Identification of the Glutaminase Genes of Aspergillus sojae Involved in Glutamate Production during Soy Sauce Fermentation

Glutaminase, an enzyme that catalyzes the conversion of L-glutamine to L-glutamate, enhances the umami taste in soy sauce. The Aspergillus sojae genome contains 10 glutaminase genes. In this study, we estimated that approximately 60% of the glutamate in soy sauce is produced through the glutaminase reaction. To determine which glutaminase is involved in soy sauce glutamate production, we prepared soy sauces using single and multiple glutaminase gene disruptants of A. sojae. The glutamate concentration in soy sauce prepared using the Delta gahA-Delta gahB-Delta ggtA-Delta gls disruptant was approximately 60% lower than that in the control strain, whereas it was decreased by approximately 20-30% in the Delta gahA-Delta gahB disruptant. However, the glutamate concentration was unchanged in the soy sauces prepared using the Delta gahA-Delta ggtA-Delta gls and Delta gahB-Delta ggtA-Delta gls disruptants. These results indicate that four glutaminases are involved in glutamate production in soy sauce, and that the peptidoglutaminase activities of GahA and GahB increase the glutamate concentration in soy sauce.