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Identification and modulation of the key amino acid residue responsible for the pH sensitivity of neoculin, a taste-modifying protein.


ABSTRACT: Neoculin occurring in the tropical fruit of Curculigo latifolia is currently the only protein that possesses both a sweet taste and a taste-modifying activity of converting sourness into sweetness. Structurally, this protein is a heterodimer consisting of a neoculin acidic subunit (NAS) and a neoculin basic subunit (NBS). Recently, we found that a neoculin variant in which all five histidine residues are replaced with alanine elicits intense sweetness at both neutral and acidic pH but has no taste-modifying activity. To identify the critical histidine residue(s) responsible for this activity, we produced a series of His-to-Ala neoculin variants and evaluated their sweetness levels using cell-based calcium imaging and a human sensory test. Our results suggest that NBS His11 functions as a primary pH sensor for neoculin to elicit taste modification. Neoculin variants with substitutions other than His-to-Ala were further analyzed to clarify the role of the NBS position 11 in the taste-modifying activity. We found that the aromatic character of the amino acid side chain is necessary to elicit the pH-dependent sweetness. Interestingly, since the His-to-Tyr variant is a novel taste-modifying protein with alternative pH sensitivity, the position 11 in NBS can be critical to modulate the pH-dependent activity of neoculin. These findings are important for understanding the pH-sensitive functional changes in proteinaceous ligands in general and the interaction of taste receptor-taste substance in particular.

SUBMITTER: Nakajima K 

PROVIDER: S-EPMC3084864 | biostudies-literature | 2011

REPOSITORIES: biostudies-literature

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Identification and modulation of the key amino acid residue responsible for the pH sensitivity of neoculin, a taste-modifying protein.

Nakajima Ken-ichiro K   Yokoyama Kanako K   Koizumi Taichi T   Koizumi Ayako A   Asakura Tomiko T   Terada Tohru T   Masuda Katsuyoshi K   Ito Keisuke K   Shimizu-Ibuka Akiko A   Misaka Takumi T   Abe Keiko K  

PloS one 20110429 4


Neoculin occurring in the tropical fruit of Curculigo latifolia is currently the only protein that possesses both a sweet taste and a taste-modifying activity of converting sourness into sweetness. Structurally, this protein is a heterodimer consisting of a neoculin acidic subunit (NAS) and a neoculin basic subunit (NBS). Recently, we found that a neoculin variant in which all five histidine residues are replaced with alanine elicits intense sweetness at both neutral and acidic pH but has no tas  ...[more]

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