title: Structural properties of the neutral and monoanionic forms of xanthosine, highly relevant to their substrate properties with various enzyme systems.
abstract: The monoanions of the 6-oxopurines guanine (Gua) and hypoxanthine (Hx), and their nucleosides, pKa approximately 9 due to dissociation of the N(1)-H, are predominantly in their neutral forms at physiological pH. By contrast, the monoanions of the 6-oxopurine xanthine (Xan) and xanthosine (Xao), were long ago proposed to involve dissociation of the N(3)-H, with pKa values of 7.5 and 5.7, respectively, so that, at physiological pH, the former is mixture of the neutral and monoanionic species, and the latter predominantly the monoanion. We have employed multi-dimensional heteronuclear NMR spectroscopy, which fully confirms the proposed mode of monoanion formation in Xao (and, by implication, in Xan), further supported by the results of ab initio quantum mechanical calculations, and additionally extended to determination of the preferred conformational parameters in solution for the neutral and monoanionic species. These findings are highly relevant to the modes of binding, and to the substrate properties, of Xan, Xao and its 5'-phosphate (XMP) in numerous enzyme systems, hitherto virtually ignored, and illustrated by several concrete examples.
authors.affiliation: Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warszawa, Poland. jarek@ibb.waw.pl
authors.forename: Jarosław
authors.initials: J
authors.lastname: Poznanski
authors.forename: Borys
authors.initials: B
authors.lastname: Kierdaszuk
authors.forename: David
authors.initials: D
authors.lastname: Shugar
mesh_terms: D004121:Dimethyl Sulfoxide / Q000737:chemistry / Q000378:metabolism; D004798:Enzymes / Q000378:metabolism*; D006147:Guanine / Q000737:chemistry / Q000378:metabolism; D006151:Guanosine / Q000737:chemistry / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007288:Inosine / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D015394:Molecular Structure; D011683:Purine-Nucleoside Phosphorylase / Q000378:metabolism; D012263:Ribonucleosides / Q000737:chemistry* / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D019820:Xanthine / Q000737:chemistry / Q000378:metabolism; D014970:Xanthines
chemical_list: D004798:Enzymes; D012263:Ribonucleosides; D014970:Xanthines; D006151:Guanosine; D019820:Xanthine; D007288:Inosine; D006147:Guanine; C005893:xanthosine; D011683:Purine-Nucleoside Phosphorylase; D004121:Dimethyl Sulfoxide