PDBsum entry 1bch

Lectin

PDB id

1bch

Contents

			Protein chains

					154 a.a. *

			Ligands

					A2G


					NGA ×2

			Metals

					_CL ×3


					_NA


					_CA ×9

			Waters ×326

* Residue conservation analysis

References listed in PDB file

Key reference

Title

Mechanism of n-Acetylgalactosamine binding to a c-Type animal lectin carbohydrate-Recognition domain.

Authors

A.R.Kolatkar, A.K.Leung, R.Isecke, R.Brossmer, K.Drickamer, W.I.Weis.

Ref.

J Biol Chem, 1998, 273, 19502-19508. [DOI no: 10.1074/jbc.273.31.19502]

PubMed id

9677372

Note In the PDB file this reference is annotated as "TO BE PUBLISHED". The citation details given above were identified by an automated search of PubMed on title and author names, giving a perfect match.

Abstract

The mammalian hepatic asialoglycoprotein receptor, a member of the C-type animal lectin family, displays preferential binding to N-acetylgalactosamine compared with galactose. The structural basis for selective binding to N-acetylgalactosamine has been investigated. Regions of the carbohydrate-recognition domain of the receptor believed to be important in preferential binding to N-acetylgalactosamine have been inserted into the homologous carbohydrate-recognition domain of a mannose-binding protein mutant that was previously altered to bind galactose. Introduction of a single histidine residue corresponding to residue 256 of the hepatic asialoglycoprotein receptor was found to cause a 14-fold increase in the relative affinity for N-acetylgalactosamine compared with galactose. The relative ability of various acyl derivatives of galactosamine to compete for binding to this modified carbohydrate-recognition domain suggest that it is a good model for the natural N-acetylgalactosamine binding site of the asialoglycoprotein receptor. Crystallographic analysis of this mutant carbohydrate-recognition domain in complex with N-acetylgalactosamine reveals a direct interaction between the inserted histidine residue and the methyl group of the N-acetyl substituent of the sugar. Evidence for the role of the side chain at position 208 of the receptor in positioning this key histidine residue was obtained from structural analysis and mutagenesis experiments. The corresponding serine residue in the modified carbohydrate-recognition domain of mannose-binding protein forms a hydrogen bond to the imidazole side chain. When this serine residue is changed to valine, loss in selectivity for N-acetylgalactosamine is observed. The structure of this mutant reveals that the beta-branched valine side chain interacts directly with the histidine side chain, resulting in an altered imidazole ring orientation.



	Figure 4. Fig. 4. Ribbon representation of crystal structure of the QPDWGH mutant of MBP complexed with GalNAc. Stereo ribbon drawing shows the vicinity of the GalNAc binding site with the sugar and selected residues drawn as balls-and-sticks. The glycine-rich loop stacked against Trp189 is highlighted in gray and Ca^2+ 1 and 2 are shown as gray spheres. The hydrogen bond between Ser154 and His202 is drawn as a dashed line.		Figure 5. Fig. 5. van der Waals dot surface representation of the GalNAc binding site of the QPDWGH mutant of MBP. Stereo pair shows the GalNAc binding site in an orientation very similar to that in Fig. 4. The His202/GalNAc contact is apparent at the bottom. The stacking of the glycine-rich loop, Trp189 ring, and the apolar face of GalNAc is seen at the top. This figure was prepared with the Xfit component of the XtalView program suite (24).