All life as we know it uses the very same energy-carrying molecule as a sort of “universal cellular fuel.” Now, ancient chemistry may explain how this all-important molecule became ATP (adenosine triphosphate), a new study suggests reports.
ATP is an organic molecule that is charged up and used in every single cell through photosynthesis or through cellular respiration (the way organisms break down food). Every day, we recycle our own 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 myriad processes, from signaling in the brain to movement and reproduction.
How ATP rose to metabolic dominance instead of many possible equivalents has long been a mystery in biology and a subject of research.
“Our results suggest that the emergence of ATP as the cell’s universal energy currency was not the result of a ‘frozen accident'” but arose from unique interactions of phosphorylation molecules, explained Evolutionary biochemist Nick Lane from University College London (UCL).
The fact that ATP is used by all living things suggests that it existed from the dawn of life and even before that, during the prebiotic conditions that preceded all living matter.
But researchers wonder how that could be the case when ATP has such an intricate structure that requires six different phosphorylation reactions and a whole lot of energy to create it from scratch.
“There is nothing special about the ‘high energy’ [phosphorus] bonds in ATP,” says biochemist Silvana Pinna, who was at UCL at the time, and colleagues in her paper.
But since ATP also helps build our cells’ genetic information, it could have been harnessed for energy via this other route, they note.
Pinna and his team suspect that originally some other molecules must have been involved in the complicated phosphorylation process. So they took a closer look at another phosphorylating molecule, AcP, which is still used by bacteria and archaea in their 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 in water to ATP. When testing the ability of other ions and minerals to catalyze ATP formation in water, the researchers were unable to replicate this with other surrogate metals or phosphorylating molecules.
“It was very surprising to discover that the reaction is so selective – in the metal ion, phosphate donor and substrate – with molecules that life still uses,” says Auricle.
“That this occurs best in water under mild, life-friendly conditions is really important to the origin of life.”
This suggests that these energy-storing reactions involving AcP under prebiotic conditions could be occurring before biological life was there to stockpile and fuel the now self-sustaining cycle of ATP production.
In addition, the experiments suggest that the formation of prebiotic ATP most likely occurred in freshwater, where photochemical reactions and volcanic eruptions, for example, could provide the right mix of ingredients, the team explains.
While this doesn’t completely rule out its occurrence in the sea, it does suggest that life’s birth may have required a strong connection to land, they note.
“Our results suggest that ATP has established itself as a universal energy currency in a prebiotic, monomeric world due to its unusual chemistry in water,” Pinna and colleagues write.
Additionally, pH gradients in hydrothermal systems may have created an uneven ratio of ATP to ADP, allowing ATP to drive work even in the prebiotic small-molecule world.
“Over time, with the advent of suitable catalysts, ATP could eventually displace AcP as the ubiquitous phosphate donor and promote the polymerization of amino acids and nucleotides to form RNA, DNA, and proteins.” explained Roadway.
This study was published in PLOS biology.