Characterization of a novel glutamate decarboxylase from Streptococcus salivarius ssp. thermophilus Y2

BACKGROUND: γ‐Aminobutyric acid with several well‐known physiological functions is biosynthesized via the irreversible α‐decarboxylation of L‐glutamate catalysed by glutamate decarboxylase (GAD). Although Streptococcus salivarius ssp. thermophilus has been widely applied to the dairy, the characterization of its GAD has not been reported. In this paper, the purification and the characterization of S. salivarius ssp. thermophilus GAD were investigated. RESULTS: GAD was purified 22‐fold from crude protein extracts with a yield of 7.8% in five steps. The final preparation gave a single band on SDS‐PAGE. The molecular weight of GAD determined by SDS‐PAGE and gel filtration was 46.9 kDa and 103.6 kDa, respectively, indicating that the enzyme exists as a dimmer of homological subunits. The optimum temperature and pH of GAD was 55 °C and pH 4.0, respectively. The enzyme reacted only with L‐glutamate among 19 α‐amino acids with apparent Km at 2.3 mmol L−1 and did not react with D‐glutamic acid. Activity of the enzyme could significantly be activated by 5 mmol L−1 of BaCl2 and inhibited by FeSO4, ZnSO4, CuSO4, MnSO4, Na2SO4, AgNO3, CoCl2, LiCl and KCl, respectively. The N‐terminal amino acid sequence of GAD was NH2‐MNEKLFREI. CONCLUSION: Both the characterization and the deduced amino sequence (ABI31651) showed the purified enzyme was a novel GAD.