Comparison of kokumi γ-[Glu](n>1)-Val and γ-[Glu](n>1)-Met synthesized through transpeptidation catalyzed by glutaminase from Bacillus amyloliquefaciens

A series of γ-[Glu](n=2,3,4)-Val or γ-[Glu](n=2,3,4)-Met were synthesized in the presence of donor (Gln) and corresponding acceptor (Val or Met) through transpeptidation catalyzed by the glutaminase from Bacillus amyloliquefaciens. Gln in excess significantly (p < .05) improved the yield of γ-[Glu](n>1)-Val/Met except for γ-Glu-Val/Met. The Km values for transpeptidase activity to yield γ-[Glu](n=0,1,2,3)-Val increased with an elevated n, but remained essentially the same irrespective of n value for γ-[Glu](n=0,1,2)-Met (which were 31-44% of that for γ-[Glu]3-Met). The highest Km value appearing when n = 3 (γ-[Glu]3-Val or γ-[Glu]3-Met) suggested the rising difficulty for synthesis when the number of donor increases. All the γ-[Glu]n-Val/Met substances exhibited kokumi properties and enhanced the continuity and umami taste of soy sauce as well as the thickness, mouthfulness and umaminess of model chicken broth. These results indicate the potential of the γ-[Glu]n-Val and γ-[Glu]n-Met as food flavor enhancers.