Sulfate as a regulator of Bacteroides thetaiotaomicron glutamate decarboxylase: Elucidating the bipartite binding and regulation mechanism.

Glutamate decarboxylase (GAD) catalyzes the conversion of glutamate to γ-aminobutyric acid (GABA), which is significant for human health and industrial GABA production. This study demonstrates that sulfate acts as an activator for Bacteroides thetaiotaomicron GAD (BTGAD) in the absence of its cofactor pyridoxal-5'-phosphate (PLP), enhancing enzymatic activity by approximately 1.4-fold. However, sulfate competitively inhibits PLP binding and suppresses BTGAD-PLP activity to 36.15%. Structural studies revealed that sulfate binds in a bipartite manner to BTGAD at PLP-binding site and N-terminal sequence. Mutagenesis studies confirm that both binding sites are essential for sulfate-dependent regulation of BTGAD. The mutants H273A and ∆N increased the activation fold of Na2SO4 on BTGAD and enhanced the inhibitory effects of Na2SO4 on BTGAD-PLP. These results establish sulfate as a bifunctional modulator with potential applications in the enzymatic production of GABA.