Proton motive efflux

Various protein production changes are also known to be implicated in the development of resistance to acetic acid. One important protein that has been identified is now known as "aconitase." It may also be possible that there exists some other mechanism, located in the bacterial cell membrane by which acetic acid resistance is conferred, as it is known that acetic acid causes toxicity by acting as an uncoupling agent, which would disturb the proton motive force. The presence of such a proton motive efflux system for acetic acid is present in A. acetii (Matsushita et al., 2005). 

Other mechanisms conferring acetic acid resistance are also located in the cell membrane, because the toxicity of organic acids may be partly a result of a disruption of the proton motive force by acetic acid, acting as an uncoupling agent. A proton motive efflux system for acetic acid has also recently been described in A. acetii (Matsushita et al., 2005). Mutation and overexpression of the aatA gene also caused resistance to formic and propionic acids simultaneously to that of acetic acid. aatA functions as an efflux pump of acetic acid. Several transporters for monocarboxylic acids are known (Nakano, Fukaya, and Horinouchi, 2006) ... 

Currently, five different molecular classes of mdr efflux pumps are known [5], While pumps of the the ATP-binding cassette (ABC) transporter superfamily are driven by ATP hydrolysis, the other four superfamilies called resistance-nodulation-division (RND), major facilitator superfamily (MFS), multidrug and toxic compound extrusion (MATE), and small multidrag resistance transporter (SMR) are driven by the proton-motive force across the cytoplasmic membrane. Usually a single pump protein is located within the cytoplasmic membrane. However, the RND-type pumps which are restricted to Gram-negative bacteria consist of two additional components, a periplasmic membrane fusion protein (MFP) which connects the efflux pump to an outer... 

Plasmid- or transposon-encoded tetracycline efflux proteins have been described in a number of bacteria. These efflux profeins are fhoughf to span fhe cytoplasmic membrane and are dependenf on the proton-motive force for their action, ft is thought that the efflux proteins bind tetracyclines and initiate proton transfer, although no functional domains have been identified. Eight distinct tetracycline efflux profeins have been idenfified thus far. 

Fig. 5.1. ATPase (FqF,) and proton motive ATP synthesis. FqF, reconstituted into liposomes transform energy of proton efflux driven by an electrochemical potential difference of protons across the membrane = FAi — 2.3/ 7 ln ApH) [3,38). F(, (right hand side of the liposome) without F, is an...

Matsushita, K., Inoue, T., Adachi, O., and Toyama, H. 2005. Acetobacter aceti possesses a proton motive force-dependent efflux system for acetic acid. Journal of Bacteriology 187 4346-4352. 

This family of transported proteins consists of proteins having approximately 110 amino acids, four TMD, and are powered by a proton motive force similar to mitochondria. These proteins must form tetramers to function as transporters. These pumps are capable of effluxing drugs, dyes, and cations. 

Transporters, which mediate multidrug efflux, have usually been divided into four classes depending on (1) whether they are conducted by proton motive force (PMF) or by ATP, and (2) whether they consist of a single protein that has any one of the 4-, 12-, or 14-transmembrane-spanning domains [194,195] or whether they are a more complex multicomponent transporter [183]. The more complex transporters, in addition to a multidrug efflux protein (MexB, for example), contain a membrane fusion protein sueh as MexA and an outer membrane protein such as OprM [194]. 

The higher the proton motive force, the larger the proton gradient, so the slower the rate of proton efflux from the matrix. Consequently, a high proton gradient slows the flux of electrons along the electron transport chain. 

Damage of cellular homeostasis and cell physiology, e.g., reduction of transmembrane electrical potentials and proton chemical potentials or proton motive forces as a result of membrane changes (efflux of ions), changes of pH gradients. 

Figure 7.1 Efflux activities of hop bitter acids by HorA and HorC. HorA was shown to act as an ABC multidrug transporter and alleviate the intrusion of hop bitter acids into the cytoplasm. On the other hand, HorC was suggested to function as a proton motive force (PMF)-dependent multidrug transporter and to extmde hop bitter acids in a manner similar to that of HorA. In addition, HorC was postulated to act as a homodimer (lijima, Suzuki, Asano, Ogata, Kitagawa, 2009). The secondary structures of HorA and HorC were described previously (Suzuki, 2012).

Fig. 10.9 Schematic representation of molecular machineries that confer acetic acid resistance in Acetobacter and Gluconacetobacter. (Schematic diagram quoted from Nakano and Fukaya 2008) THBH and phosphatidylcholine on the membrane and polysaccharide on the surface of the cells are suggested to be involved in acetic acid resistance. Acetic acid, which penetrates into the cytoplasm, is assumed to be metabolized through the TCA cycle by the actions of enzymes typical for AAB. Furthermore, intracellular acetic acid is possibly pumped out by a putative ABC transporter and proton motive force-dependent efflux pump using energy produced by ethanol oxidation or acetic acid overoxidation. Intracellular cytosolic enzymes are intrinsically resistant to low pH and are protected against denaturation by stress proteins such as molecular chaperones. ADH membrane-bound alcohol dehydrogenase, ALDH membrane-bound aldehyde dehydrogenase, CS citrate synthase, ACN aconitase, PC phosphatidylcholine...

Acetic acid efflux by transporter or pump two types of efflux system, a putative ABC transporter and a proton motive force-dependent efflux pump, function to pump out intracellular acetic acid. 

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