Why did a weird little molecule called ATP become so crucial for all life on Earth?

Adenosine triphosphate – ATP – is the reason plants grow, bodies move, and neurons fire.

Called the “universal energy currency”, the molecule appears in all forms of life and plays a key role in transferring energy from one part of a cell to another.

But what made ATP so special?

A team of UK researchers think they’ve landed on the answer.

According to their study, published in PLOS Biology, ATP didn’t become dominant by accident: it comes from a very favourable chemical combination of its precursors.

ATP is a complicated molecule to make, but it must have been made very early in the evolutionary process, because it appears in every organism on Earth.

It stores and generates energy by switching between adenosine diphosphate and monophosphate – ADP and AMP – and ATP.

ATP drives its own synthesis: having ATP molecules present makes it more likely that even more ATP will be generated from ADP.

But what was its precursor in the prebiotic soup, before there was a lot of ATP around?

The researchers suspected, based on earlier studies, that prebiotic ATP came from a reaction between ADP, iron, and a compound called acetyl phosphate (AcP).

Adenosine diphosphate and acetyl phosphate combining to make atp
The simulation of ADP and AcP interacting, about to make ATP. Credit: Aaron Halpern, UCL (CC-BY 4.0, https://creativecommons.org/licenses/by/4.0/)

In this study, they wanted to see if AcP and iron were necessary for making ATP, and whether they could make other molecules that worked as good energy carriers.

The researchers ran a series of experiments in water, replicating the conditions of prebiotic Earth.

They tested a number of different minerals and found that iron ions were way more effective at making ATP than any other substance.

Next, they tested whether other molecules similar to AcP could generate ATP too. They found that AcP was the best ATP precursor.

More on ATP: Life without phosphate – mystery solved?

Based on these experimental results, and computer modelling, the researchers developed a mechanism for the AcP-ADP-iron reaction, showing why it was particularly favourable without anything living to help it along.

“ATP is so central to metabolism that I thought it might be possible to form it from ADP under prebiotic conditions,” says lead author Dr Silvana Pinna, who did the research during a PhD at University College London, UK.

“But I also thought that several phosphorylating agents and metal ion catalysts would work, especially those conserved in life.

“It was very surprising to discover the reaction is so selective – in the metal ion, phosphate donor, and substrate – with molecules that life still uses.

“The fact that this happens best in water under mild, life-compatible conditions is really quite significant for the origin of life.”

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