Synthesis and Sensory Characteristics of Kokumi γ-[Glu]n-Phe in the Presence of Glutamine and Phenylalanine: Glutaminase from Bacillus amyloliquefaciens or Aspergillus oryzae as the Catalyst

The transpeptidase activity of glutaminase from Bacillus amyloliquefaciens (GBA) and Aspergillus oiyzae (GAO) to yield gamma-[Glu](n)-Phe peptides were verified for the first time. In the presence of Gin and Phe, gamma-Glu-Phe and gamma-Glu-gamma-Glu-Phe were synthesized by GAO, and gamma-Glu-Phe, gamma-Glu-gamma-Glu-Phe, gamma-Glu-gamma-Glu-gamma-Glu-Phe, gamma-Glu-gamma-Glu-gamma-Glu-gamma-Glu-Phe, and gamma-Glu-gamma-Glu-gamma-Glu-gamma-Glu-gamma-Glu-Phe were synthesized by GBA. The K-m values for the transpeptidation catalyzed by GBA and GAO were 47.88 and 153.92 mM (Phe as the acceptor), 84.89 and 236.47 mM (gamma-Glu-Phe as the acceptor), indicating that GBA had a greater affinity than GAO for Phe and gamma-Glu-Phe in the transpeptidation reaction. The K-m values for the transpeptidation catalyzed by GBA against acceptors, Phe and gamma-[Glu]((1 <= n<5))-Phe (47.88-206.47 mM), increased with an elevated number of gamma-glutamyl residue within the acceptor. The optimal conditions for gamma-[Glu](n)-Phe synthesis were pH 10 and 37 degrees C for 3 h, 300 mM Gln, 100 mM Phe, 0.05 U/mL GBA. All the gamma-[Glu]((1 <= n<5))-Phe exhibited astringency in water and imparted a kokumi taste to commercial soy sauce and model chicken broth. The astringent threshold values (2.5-3.92 mM) were approximately 3-fold of the kokumi threshold concentrations (0.78-1.53 mM). gamma-[Glu](n)-Phe or the post-enzymatic reaction mixture enhanced the umami intensity of commercial soy sauce and model chicken broth.