All life as we know it uses the exact same energy molecule as a kind of “universal cellular fuel”. Now, ancient chemistry can explain how this all-important molecule came to be ATP (adenosine triphosphate), according to a new study.
ATP is an organic molecule, charged by photosynthesis or cellular respiration (the way organisms break down food) and used in every cell. Every day, we recycle our body weight into ATP.
In both of the above systems, a phosphate molecule is added to ADP (adenosine diphosphate) through a reaction called phosphorylation – resulting in ATP.
Reactions that release the same phosphate (in another process called hydrolysis) provide chemical energy that our cells use for countless processes, from brain signaling to movement and reproduction.
How ATP rose to metabolic dominance, in place of many possible equivalents, has been a long-standing mystery in biology and the focus of research.
“Our results suggest … that the emergence of ATP as the global energy currency of the cell was not the result of a ‘frozen accident,'” but arose from unique interactions of phosphorylating molecules, explains evolutionary biochemist Nick Lane from University College London (UCL ). ).
The fact that ATP is used by all living things suggests that it has been around since the beginning of life and even before, during the prebiotic conditions that preceded all of us living matter.
But researchers are puzzled as to how this could be when ATP has such a complex structure that involves six different phosphorylation reactions and a lot of energy to create it from scratch.
“There is nothing special about ‘high energy’ [phosphorus] bonds in ATP,” says biochemist Silvana Pinna who was at UCL at the time and colleagues in their paper.
But since ATP also helps create our cells’ genetic information, it may have been used for energy use through this other pathway, they note.
Pinna and team suspect that some other molecule must be involved in the complex phosphorylation process in the first place. So they looked closely at another phosphorylating molecule, AcP, which is still used by bacteria and archaea in the metabolism of chemicals, including phosphate and thioester—a chemical thought to have been abundant at the beginning of life.
In the presence of iron ions (Fe3+), AcP can phosphorylate ADP to ATP in water. After testing the ability of other ions and metals to catalyze the formation of ATP in water, the researchers were unable to replicate this with other metal substitutes or phosphorylating molecules.
“It was very surprising to find that the reaction is so selective – to the metal ion, the phosphate donor and the substrate – with molecules that life still uses,” says Pinna.
“The fact that this happens best in water under mild, life-compatible conditions is really very important for the origin of life.”
This suggests that with AcP, these energy-storing reactions could take place in prebiotic conditions, before biological life was there to accumulate and fuel the now self-perpetuating cycle of ATP production.
Furthermore, the experiments show that the creation of prebiotic ATP was most likely to take place in fresh water, where photochemical reactions and volcanic eruptions, for example, could provide the right mix of ingredients, the team explains.
While this does not completely rule out its occurrence in the sea, it does imply that the birth of life may have required a strong link with land, they note.
“Our results suggest that ATP established itself as the global energy currency in a probiotic, monomeric world, based on its unusual chemistry in water,” Pinna and colleagues write.
Furthermore, pH gradients in hydrothermal systems could have created an unequal ATP to ADP ratio, allowing ATP to drive work even in the prebiotic world of small molecules.
“Over time, with the emergence of appropriate catalysts, ATP could eventually displace AcP as the ubiquitous phosphate donor and drive the polymerization of amino acids and nucleotides to form RNA, DNA, and proteins,” Lane explains.
This research was published in PLOS Biology.