Enzymes can generate energy for living organisms. Adenosine triphosphate, abbreviated as ATP, is the main storage form of chemical energy. ATP is a charged battery that can release energy to power for biological activities. Enzymes are the transformer to turn energy into proper chemical forms and store it in ATP molecules. Most of these enzymes are called ATP synthases, which is found in all life forms and supports all cellular activities by directly creating the energy storage molecule ATP that is the most commonly utilized "energy currency" of cells for all organisms. ATP is formed via a process of enzymes oxidizing nutrients adenosine diphosphate (ADP) and inorganic phosphate (P_i), which is called oxidative phosphorylation.

The synthesis of ATP from ADP and P_i is energetically unsupported and normally proceeds in a reverse direction. To drive a forward reaction, ATP synthase conducts ATP synthesis during cellular resP_iration through an electrochemical gradient that is resulted from the difference in proton concentration across the mitochondrial membrane in eukaryotes or the plasma membrane in bacteria. However, the production of ATP during photosynthesis in plants is achieved by using a proton gradient present in the thylakoid lumen through the thylakoid membrane and into the chloroplast stroma.

Figure 1. The overall reaction catalyzed by ATP synthase.

Structure of ATP Synthase

ATP synthase includes two main subunits of F₀ and F₁ and acts through a rotational motor mechanism to finish ATP production. The part inserted within the membrane of the mitochondria in eukaryotes, plasma membrane in prokaryotes, or thylakoid membrane of the chloroplast in plants, is named F₀, which is a motor powered by H⁺ ions flowing across the membrane. The part inside the mitochondria, the chloroplast stroma, or the prokaryotic cells is called F₁-ATPase, another motor that is applied to generate ATP. The F₀ region is similar to DNA helicases that unzip DNA, while the F₁-ATPase region resembles the H⁺ motors that drive flagella on some bacteria to move, and it has a central stalk and rotor, whose turn could convert ADP and Pi into ATP. These two parts as two separate structures with two different functions eventually evolve into ATP synthase, where the two components would rotate in response to a proton flow, and this rotational energy is then coupled to ATP synthesis. ATP synthase is accepted as a molecular machine owing to its rotating subunit.

Function of ATP Synthase

ATP synthases are an ancient family of proteins that keep basically the same structure and function and are highly conserved throughout all kingdoms of life. ATP synthases mainly function as molecular engines that utilize the energy derived from proton flow to fuel the phosphorylation of ADP, thus producing ATP to be utilized in all cellular processes. About 100 molecules of ATP could be produced by an ATP synthase every second. Mitochondria containing eukaryotes, such as plants, animals, and fungi, possess large quantities of ATP synthases to produce ATP, where ATP synthase is situated in the inner mitochondrial membrane and the F₁-part sticks into mitochondrial matrix. Chloroplasts in plants also contain ATP synthase to synthesize ATP from sunlight and carbon dioxide. Prokaryotes, mainly including bacteria and archaea, without mitochondria produce ATP in similar cellular respiration manner in their plasma membrane. In aerobic bacteria under physiological conditions, ATP synthase generally runs in the opposite direction to create ATP by taking the proton motive force generated from the electron transport chain as a source of energy. Fermenting bacteria that do not have an electron transport chain could use large quantities of ATP to create a transmembrane proton gradient, where ATP synthase generates a proton motive force anaerobically by the expulsion of protons to power the movement of flagella and the transport of nutrients into the cell. As a consequence of this mechanism, it is considered that the ATP synthase can lead to an increase in intracellular pH under situations where it is acidified.