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ATP synthase: a brief introduction

The text below is a general introduction to ATP synthase. A brief history of ATP synthase research is also available.
It is supposed that the reader has some basic background knowledge in biology and biochemistry. If you need more detailed information on ATP synthase, see the FAQ list, or try the wonderful lecture of A. Crofts.

This enzyme is the primary source of ATP in a vast majority of living species on Earth, including us. In human body it daily generates over 50 kg of ATP, which is subsequently used to provide energy for various biochemical reactions, including DNA and protein synthesis, muscle contraction, transport of nutrients and neural activity, to name just a few.
In plants and photosynthetic bacteria ATP synthase is essential for solar energy conversion and carbon fixation. This is one of the oldest enzymes on Earth, which appeared earlier then photosynthetic or respiratory enzyme machinery.

ATP synthase is a membrane enzyme. It is found in eu- and archebacteria in the plasma membrane; it is present in the thylakoid membrane in chloroplasts and in the inner mitochondrial membrane of eukaryotic cells. Enzymes from different organisms show striking homology in the primary structure of subunits essential for catalysis, and have the same core catalytic mechanism. The inter-species differences are mainly in auxiliary subunits and in the regulatory features of ATP synthase.

As could be deduced from the name of the enzyme, it catalyses the reaction of ATP synthesis. The catalytic act is coupled with transmembrane translocation of several protons driven by protonmotive force (pmf) generated by respiratory or photosynthetic enzymes.
As mentioned above, ATP synthesis is the main function of the enzyme in most eukaryotic organisms. However, in many bacterial species (mostly anaerobic) the reverse reaction is vitally important. When neither respiratory chain nor photosynthetic proteins can generate pmf, ATP synthase works as a proton pump, generating pmf at the expense of ATP hydrolysis. In this way many important cellular functions, such as flagella motility or ion\nutrients transmembrane transport are supported.
Clearly, ATP hydrolysis activity is always a potential danger to a living cell, so ATP synthase has several regulatory mechanisms to prevent futile ATP wasting. Regulation of ATP hydrolysis activity is particularly strong in chloroplast ATP synthase - during the night it is important to prevent consumption of ATP synthesized in daytime.

 Cartoon illustration of ATP synthase
 Scheme of energy conversion during ATP synthesis

The overall equation is:

    ADP3- + HPO42- + H+ + nH+out   <=>  ATP4- + H2O + nH+in

where indices "out" and "in" denote the outer (positively charged) and the inner (negatively charged) side of the membrane, respectively.

The structure of this enzyme is rather complex. It is an asymmetric multisubunit protein complex of about 500 kDa. It consists of two distinct (both structurally and functionally) multisubunit portions. Hydrophobic Fo portion is embedded into the membrane and performs proton translocation, while hydrophillic F1 portion protrudes into the aqueous phase and performs ATP synthesis/hydrolysis.
During catalysis a complex formed by certain subunits rotate relative to the rest of the enzyme. This feature makes ATP synthase the smallest rotary machine ever known.
In 1997 the Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker "for their elucidation of the enzymatic mechanism underlying the synthesis of adenosine triphosphate (ATP)", and to Jens C. Skou "for the first discovery of an ion-transporting enzyme, Na+, K+ -ATPase".

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