The proton motive force generated in Leuconostoc oenos by L-malate fermentation

In cells of Leuconostoc oenos, the fermentation of L-malic acid generates both a transmembrane pH gradient, inside alkaline, and an electrical potential gradient, inside negative. In resting cells, the proton motive force ranged from -170 mV to -88 mV between pH 3.1 and 5.6 in the presence of L-malate. Membrane potentials were calculated by using a model for probe binding that accounted for the different binding constants at the different pH values at the two faces of the membrane. The Δψ generated by the transport of monovalent malate, H-malate^-, controlled the rate of fermentation. The fermentation rate significantly increased under conditions of decreased Δψ, i.e., upon addition of the ionophore valinomycin in the presence of KCl, whereas in a buffer depleted of potassium, the addition of valinomycin resulted in a hyperpolarization of the cell membrane and a reduction of the rate of fermentation. At the steady state, the chemical gradient for H-malate^- was of the same magnitude as Δψ. Synthesis of ATP was observed in cells performing malolactic fermentation.