A simple strategy to differentiate between H+- and Na+-transporting NADH:quinone oxidoreductases

Na+ Translocation Dominates over H+-Translocation in the Membrane Pyrophosphatase with Dual Transport Specificity

Cation-pumping membrane pyrophosphatases (mPPases; EC 7.1.3.1) vary in their transport specificity from obligatory H+ transporters found in all kingdoms of life, to Na+/H+-co-transporters found in many prokaryotes. The available data suggest a unique "direct-coupling" mechanism of H+ transport, in which the transported proton is generated from nucleophilic water molecule. Na+ transport is best rationalized by assuming that the water-borne proton propels a prebound Na+ ion through the ion conductance channel ("billiard" mechanism). However, the "billiard" mechanism, in its simple form, is not applicable to the mPPases that simultaneously transport Na+ and H+ without evident competition between the cations (Na+,H+-PPases). In this study, we used a pyranine-based fluorescent assay to explore the relationship between the cation transport reactions catalyzed by recombinant Bacteroides vulgatus Na+,H+-PPase in membrane vesicles. Under appropriately chosen conditions, including the addition of an H+ ionophore to convert Na+ influx into equivalent H+ efflux, the pyranine signal measures either H+ or Na+ translocation. Using a stopped-flow version of this assay, we demonstrate that H+ and Na+ are transported by Na+,H+-PPase in a ratio of approximately 1:8, which is independent of Na+ concentration. These findings were rationalized using an "extended billiard" model, whose most likely variant predicts the kinetic limitation of Na+ delivery to the pump-loading site.

Bioenergetics and Reactive Nitrogen Species in Bacteria

The production of reactive nitrogen species (RNS) by the innate immune system is part of the host's defense against invading pathogenic bacteria. In this review, we summarize recent studies on the molecular basis of the effects of nitric oxide and peroxynitrite on microbial respiration and energy conservation. We discuss possible molecular mechanisms underlying RNS resistance in bacteria mediated by unique respiratory oxygen reductases, the mycobacterial bcc-aa3 supercomplex, and bd-type cytochromes. A complete picture of the impact of RNS on microbial bioenergetics is not yet available. However, this research area is developing very rapidly, and the knowledge gained should help us develop new methods of treating infectious diseases.

The flavin transferase ApbE flavinylates the ferredoxin:NAD+-oxidoreductase Rnf required for N2 fixation in Azotobacter vinelandii

Azotobacter vinelandii, the model microbe in nitrogen fixation studies, uses the ferredoxin:NAD+-oxidoreductase Rnf to regenerate ferredoxin (flavodoxin), acting as an electron donor for nitrogenase. However, the relative contribution of Rnf to nitrogenase functioning is unknown because this bacterium contains another ferredoxin reductase, FixABCX. Furthermore, Rnf is flavinylated in the cell, but the importance and pathway of this modification reaction also remain largely unknown. We constructed A. vinelandii cells with impaired activities of FixABCX and/or putative flavin transferase ApbE. The ApbE-deficient mutant could not produce covalently flavinylated membrane proteins and demonstrated markedly decreased flavodoxin:NAD+ oxidoreductase activity and significant growth defects under diazotrophic conditions. The double ΔFix/ΔApbE mutation abolished the flavodoxin:NAD+ oxidoreductase activity and the ability of A. vinelandii to grow in the absence of a fixed nitrogen source. ApbE flavinylated a truncated RnfG subunit of Rnf1 by forming a phosphoester bond between flavin mononucleotide and a threonine residue. These findings indicate that Rnf (presumably its Rnf1 form) is the major ferredoxin-reducing enzyme in the nitrogen fixation system and that the activity of Rnf depends on its covalent flavinylation by the flavin transferase ApbE.

The three NADH dehydrogenases of Pseudomonas aeruginosa: Their roles in energy metabolism and links to virulence

Pseudomonas aeruginosa is a ubiquitous opportunistic pathogen which relies on a highly adaptable metabolism to achieve broad pathogenesis. In one example of this flexibility, to catalyze the NADH:quinone oxidoreductase step of the respiratory chain, P. aeruginosa has three different enzymes: NUO, NQR and NDH2, all of which carry out the same redox function but have different energy conservation and ion transport properties. In order to better understand the roles of these enzymes, we constructed two series of mutants: (i) three single deletion mutants, each of which lacks one NADH dehydrogenase and (ii) three double deletion mutants, each of which retains only one of the three enzymes. All of the mutants grew approximately as well as wild type, when tested in rich and minimal medium and in a range of pH and [Na+] conditions, except that the strain with only NUO (ΔnqrFΔndh) has an extended lag phase. During exponential phase, the NADH dehydrogenases contribute to total wild-type activity in the following order: NQR > NDH2 > NUO. Some mutants, including the strain without NQR (ΔnqrF) had increased biofilm formation, pyocyanin production, and killed more efficiently in both macrophage and mouse infection models. Consistent with this, ΔnqrF showed increased transcription of genes involved in pyocyanin production.