The majority of cellular energy in the form of adenosine triphosphate (ATP) is synthesized by the ubiquitous F(1)F(0) ATP synthase. Power for ATP synthesis derives from an electrochemical proton (or Na(+)) gradient, which drives rotation of membranous F(0) motor components. Efficient rotation not only requires a significant driving force (DeltamuH(+)), consisting of membrane potential (Deltapsi) and proton concentration gradient (DeltapH), but also a high proton concentration at the source P side. In vivo this is maintained by dynamic proton movements across and along the surface of the membrane. The torque-generating unit consists of the interface of the rotating c ring and the stator a subunit. Ion translocation through this unit involves a sophisticated interplay between the c-ring binding sites, the stator arginine, and the coupling ions on both sides of the membrane. c-ring rotation is transmitted to the eccentric shaft gamma-subunit to elicit conformational changes in the catalytic sites of F(1), leading to ATP synthesis.

Sodium/proton (Na+/H+) antiporters, located at the plasma membrane in every cell, are vital for cell homeostasis1. In humans, their dysfunction has been linked to diseases, such as, hypertension, heart failure and epilepsy and they are well-established drug targets2. The best understood model system for Na+/H+ antiport is NhaA from Escherichia coli1,3, where both EM and crystal structures are available4-6. NhaA is made up of two distinct domains, a Core domain and a Dimerisation domain. In the NhaA crystal structure a cavity is located between the two domains providing access to the ion-binding site from the inward-facing surface of the protein1,4. Like many Na+/H+ antiporters, the activity of NhaA is regulated by pH, only becoming active above pH 6.5, where a conformational change is thought to occur7. To date, the only reported NhaA crystal structure is of the low pH inactivated form4. Here, we describe the active-state structure of a Na+/H+ antiporter, NapA from Thermus thermophilus at 3 Å resolution, solved from crystals grown at pH 7.8. In the NapA structure, the Core and Dimerisation domains are in different positions to those seen in NhaA and a negatively charged cavity has now opened to the outside. The extracellular cavity allows access to a strictly conserved aspartate residue thought to directly coordinate ion-binding1,8,9, a role supported here by molecular dynamics simulations. To alternate access to this ion-binding site, however, requires a surprisingly large rotation of the Core domain, some 20° against the Dimerisation interface. We conclude that despite their fast transport rates of up to 1500 ions/sec3, Na+/H+ antiporters operate by a two-domain rocking bundle model, revealing themes relevant to secondary-active transporters in general.

A dimeric structure of the sodium–proton antiporter NhaA provides insight into the roles of Asp163 and Lys300 in the transport mechanism.

F1F0 ATP synthases convert energy stored in an electrochemical gradient of H+ or Na+ across the membrane into mechanical rotation, which is subsequently converted into the chemical bond energy of ATP. The majority of cellular ATP is produced by the ATP synthase in organisms throughout the biological kingdom and therefore under diverse environmental conditions. The ATP synthase of each particular cell is confronted with specific challenges, imposed by the specific environment, and thus by necessity must adapt to these conditions for optimal operation. Examples of these adaptations include diverse mechanisms for regulating the ATP hydrolysis activity of the enzyme, the utilization of different coupling ions with distinct ion binding characteristics, different ion-to-ATP ratios reflected by variations in the size of the rotor c ring, the mode of ion delivery to the binding sites, and the different contributions of the electrical and chemical gradients to the driving force.

Cycles have a profound role in cellular life at all levels of organization. Well-known cycles in cell metabolism include the tricarboxylic acid and the urea cycle, in which a specific carrier substrate undergoes a sequence of chemical transformations and is regenerated at the end. Other examples include the interconversions of cofactors, such as NADH or ATP, which are present in the cell in limiting amounts and have to be recycled effectively for metabolism to continue. Every living cell performs a rapid turnover of ATP to ADP to fulfil various energetic demands and effectively regenerates the ATP from ADP in an energy-consuming process. The turnover of the ATP cycle is impressive; a human uses about its body weight in ATP per day. Enzymes perform catalytic reaction cycles in which they undergo several chemical and physical transformations before they are converted back to their original states. The ubiquitous F 1 F o ATP synthase is of particular interest not only because of its biological importance, but also owing to its unique rotational mechanism. Here, we give an overview of the membrane-embedded F o sector, particularly with respect to the recent crystal structure of the c ring from Ilyobacter tartaricus, and summarize current hypotheses for the mechanism by which rotation of the c ring is generated.

The AcrA-AcrB-TolC complex is the major multidrug efflux pump in Escherichia coli. The asymmetric structure of the trimeric inner-membrane component AcrB implies functional rotation of the monomers and a peristaltic mode of drug efflux. This mechanism suggests the occurrence of conformational changes in the periplasmic pore domain through the movements of subdomains during cycling of the monomers through the different states loose (L), tight (T) and open (O). We introduced cysteines at the interfaces of potentially moving subdomains, leading to disulfide bond formation as quantified by alkylation of free cysteines and MALDI-TOF analysis. Inhibition of pump function as a result of cross-linking caused increased susceptibility to noxious compounds and reduction of N-phenylnaphthylamine efflux. Regain of function for impaired mutants was obtained upon exposure to the reducing agent DTT. The results support the presence of the asymmetric AcrB trimer in E. coli membranes and the functional rotation mechanism.

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The three-component AcrA/AcrB/TolC efflux system of Escherichia coli catalyzes the proton motive force-driven extrusion of a variety of cytotoxic compounds. The inner membrane pump component AcrB belongs to the resistance nodulation and cell division (RND) superfamily and is responsible for drug specificity and energy transduction of the entire tripartite efflux system. Systematic mutational analysis of titratable and polar membrane-located amino acids revealed four residues, D407, D408, K940, and, R971, to be of prime importance for AcrB function. Using matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, D408 was shown to specifically react with dicyclohexylcarbodiimide (DCCD) in a pH-dependent manner. The apparent pK(a) of D408 of 7.4 would enable binding and release of protons under physiological conditions. In contrast to other secondary transporters, D408 was not protected from carbodiimide modification in the presence of drugs, which supports the notion of spatially separated transport pathways for drugs and protons. This study provides evidence for a substantial role of membrane-located carboxylates as a central element of the proton translocation pathway in AcrB and other members of the RND superfamily.