title: Structural Basis of pH Dependence of Neoculin, a Sweet Taste-Modifying Protein.
abstract: Among proteins utilized as sweeteners, neoculin and miraculin are taste-modifying proteins that exhibit pH-dependent sweetness. Several experiments on neoculin have shown that His11 of neoculin is responsible for pH dependence. We investigated the molecular mechanism of the pH dependence of neoculin by molecular dynamics (MD) calculations. The MD calculations for the dimeric structures of neoculin and His11 mutants showed no significant structural changes for each monomer at neutral and acidic pH levels. The dimeric structure of neoculin dissociated to form isolated monomers under acidic conditions but was maintained at neutral pH. The dimeric structure of the His11Ala mutant, which is sweet at both neutral and acidic pH, showed dissociation at both pH 3 and 7. The His11 residue is located at the interface of the dimer in close proximity to the Asp91 residue of the other monomer. The MD calculations for His11Phe and His11Tyr mutants demonstrated the stability of the dimeric structures at neutral pH and the dissociation of the dimers to isolated monomers. The dissociation of the dimer caused a flexible backbone at the surface that was different from the dimeric interface at the point where the other monomer interacts to form an oligomeric structure. Further MD calculations on the tetrameric structure of neoculin suggested that the flexible backbone contributed to further dissociation of other monomers under acidic conditions. These results suggest that His11 plays a role in the formation of oligomeric structures at pH 7 and that the isolated monomer of neoculin at acidic pH is responsible for sweetness.
authors.affiliation: Department of Applied Life Sciences, Niigata University of Pharmacy and Applied Life Sciences, Higashijima, Akiha-ku, Niigata, Japan.
authors.forename: Takayuki
authors.initials: T
authors.lastname: Ohkubo
authors.affiliation: Department of Applied Life Sciences, Niigata University of Pharmacy and Applied Life Sciences, Higashijima, Akiha-ku, Niigata, Japan.
authors.forename: Minoru
authors.initials: M
authors.lastname: Tamiya
authors.affiliation: Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo, Japan.
authors.forename: Keiko
authors.initials: K
authors.lastname: Abe
authors.affiliation: Department of Applied Life Sciences, Niigata University of Pharmacy and Applied Life Sciences, Higashijima, Akiha-ku, Niigata, Japan.
authors.forename: Masaji
authors.initials: M
authors.lastname: Ishiguro
mesh_terms: D000596:Amino Acids / Q000737:chemistry; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D056004:Molecular Dynamics Simulation; D050505:Mutant Proteins / Q000737:chemistry; D009154:Mutation / Q000235:genetics; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism*; D055503:Protein Multimerization; D013649:Taste
chemical_list: D000596:Amino Acids; D050505:Mutant Proteins; D010940:Plant Proteins; C511659:neoculin protein, Curculigo latifolia