| Summary: | Archaea are microorganisms which thrive under extreme environmental conditions. These organisms are placed at the bottom of the evolutionary tree of life and retain some of the efficient energy conserving mechanisms. Archaeal A1AO ATP synthases (A-ATP synthases) are a separate class of energy converters, synthesizing adenosine triphosphate (ATP) by means of ion gradient-driven phosphorylation. These enzymes possess the unique capability to couple ATP synthesis to the transport of both, H+ and Na+ ions. An A-ATP synthase is composed of a total of nine subunits in the stoichiometry of A3:B3:C:D:E:F:H2:a:cx. Adenosine triphosphate is synthesized in the A3B3 headpiece of the A1 domain, and the energy provided for this process is transmitted to the membrane-bound AO domain. The energy coupling between the A3B3 hexamer and the AO sector occurs via the stalk subunits C, D and F. In the intact A-ATP synthase, the A-B dimer interface in the A3B3 hexamer is the nucleotide binding site, wherein subunit A and B are catalytic and non-catalytic/regulatory subunits, respectively. Recently, the structures of subunit A from thermophilic archaea Pyrococcus horikoshii OT3 and subunit B from a methanogenic archaea Methanosarcina mazei Gö1 have been determined in the absence of nucleotides. In an attempt to understand the nucleotide binding and catalytic mechanism, crystal structures of subunit A from P. horikoshii OT3 are determined and presented in complex with a phosphate analog-, AMPPNP- and ADP to resolutions of 2.47 Å and 2.4 Å, respectively.
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