Characterization and induction of phenolic acid decarboxylase from Aspergillus luchuensis

Awamori is a traditional distilled liquor in the Ryukyu Islands, made from steamed rice by the action of the black-koji mold Aspergillus luchuensis and awamori yeast Saccharomyces cerevisiae. One of the specific flavors in aged awamori kusu is vanillin, which is derived from ferulic acid (FA) in rice grains. FA is released from the cell wall material in the rice grain by ferulic acid esterase produced by A. luchuensis. Through decarboxylation of FA, 4-vinylguaiacol (4-VG) is produced, which is transferred to the distilled liquor, and converted to vanillin by natural oxidization during the aging process. However, the actual mechanism for conversion of FA to 4-VG in the awamori brewing process is unknown. A genetic sequence having homology to the phenolic acid decarboxylase (PAD)-encoding region from bacteria and the yeast Candida guilliermondii has been identified in A. luchuensis mut. kawachii. In the present study, recombinant PAD from A. luchuensis, designated as AlPAD, expressed as a homodimer, catalyzed the conversion of FA to 4-VG, displayed optimal catalytic activity at pH 5.7 and 40°C, and was stable up to 50°C. Both rice bran and FA could induce the bioconversion of FA to 4-VG and the expression of AlPAD in A. luchuensis. The amount of AlPAD determined using western blotting correlated with the level of FA decarboxylase activity during koji production. In awamori brewing process, AlPAD might be responsible for a part of the conversion of FA to 4-VG.