Mechanism of pH-dependent N-acetylgalactosamine binding by a functional mimic of the hepatocyte asialoglycoprotein receptor.
Efficient release of ligands from the Ca(2+)-dependent carbohydrate-recognition domain (CRD) of the hepatic asialoglycoprotein receptor at endosomal pH requires a small set of conserved amino acids that includes a critical histidine residue. When these residues are incorporated at corresponding positions in an homologous galactose-binding derivative of serum mannose-binding protein, the pH dependence of ligand binding becomes more like that of the receptor. The modified CRD displays 40-fold preferential binding to N-acetylgalactosamine compared with galactose, making it a good functional mimic of the asialoglycoprotein receptor. In the crystal structure of the modified CRD bound to N-acetylgalactosamine, the histidine (His(202)) contacts the 2-acetamido methyl group and also participates in a network of interactions involving Asp(212), Arg(216), and Tyr(218) that positions a water molecule in a hydrogen bond with the sugar amide group. These interactions appear to produce the preference for N-acetylgalactosamine over galactose and are also likely to influence the pK(a) of His(202). Protonation of His(202) would disrupt its interaction with an asparagine that serves as a ligand for Ca(2+) and sugar. The structure of the modified CRD without sugar displays several different conformations that may represent structures of intermediates in the release of Ca(2+) and sugar ligands caused by protonation of His(202).
Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305, USA.
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Feinberg
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Torgersen
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Drickamer
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D000116:Acetylgalactosamine / Q000737:chemistry* / Q000378:metabolism*; D000596:Amino Acids / Q000737:chemistry; D001120:Arginine / Q000737:chemistry; D037263:Asialoglycoprotein Receptor; D001224:Aspartic Acid / Q000737:chemistry; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000378:metabolism; D005690:Galactose / Q000378:metabolism; D005998:Glycine / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D037601:Mannose-Binding Lectin; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D011487:Protein Conformation; D011956:Receptors, Cell Surface / Q000737:chemistry* / Q000378:metabolism*; D014443:Tyrosine / Q000737:chemistry; D014867:Water / Q000378:metabolism
D000596:Amino Acids; D037263:Asialoglycoprotein Receptor; D002352:Carrier Proteins; D037601:Mannose-Binding Lectin; C417247:QPDWG-HDRPY protein, rat; D011956:Receptors, Cell Surface; D014867:Water; D001224:Aspartic Acid; D014443:Tyrosine; D006639:Histidine; D001120:Arginine; D000116:Acetylgalactosamine; D002118:Calcium; D005998:Glycine; D005690:Galactose