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.
Highlights
Humans are able to sense sweetness when tasting a variety of compounds, including sugars, amino acids, peptides, glycosides and sweet-tasting proteins
We examined whether bacterially produced wild-type (WT) neoculin and the 5HA variant had equivalent activity to the native protein and to the protein produced by A. oryzae, respectively [17]
How does neoculin basic subunit (NBS) His11 function in the taste-modifying activity of neoculin? One possibility is that His11 interacts with the sweet taste receptor directly to modulate its activation; the other is that
Summary
Humans are able to sense sweetness when tasting a variety of compounds, including sugars, amino acids, peptides, glycosides and sweet-tasting proteins. While the majority of sweet substances have low molecular weights, eight proteins are known to elicit sweetness: brazzein [4], lysozyme [5,6], mabinlin [7], monellin [8], pentadin [9], thaumatin [10], miraculin [11], and neoculin [12,13]. Among these proteins, neoculin, isolated from the edible fruit of Curculigo latifolia that grows in West Malaysia, induces an unusual taste sensation. Both tastemodifying proteins might be used as unique, non-glycemic taste improvers for sour foods
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