F1F0-ATP Synthases of Alkaliphilic Bacteria: Lessons from Their Adaptations David B

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Biochimica et Biophysica Acta 1797 (2010) 1362–1377 Contents lists available at ScienceDirect Biochimica et Biophysica Acta journal homepage: www.elsevier.com/locate/bbabio Review F1F0-ATP synthases of alkaliphilic bacteria: Lessons from their adaptations David B. Hicks a, Jun Liu a, Makoto Fujisawa b, Terry A. Krulwich a,⁎ a Department of Pharmacology and Systems Therapeutics, Mount Sinai School of Medicine, New York, NY 10029, USA b Faculty of Food Life Sciences, Toyo University, Ora-gun, Gunma 374-0193, Japan article info abstract Article history: This review focuses on the ATP synthases of alkaliphilic bacteria and, in particular, those that successfully Received 19 January 2010 overcome the bioenergetic challenges of achieving robust H+-coupled ATP synthesis at external pH values Received in revised form 22 February 2010 N10. At such pH values the protonmotive force, which is posited to provide the energetic driving force for Accepted 23 February 2010 ATP synthesis, is too low to account for the ATP synthesis observed. The protonmotive force is lowered at a Available online 1 March 2010 very high pH by the need to maintain a cytoplasmic pH well below the pH outside, which results in an energetically adverse pH gradient. Several anticipated solutions to this bioenergetic conundrum have been Keywords: ATPase ruled out. Although the transmembrane sodium motive force is high under alkaline conditions, respiratory + + Oxidative phosphorylation alkaliphilic bacteria do not use Na - instead of H -coupled ATP synthases. Nor do they offset the adverse pH c-ring gradient with a compensatory increase in the transmembrane electrical potential component of the Bacillus pseudofirmus OF4 protonmotive force. Moreover, studies of ATP synthase rotors indicate that alkaliphiles cannot fully resolve Bacillus TA2.A1 the energetic problem by using an ATP synthase with a large number of c-subunits in the synthase rotor ring. Spirulina platensis Increased attention now focuses on delocalized gradients near the membrane surface and H+ transfers to ATP synthases via membrane-associated microcircuits between the H+ pumping complexes and synthases. Microcircuits likely depend upon proximity of pumps and synthases, specific membrane properties and specific adaptations of the participating enzyme complexes. ATP synthesis in alkaliphiles depends upon alkaliphile-specific adaptations of the ATP synthase and there is also evidence for alkaliphile-specific adaptations of respiratory chain components. © 2010 Elsevier B.V. All rights reserved. 1. Introduction reports of ectopic F1F0-ATP synthases in caveolae/lipid rafts on the surface of a variety of mammalian cell types, including cancer cells [2–5]. ATP is the dominant chemical energy currency of living cells. A The homologous but structurally simpler bacterial ATP synthases are minority of cells, such as mature red blood cells and some non- found in the cytoplasmic membrane or thylakoid membranes [6–8]. respiratory microbial cells, generate ATP exclusively through substrate ATP synthesis by ATP synthases is energized by electrochemical level phosphorylation. By contrast, the majority of ATP synthesized by gradients of H+ or Na+ across the membrane. ATP synthesis is coupled animal, plant and microbial cells is obtained at higher energy yield from to downhill movement of ions fostered by those gradients across catabolic substrates or light via the use of oxidative phosphorylation mitochondrial, thylakoid and bacterial cell membranes [6,8].ATP (OXPHOS) or photo-phosphorylation, which depend upon the catalytic synthases are reversible, i.e., they can function as ATPases that hydrolyze activity of F1F0-ATP synthases (which will be called ATP synthases in this ATP in concert with ion pumping (energetically uphill) in the opposite article). These synthases have been part of the physiological landscape direction of ion uptake during synthesis. For many fermentative through much of the evolutionary path of contemporary organisms, bacteria, ATPase activity is the only physiological activity of the enzyme. perhaps as far back as LUCA, the putative “last universal common By contrast, the hydrolytic activity of ATP synthases in most other ancestor” of living organisms [1]. ATP synthases of animals and plants settings is carefully controlled, or even latent, in order to protect the are embedded by one of their domains, the F0 domain, in the inner cytoplasmic ATP pool [9–11]. Structure–function information that membranes of mitochondria or in chloroplast thylakoid membranes. An emerged over the past decade and a half supports a rotary mechanism interesting exception to these classic organelle localizations are recent for ATP synthesis that involves sequential conformational changes in the three catalytic sites of the synthase. Experiments on single synthase complexes have captured steps and sub-steps of the rotation. An ⁎ Corresponding author. Department of Pharmacology and Systems Therapeutics, extensive collection of high resolution structures of major portions of Box 1603, Mount Sinai School of Medicine, 1 Gustave L. Levy Place, New York, NY ATP synthases has also emerged. Although not yet representing the 10029-6500, USA. Tel.: +1 212 241 7280; fax: +1 212 996 7214. entire structure, the structural information has enormously fostered E-mail addresses: [email protected] (D.B. Hicks), [email protected] (J. Liu), [email protected] (M. Fujisawa), [email protected] hypothesis-building and experimental work aimed at greater mecha- (T.A. Krulwich). nistic understanding [12–24]. 0005-2728/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.bbabio.2010.02.028 D.B. Hicks et al. / Biochimica et Biophysica Acta 1797 (2010) 1362–1377 1363 The importance of resolving the mechanistic details of ATP the F0 and the catalytic domain of the F1 that contains the three α- and synthase function arises from the enzyme's central role in cell phys- β-subunit pairs [50–54]. The rotor element found in the F0 (highlight- iology. The importance of further insights is underscored by the ed in green in Fig. 1) is a homo-oligomer of hairpin-like c-subunits serious consequences of genetic defects in ATP synthase structural in most ATP synthases. The cytoplasmic loops that are exposed on the genes, regulatory genes or assembly factors [25,26]. Dysfunctions in N-side of the membrane (the electrically negative side) interact with respiration-dependent ATP synthesis, i.e. oxidative phosphorylation the γε sub-complex which forms the central stalk, with the γ-subunit (OXPHOS), are further correlated with complex disorders whose extending asymmetrically into the F1 catalytic domain [55] (see frequency increases with ageing, such as Type 2 diabetes [27]. The Section 2.3). mechanistic details of ATP synthase function will also enhance our The number of c-subunits observed per c-rotor varies from 10 to 15, ability to inactivate the ATP synthase of bacterial pathogens in which depending upon the organism [18,20,56–65]. The two c-subunit the enzyme plays an essential role in part of their pathogenic life stoichiometries so far determined for eukaryotic c-rotors are at opposite cycle. This is the promising basis for the use of diarylquinolines that ends of those ranges with the yeast stoichiometry at 10 [66] and the target the ATP synthase of pathogenic Mycobacterium tuberculosis and spinach chloroplast stoichiometry at 14 [67–69]. By contrast to the its multi-drug resistant forms [28–31]. The resulting inhibition of ATP typical homo-oligomeric rotor, the Na+-coupled ATP synthase of the synthase reduces viability of the pathogens under hypoxic conditions anaerobic, acetogenic bacterium Acetobacterium woodii instead has a in which they otherwise persist in the host [32]. Analogously, growing hetero-oligomeric rotor with two different c-subunits. The A. woodii c- insights into the mechanisms of natural ATP synthase modulators, and rotor has 10 c-subunits/rotor; 9 of them are the products of two inhibitors of its hydrolytic mode, could lead to rational design of identical genes in the atp operon that have the usual 2-helix hairpin therapeutic strategies for human disorders that are linked to ATP structure of other F-type ATP synthase c-subunits. The remaining rotor synthase and to strategies for inhibiting the ATP synthases that are subunit is encoded by a third c-subunit gene and is a 4-transmembrane ectopically expressed on cancer cell surfaces [10,33,34]. helix (TMH) “c”-subunit of the type found in V-type ATP synthases/ There is another gateway to progress on the structure–function ATPases [70]. The V-type ATPase from Enterococcus hirae has 10 of the V- underpinnings of ATP synthase activity and its contributions to phys- type subunit in its K-rotor, the V-type ATPase equivalent of the c-rotor iology. This is the growing awareness that ATP synthases from higher [71]. The larger V-type subunit has only one ion-binding site, like that of organisms, as well as the cytochrome oxidases that participate in ATP the smaller ones. Its presence in the A. woodii rotor is expected to impact synthase-dependent OXPHOS, have multiple isoforms of important the functional properties of the enzyme. Perhaps the V-type subunit subunits [35–38]. Some of these have been shown to relate to tissue- enhances the propensity for ATP hydrolysis in a manner that relates to specific functional difference [39–41]. Among bacteria, the pace of its specialized physiological circumstances [70,72]. evolutionary adaptations and the extraordinary range of environ- ments in which ATP synthases function have yielded a wealth of 2.2. atp operons adaptations and variants in the OXPHOS machinery [42–44]. Microbial ATP synthases exhibit variations that support their adap- Bacterial ATP synthases are most often encoded in single operons tation to very distinct conditions of pH, temperature, oxygen levels, that have the eight structural genes in the order atp (or unc) BEFHAGDC, salt etc.

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