolution of 1.12 A using synchrotron data collected at 263 K. Both of the two putative N-glycosylation sites at Asn104 and Asn182 are glycosylated and, owing to crystal contacts, the glycan structure at Asn182 is exceptionally well defined in the electron-density maps, showing the six-carbohydrate structure Manalpha1-6(Manalpha1-3)Manalpha1-6Manbeta1-4GlcNAcbeta1-4GlcNAcbeta-Asn182. Equivalent carbohydrate residues were restrained to have similar geometries, but were refined without target values. The refined bond lengths and angles were compared with the values obtained from small-molecule studies that form the basis for the dictionaries used for glycoprotein refinement.