Even before the first cells appeared, a simple reaction between water, phosphorus and metal could have provided an initial source of chemical energy.
Today, all cells use energy to function. To do so, they rely on a central molecule: ATP. It can be viewed as a small rechargeable energy reserve. When it loses a phosphate, it becomes ADP and releases energy. ADP can then acquire a phosphate and become ATP again.
Diagram of the proposed mechanism for prebiotic phosphorylation in a hydrothermal vent producing hydrogen. Phosphite provides the energy, and palladium catalyzes the formation of phosphorylated compounds.
Credit: HHU / William Martin.
But this mechanism raises a problem when we look back to the origins of life. In present-day cells, highly elaborate enzymes enable these transformations. Yet these enzymes did not exist before life. It is therefore necessary to explain how molecules capable of transporting energy could have appeared spontaneously.
The researchers explored a possibility linked to hydrothermal vents on the early Earth. In some of these environments, hot water reacts with rocks. These reactions release hydrogen and bring metals and different forms of phosphorus together. Such an environment could have naturally provided the necessary ingredients.
In the laboratory, the researchers mixed water, phosphite and palladium. Phosphite is a form of phosphorus that can release energy when it reacts. The metal facilitates this reaction. Most importantly, this energy can be used to attach a phosphate to other molecules, without the intervention of an enzyme.
This is exactly what happened with AMP. This molecule has only one phosphate. By adding a second, the reaction transforms it into ADP, one of the molecules used by cells today to manage their energy. At 50 °C, up to 7.3% of the AMP was converted into ADP after 72 hours.
The experiment was not limited to this transformation. The same chemistry made it possible to produce several other phosphate-containing molecules capable of participating in energy transfers. Some are still part of the mechanisms used by cells today.
The pure palladium used initially might seem far removed from natural conditions. The researchers therefore repeated the experiment with a mixture of palladium, nickel and iron, similar to certain minerals associated with hydrothermal environments. Once again, ADP formed from AMP.
This experiment offers a possible answer to a step that must have preceded the emergence of life: how to obtain molecules in water capable of capturing and transporting energy, even before enzymes existed.