Glutaminase

Green vegetable proteins such as soybeans and wheat contain quite a lot of glutamines instead of glutamic acid, glutaminase in koji deserves special mention. Glutaminases are used in the hydrolysis of vegetable proteins to produce L-glutamic acid, which imparts a ''umami'' taste in soy sauce and improves the flavor (52). Production of glutaminase by yellow-green aspergilli is much larger in a solid culture than in a liquid one. Koji is the most adequate enzyme source for the hydrolysis of cereal proteins because it has a high content of glutaminase as well as the composite peptidases. Green glutaminase is fairly sensitive to acidic pH as well as to salt (53), the brine fermentation in soy sauce making is carried out at a lower temperature (15-20 °C) for the first month to prevent the loss of glutaminase activity. A glutaminase from Aspergillus oryzae was purified and its molecular weight was determined to be 82,091. On the basis of its internal amino acid sequences, the glutaminase gene (gtaA) from A. oryzae was isolated and characterized. The gtaA gene had an open reading frame coding for 690 amino acid residues, including a signal peptide of 20 amino acid residues and a mature protein of 670 amino acid residues. In the 5'-flanking region of the gene, there were three putative CreAp binding sequences and one putative AreAp binding sequence. The gtaA structural gene was introduced into A. oryzae NS4 and a marked increase in activity was detected in comparison with the control strain (54).

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