Everything you do needs energy to drive it, whether it is running up a flight of stairs, eating breakfast, fighting off an infection, or even just producing new proteins for hair growth. The energy currency used by all cells from bacteria to man is adenosine triphosphate (ATP). Every process within an organism – DNA and protein synthesis, muscle contraction, active transport of nutrients and ions, neural activity, maintenance of osmosis, carbon fixation – requires a source of ATP.
Why is ATP such a good source of energy? ATP is a nucleoside triphosphate (ribose sugar, adenine base and three phosphate groups), where a high-energy bond attaches the third phosphate group to the molecule. This bond is highly unstable, and when it is hydrolysed it releases a substantial amount of free energy (approximately 7 kcal/mole). In addition to providing energy, ATP has other essential roles within cells: it is one of the four nucleotides required for the synthesis of DNA (replication) and RNA (protein synthesis); it regulates certain biochemical pathways; in mammals it is released from damaged cells to elicit a pain response; and in photosynthetic organisms it drives carbon fixation. However, as it is unstable (cannot be stored for long) and is used for almost every conceivable process, each cell in the body must constantly produce ATP to supply its needs. In total, an organism’s requirement for ATP is substantial: the average human body generates over 100 kg of ATP per day. ATP synthase is the prime producer of ATP in cells, catalysing the combination of ADP (adenosine diphosphate) with inorganic phosphate to make ATP:
ADP + Pi ŕ ATP + H2O
ATP Synthase, an Early Enzyme of Life
Because of its fundamental importance in sustaining life, organisms evolved ATP synthase (ATPase) early during evolution, making it one of the oldest of enzymes - even predating photosynthetic and respiratory enzyme machinery. As a result, ATPase has remained a highly conserved enzyme throughout all kingdoms of life: the ATPases found in the thylakoid membranes of chloroplasts and in the inner membranes of mitochondria in eukaryotes retain essentially the same structure and function as their enzymatic counterparts in the plasma membranes of bacteria. In particular, the subunits that are essential for catalysis show striking homology between species.