Formation of taste-active pyroglutamyl peptide ethyl esters in sake by rice koji peptidases.

Formation of taste-active pyroglutamyl (pGlu) peptide ethyl esters in sake was investigated: 2 enzymes (A and B) responsible for the esterification were purified from a rice koji extract. MADLI-TOF/TOF analysis after deglycosylation identified enzyme (A) as peptidase S28 (GenBank accession number OOO13707.1) and enzyme (B) as serine-type carboxypeptidase (accession number AO090010000534). Both enzymes hydrolyzed pGlu peptides and formed ethyl esters under sake mash conditions: acidic pH (3-4) and in ethanol (5%-20% v/v) aqueous solutions. Enzyme (A) formed pGlu penta-peptide ethyl esters from pGlu undeca-peptides by a prolyl endo-type reaction. Enzyme (B) formed (pGlu) deca-peptide and its ethyl esters from pGlu undeca-peptides in an exo-type reaction. We are the first to report the enzymatic ethyl esterification reaction in the formation of pGlu peptides by rice koji peptidases.