The Na(+)-translocating F(1)F(0) ATP synthase of Propionigenium modestum: mechanochemical insights into the F(0) motor that drives ATP synthesis

Directed proton transfer from Fo to F1 extends the multifaceted proton functions in ATP synthase

The role of protons in ATP synthase is typically considered to be energy storage in the form of an electrochemical potential, as well as an operating element proving rotation. However, this review emphasizes that protons also act as activators of conformational changes in F1 and as direct participants in phosphorylation reaction. The protons transferred through Fo do not immediately leave to the bulk aqueous phase, but instead provide for the formation of a pH gradient between acidifying Fo and alkalizing F1. It facilitates a directed inter-subunit proton transfer to F1, where they are used in the ATP synthesis reaction. This ensures that the enzyme activity is not limited by a lack of protons in the alkaline mitochondrial matrix or chloroplast stroma. Up to one hundred protons bind to the carboxyl groups of the F1 subunit, altering the electrical interactions between the amino acids of the enzyme. This removes the inhibition of ATP synthase caused by the electrostatic attraction of charged amino acids of the stator and rotor and also makes the enzyme more prone to conformational changes. Protonation occurs during ATP synthesis initiation and during phosphorylation, while deprotonation blocks the rotation inhibiting both synthesis and hydrolysis. Thus, protons participate in the functioning of all main components of ATP synthase molecular machine making it effectively a proton-driven electric machine. The review highlights the key role of protons as a coupling factor in ATP synthase with multifaceted functions, including charge and energy transport, torque generation, facilitation of conformational changes, and participation in the ATP synthesis reaction.

Short-Term Temporal Metabolic Behavior in Halophilic Cyanobacterium Synechococcus sp. Strain PCC 7002 after Salt Shock

In response to salt stress, cyanobacteria increases the gene expression of Na⁺/H⁺ antiporter and K⁺ uptake system proteins and subsequently accumulate compatible solutes. However, alterations in the concentrations of metabolic intermediates functionally related to the early stage of the salt stress response have not been investigated. The halophilic cyanobacterium Synechococcus sp. PCC 7002 was subjected to salt shock with 0.5 and 1 M NaCl, then we performed metabolomics analysis by capillary electrophoresis/mass spectrometry (CE/MS) and gas chromatography/mass spectrometry (GC/MS) after cultivation for 1, 3, 10, and 24 h. Gene expression profiling using a microarray after 1 h of salt shock was also conducted. We observed suppression of the Calvin cycle and activation of glycolysis at both NaCl concentrations. However, there were several differences in the metabolic changes after salt shock following exposure to 0.5 M and 1 M NaCl: (i): the main compatible solute, glucosylglycerol, accumulated quickly at 0.5 M NaCl after 1 h but increased gradually for 10 h at 1 M NaCl; (ii) the oxidative pentose phosphate pathway and the tricarboxylic acid cycle were activated at 0.5 M NaCl; and (iii) the multi-functional compound spermidine greatly accumulated at 1 M NaCl. Our results show that Synechococcus sp. PCC 7002 acclimated to different levels of salt through a salt stress response involving the activation of different metabolic pathways.

Metagenomic cloning and characterization of Na⁺ transporters from Huamachi Salt Lake in China

Moderately halophilic bacteria are a kind of extreme environment microorganism that can tolerate moderate salt concentrations ranging from 0.5M to 2.5M. Here, via a metagenomic library screen, we identified four putative Na(+) transporters, designated H7-Nha, H16-Mppe, H19-Cap and H35-Mrp, from moderately halophilic community in the hypersaline soil of Huamachi Salt Lake, China. Functional complementation observed in a Na(+)(Ca(2+))/H(+) antiporter-defective Escherichia coli mutant (KNabc) suggests that the four putative Na(+) transporters could confer cells a capacity of Na(+) resistance probably by enhancing Na(+) or Ca(2+) efflux, but not Li(+) or K(+) exchange. Blastp analysis of the deduced amino-acid sequences indicates that H7-Nha has 71% identity to the NhaG Na(+)/H(+) antiporter of Bacillus subtilis, while H19-Cap shows 99% identity to Enterobacter cloacae Ca(2+) antiporter. Interestingly, H16-Mppe shares 59% identity to the metallophosphoesterase of Bacillus cellulosilyticus and H35-Mrp shows 68% identity to multidrug resistance protein of Lysinibacillus sphaericus. This is the first report that predicts a potential role of metallophosphoesterase in Na(+) resistance in halophilic bacteria. Furthermore, everted membrane vesicles prepared from E. coli cells harboring H7-Nha exhibit Na(+)/H(+) antiporter activity, but not Li(+) (K(+))/H(+) antiporter activity, confirming that H7-Nha supports Na(+) resistance mainly via Na(+)/H(+) antiport. Our report also demonstrates that metagenomic library screen is a convenient and effective way to explore more novel types of Na(+) transporters.

Insights into the molecular mechanism of rotation in the Fo sector of ATP synthase

F(1)F(o)-ATP synthase is a ubiquitous membrane protein complex that efficiently converts a cell's transmembrane proton gradient into chemical energy stored as ATP. The protein is made of two molecular motors, F(o) and F(1), which are coupled by a central stalk. The membrane unit, F(o), converts the transmembrane electrochemical potential into mechanical rotation of a rotor in F(o) and the physically connected central stalk. Based on available data of individual components, we have built an all-atom model of F(o) and investigated through molecular dynamics simulations and mathematical modeling the mechanism of torque generation in F(o). The mechanism that emerged generates the torque at the interface of the a- and c-subunits of F(o) through side groups aSer-206, aArg-210, and aAsn-214 of the a-subunit and side groups cAsp-61 of the c-subunits. The mechanism couples protonation/deprotonation of two cAsp-61 side groups, juxtaposed to the a-subunit at any moment in time, to rotations of individual c-subunit helices as well as rotation of the entire c-subunit. The aArg-210 side group orients the cAsp-61 side groups and, thereby, establishes proton transfer via aSer-206 and aAsn-214 to proton half-channels, while preventing direct proton transfer between the half-channels. A mathematical model proves the feasibility of torque generation by the stated mechanism against loads typical during ATP synthesis; the essential model characteristics, e.g., helix and subunit rotation and associated friction constants, have been tested and furnished by steered molecular dynamics simulations.

Aqueous access channels in subunit a of rotary ATP synthase

The role of subunit a in proton translocation by the Escherichia coli F(1)F(o) ATP synthase is poorly understood. In the membrane-bound F(o) sector of the enzyme, H(+) binding and release occurs at Asp(61) in the middle of the second transmembrane helix (TMH) of subunit c. Protons are thought to reach Asp(61) via an aqueous access pathway formed at least in part by one or more of the five TMHs of subunit a. In this report, we have substituted Cys into a 19-residue span of the fourth TMH of subunit a and used chemical modification to obtain information about the aqueous accessibility of residues along this helix. Residues 206, 210, and 214 are N-ethylmaleimide-accessible from the cytoplasmic side of the membrane and may lie on the H(+) transport route. Residues 215 and 218 on TMH4, as well as residue 245 on TMH5, are Ag(+)-accessible but N-ethylmaleimide-inaccessible and may form part of an aqueous pocket extending from Asp(61) of subunit c to the periplasmic surface.

Reconstitution of Fo of the sodium ion translocating ATP synthase of Propionigenium modestum from its heterologously expressed and purified subunits

The atpB and atpF genes of Propionigenium modestum were cloned as His-tag fusion constructs and expressed in Escherichia coli. Both recombinant subunits a and b were purified via Ni(2+) chelate affinity chromatography. A functionally active Fo complex was reassembled in vitro from subunits a, b and c, and incorporated into liposomes. The F(o) liposomes catalysed (22)Na(+) uptake in response to an inside negative potassium diffusion potential, and the uptake was prevented by modification of the c subunits with N,N'-dicyclohexylcarbodiimide (DCCD). In the absence of a membrane potential the Fo complexes catalysed (22)Na(+)(out)/Na(+)(in)-exchange. After F(1) addition the F(1)F(o) complex was formed and the holoenzyme catalysed ATP synthesis, ATP dependent Na(+) pumping, and ATP hydrolysis, which was inhibited by DCCD. Functional F(o) hybrids were reconstituted with recombinant subunits a and b from P. modestum and c(11) from Ilyobacter tartaricus. These Fo hybrids had Na(+) translocation activities that were not distinguishable from that of P. modestum F(o).