Bypassing Isophthalate Inhibition by Modulating Glutamate Dehydrogenase (GDH): Purification and Kinetic Characterization of NADP-GDHs from Isophthalate-Degrading Pseudomonas aeruginosa Strain PP4 and Acinetobacter lwoffii Strain ISP4

摘要

Pseudomonas aeruginosa strain PP4 and Acinetobacter lwoffii strain ISP4 metabolize isophthalate as a sole source of carbon and energy. Isophthalate is known to be a competitive inhibitor of glutamate dehydrogenase (GDH), which is involved in C and N metabolism. Strain PP4 showed carbon source-dependent modulation of NADP-GDH; GDHI was produced when cells were grown on isophthalate, while GDHII was produced when cells were grown on glucose. Strain ISP4 produced a single form of NADP-GDH, GDHP, when it was grown on either isophthalate or rich medium (2YT). All of the forms of GDH were purified to homogeneity and characterized. GDHI and GDHII were found to be homotetramers, while GDHP was found to be a homohexamer. GDHII was more sensitive to inhibition by isophthalate (2.5- and 5.5-fold more sensitive for amination and deamination reactions, respectively) than GDHI. Differences in the N-terminal sequences and electrophoretic mobilities in an activity-staining gel confirmed the presence of two forms of GDH, GDHI and GDHII, in strain PP4. In strain ISP4, irrespective of the carbon source, the GDHP produced showed similar levels of inhibition with isophthalate. However, the specific activity of GDHP from isophthalate-grown cells was 2.5- to 3-fold higher than that of GDHP from 2YT-grown cells. Identical N-terminal sequences and electrophoretic mobilities in the activity-staining gel suggested the presence of a single form of GDHP in strain ISP4. These results demonstrate the ability of organisms to modulate GDH either by producing an entirely different form or by increasing the level of the enzyme, thus enabling strains to utilize isophthalate more efficiently as a sole source of carbon and energy.