Ferrous-iron-dependent Uptake of L-Glutamate by a Mesophilic, Mixotrophic Iron-oxidizing Bacterium Strain OKM-9

Strain OKM-9 is a mesophilic, mixotrophic iron-oxidizing bacterium that absolutely requires ferrous iron as its energy source and L-amino acids (including L-glutamate) as carbon sources for growth. The properties of the L-glutamate transport system were studied with OKM-9 resting cells, plasma membranes, and actively reconstituted proteoliposomes. L-Glutamate uptake into resting cells was totally dependent on ferrous iron that was added to the reaction mixture. Potassium cyanide, an iron oxidase inhibitor, completely inhibited the activity at 1 mM. The optimum pH for Fe²⁺-dependent uptake activity of L-glutamate was 3.5-4.0. Uptake activity was dependent on the concentration of the L-glutamate. The K_m and V_max for L-glutamate were 0.4 mM and 11.3 nmol•min⁻¹•mg⁻¹, respectively. L-Aspartate, D-aspartate, D-glutamate, and L-cysteine strongly inhibited L-glutamate uptake. L-Aspartate competitively inhibited the activity, and the apparent K_i for this amino acid was 75.9 μM. 2,4-Dinitrophenol, carbonyl cyanide m-chlorophenylhydrazone, gramicidin D, valinomycin, and monensin did not inhibit Fe²⁺-dependent L-glutamate uptake. The OKM-9 plasma membranes had approximately 40% of the iron-oxidizing activity of the resting cells and approximately 85% of the Fe²⁺-dependent uptake activity. The glutamate transport system was solubilized from the membranes with 1% n-octyl-β-D-glucopyranoside and reconstituted into a lecithin liposome. The L-glutamate transport activity of the reconstituted proteoliposomes was 8-fold than that of the resting cells. The Fe²⁺-dependent L-glutamate uptake observed here seems to explain the mixotrophic nature of this strain, which absolutely requires Fe²⁺ oxidation when using amino acids as carbon sources.