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Background

ATP synthesis is the most prevalent chemical reaction in the biological world and ATP synthase is one of the most ubiquitous, abuntant proteins on earth. From Escherichia coli to plants and mammals, this enzyme is one of the most conserved during evolution^[1]. PDB code of ATP synthase is 1c17. The molecular study of ATP synthase was initiated in 1960 when Efraim Racker and his colleagues reported the isolation of soluable factor from beef heart mitochondria. ATP synthase produces ATP from adenosine diphosphate (ADP) and inorganic phosphate with the use of energy from a transmembrane proton-motive force generated by respiration or photosynthase^[2].

Structure of ATP synthase

Strucutre of ATP synthases are basically similiar whatever the source. In their simplest form in prokaryotes, they contain eight different subunits, with stoichiometry αβ₃γδεab₂c_10-15. The total molecular size is about 530kDa^[3]. The enzyme consists of the extramembranous F₁ catalytic domain linked by means of a central stalk to an intrinsic membrane domain called F₀. In the atomic structure of F₁, the α and β subunits are arranged alternately around a coiled coil of two antiparallel α helices in the γ subunit ^[4]. ATP synthase has many and . The catalytic sites are in the β subunits at the α/β subunit interface. The remainder of the γ subunit protrudes from the α₃β₃ assembly and can be cross linked to the polar loop region of the c subunits in F₀. In mitochondria, the δ and ε subunits are associated with the γ subunit in the central stalk assembly, as are the bacterial and chloroplast ε subunits, the counterparts of mitochondrial δ. ATP-dependent rotation of γ and ε within an immobilized α₃β₃ complex from the thermophilic bacterium Bacillus PS3 has been observed directly.

Two structural domains of ATP synthase^[4].

High resolution structure analysis

The first atomic-level (2.8Å) resolution structures by X-ray, strucutures was of bovine mitochondrial F₁ in 1994. The α and β subunits each had similiar three-domain structure, with an N-terminal β-barrel furthest away from the membrane surface, a central nucleotide-binding domain, and a C-terminal helical domain. Also, an NMR structure of isolated E. coli ε subunit is in good agreement with X-ray structure. The structure of the N-terminal domain of E. coli δ subunit consisting of residues 1 through 134 was solved also by NMR ^[3]. In addition, according to analyses by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) , high-performance liquid chromatography (HPLC) analysis, and NH₂-terminal sequencing, the purified complex used here for crystallization consists of subunis α, β, γ, δ, ε, b, d, a, h, f, ATP8, and c (in diminishing apparent molecular weight order for F₁ and F₀ on SDS gels) is similar to other preparations.

Reaction analysis of the catalytic sites

The energy for ATP synthesis is provided from proton transport along the gradient of electrochemical potential of protons across membranes (△H). When the magnitude of △H is large, as in functional mitochondrial, downhill proton flow through F₀ causes rotation of the F₀ rotor and hence rotation of the γε-subunits of F₁. The rotary motion of the γ alternates the structure of the β-subunit so the ATP is synthesized^[5]. Mutagenesis of catalyticsites of ATP synthase have proceeded extensively in the E. coli enzymes, somewhat less so in Saccharomyces cerevisiae and Bacillus PS3, and to far lesser extent in other species. Almost without exception, detailed in vitro biochemical analyses of purified mutant enzymes have been limited to the ATP hydrolysis reaction. However, in both yeast and E. coli, growth tests on nonfermentable substrates provide sensitive if quantative assays of ATP synthesis in the cell. Ligands to the Mg²⁺ cation have been studied in detail by mutagenesis and functional analysis ^[6].

Octahedral coordination of Mg²⁺ in the catalytic site of ATP synthase^[3].

Perspective

As a motor protein, ATP synthase offers a rare research opportunity. Structure of the F₁ motor, both rotor and stator in the same assembly, are known in atomic detail for the first time, and rotation can be analyzed at sub-millisecond time resolution ^[7]. An emerging possibility of step-size mismatch between the F₁ and F₀ motors provides an opportunity to find a noval coupling mechanism of the two motors that will explain why the mismatch is good for the enzyme. Finally, one can even dream of using this, the world's tiniest motor, as an engine part in the fabrication of nano-machines. The marvel of ATP will continue^[1].

Additional Resources

For additional information, see: Photosynthesis

References

1. ↑ ^{1.0 1.1} Yoshida M, Muneyuki E, Hisabori T. ATP synthase--a marvellous rotary engine of the cell. Nat Rev Mol Cell Biol. 2001 Sep;2(9):669-77. PMID:11533724 doi:10.1038/35089509
2. ↑ Stock D, Leslie AG, Walker JE. Molecular architecture of the rotary motor in ATP synthase. Science. 1999 Nov 26;286(5445):1700-5. PMID:10576729
3. ↑ ^{3.0 3.1 3.2} Senior AE, Nadanaciva S, Weber J. The molecular mechanism of ATP synthesis by F1F0-ATP synthase. Biochim Biophys Acta. 2002 Feb 15;1553(3):188-211. PMID:11997128
4. ↑ ^{4.0 4.1} von Ballmoos C, Wiedenmann A, Dimroth P. Essentials for ATP synthesis by F1F0 ATP synthases. Annu Rev Biochem. 2009;78:649-72. PMID:19489730 doi:10.1146/annurev.biochem.78.081307.104803
5. ↑ Weber J, Senior AE. ATP synthesis driven by proton transport in F1F0-ATP synthase. FEBS Lett. 2003 Jun 12;545(1):61-70. PMID:12788493
6. ↑ Weber J, Hammond ST, Wilke-Mounts S, Senior AE. Mg2+ coordination in catalytic sites of F1-ATPase. Biochemistry. 1998 Jan 13;37(2):608-14. PMID:9425083 doi:10.1021/bi972370e
7. ↑ Adachi K, Yasuda R, Noji H, Itoh H, Harada Y, Yoshida M, Kinosita K Jr. Stepping rotation of F1-ATPase visualized through angle-resolved single-fluorophore imaging. Proc Natl Acad Sci U S A. 2000 Jun 20;97(13):7243-7. PMID:10840052 doi:10.1073/pnas.120174297

--Hao Lu