Models and thermodynamics of PCA oxidation.

Abstract

(A) Potential models of PCA oxidation in Gram-negative bacteria capable of respiration. In aqueous environments, PCA redox reactions are two-electron, two-proton processes. Reduced PCA is shown in green, reflecting its true color. PCA oxidation can theoretically be coupled to the respiratory electron transport chain in a couple ways: (Category 1, electron transfer to terminal reductase) PCA may donate electrons to the terminal reductase (grey shape) for a respirable terminal electron acceptor (T.E.A.), thus contributing two protons to the periplasm; (Category 2, electron transfer to quinol pool) PCA may donate two electrons and two protons to quinones, thus regenerating the quinol pool. (Category 3, electron transfer to terminal electron acceptor) Alternatively, PCA directly reduces the terminal electron acceptor. This may happen externally to the cell, or PCA may enter the cytoplasm and react with the terminal electron acceptor independently of the electron transport chain, as depicted. In this illustration, the arrows may represent direct reactions or ones mediated by enzymes or other factors. Transferring electrons to the terminal reductase or quinol pool (Categories 1 and 2) represents scenarios that may be energetically beneficial for a respiring bacterial cell, whereas transferring electrons to the terminal electron acceptor (Category 3) may be detrimental. This would require the transport of PCA across the inner membrane because its carboxylic acid moiety is negatively charged at circumneutral pH, and it cannot passively cross the membrane. For simplicity, this illustration does not show potential reactions with a periplasmic reductase, but the logic would remain the same, only with no involvement of the cytoplasmic space. (B) Electron tower of relevant half-reactions. Reactions are ordered by their relative standard midpoint potentials with more negative values on top and more positive ones on the bottom (not to scale). Thermodynamically favorable pairings comprise more positive half-reactions with more negative ones in reverse. The theoretical limit for energy that can be conserved from a pairing correlates with the magnitude of the difference in half-reaction potentials. PCA: phenazine-1-carboxylic acid. MQ: menaquinone. DMQ: demethylmenaquinone. UQ: ubiquinone. Fum2-: fumarate. Succ2-: succinate. TMAO: trimethylamine-N-oxide. TMA: trimethylamine. DMSO: dimethyl sulfoxide. DMS: dimethyl sulfide. NO2-: nitrite. NO: nitric oxide. NO3-: nitrate.</p