On the other hand, amino acids with sweet, bitter, or umami (the pleasant savory taste of foods, such as seaweed, cured fish, aged cheeses, and meats, elicited by glutamate) taste may impact some sensory qualities of sake, but too much amino acid content is often thought to produce an unfavorable taste in sake. In sake mash, amino acids are derived mainly from the digestion of rice proteins by sake koji enzymes; however, yeast cells also synthesize them during fermentation. In yeast, amino acid metabolism and its regulatory mechanisms vary under different growth environments by regulating anabolic and catabolic processes, including uptake and export, and the metabolic styles form a complicated and robust network. There is also crosstalk with various metabolic pathways, products, and signal molecules. The elucidation of metabolic regulatory mechanisms and physiological roles is important fundamental research for understanding life phenomena [8, 21]. However, a lack of knowledge concerning the mechanism underlying amino acid production during sake fermentation has made it difficult to develop yeast strains with different amino acid profiles. The development of strains that can produce specific or various amino acids could enable the production of sake with distinctive tastes.
a Partial amino acid sequence alignment of GKs among various microorganisms. The amino acid sequence of the S. cerevisiae GK was compared to Komagataella phaffii (UniProt ID code: F2QWU5), Schizosaccharomyces pombe (O13810), Bacillus subtilis (P39820), Burkholderia thailandensis (Q2SZF9), and Escherichia coli (P0A7B5) homologues. Numbering of residues is in ScGK and conserved residues were highlighted in black boxes. Gln79 and Ile150 are shown in red and light blue, respectively. Gray bars and orange arrows above the alignment represented the hypothetical α-helices and β-sheets of ScGK, predicted by Jpred 4 (A Protein Secondary Structure Prediction Server, ); those under the alignment were the secondary structure in the crystal structure of EcGK (PDB ID code:2J5T) [9]. b Effect of proline on GK activity. The GK activities of the wild-type (open circle), Q79H (filled triangle), and I150T (open square) variant GKs were measured in the presence of proline. The relative activities are expressed corresponding to the parameters in the absence of proline. The values for the wild-type and Q79H variant GKs are the means and standard deviations of results from three independent experiments. The value for the I150T variant GK was obtained from one experiment
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Crystal structure of PhaB and mutants. (A) Ribbon diagram of the PhaB monomer. The ribbon model is colored according to the sequence, from blue at the N terminus to red at the C terminus. (B) Tetrameric structure of PhaB. The model is colored according to the subunit. Q47 (purple) and T137 (cyan) are also shown as spherical models. (C) Structure superimposition among the wild type (red), Q47L (blue), and T137S (orange). For clarity, only a monomer of the superimposed tetramer is shown. (D) Temperature factor of wild-type PhaB. The tube model is colored according to the temperature factor, from blue at 20 Å2 to red at 50 Å2. The width of the tube also corresponds to the temperature factor. In short, red thick regions imply high flexibility and blue thin regions imply low flexibility. Q47 is also shown as green sticks. The flexible α7-α8 and α2 regions are indicated. 2ff7e9595c
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