A simple two-carbon compound may have played a crucial role in the evolution of metabolism before the advent of cells, according to a new study published Oct. 4 in the open-access journal PLOS Biology, by Nick Lane and colleagues at University College London, UK. The discovery potentially sheds light on the early stages of prebiotic biochemistry and suggests how ATP became the universal energy carrier for all cellular life today.
ATP, adenosine triphosphate, is used by all cells as an energy intermediary. During cellular respiration, energy is captured when a phosphate is added to ADP (adenosine diphosphate) to generate ATP; the cleavage of this phosphate releases energy to power most types of cellular functions. But building ATP’s complex chemical structure from scratch is energy-intensive and requires six distinct ATP-driven steps; Although convincing models allow for the prebiotic formation of the ATP backbone without energy from already formed ATP, they also suggest that ATP was probably quite rare and that another compound may have played a central role in the formation. conversion from ADP to ADP at this stage of evolution.
According to Lane and colleagues, the most likely candidate was the two-carbon compound acetyl phosphate (AcP), which today functions in bacteria and archaea as a metabolic intermediate. AcP has been shown to phosphorylate ADP to ATP in water in the presence of iron ions, but a host of questions remained after this demonstration, including whether other small molecules might also work, whether the AcP is specific for ADP or might work just as well. well with diphosphates of other nucleosides (such as guanosine or cytosine), and whether iron is unique in its ability to catalyze the phosphorylation of ADP in water.
In their new study, the authors explored all of these questions. Based on data and assumptions about the chemical conditions of the Earth before the appearance of life, they tested the ability of other ions and minerals to catalyze the formation of ATP in water; none were as effective as iron. Next, they tested a panel of other small organic molecules for their ability to phosphorylate ADP; none was as effective as AcP, and only one other (carbamoyl phosphate) had significant activity. Finally, they showed that none of the other nucleoside diphosphates accepted an AcP phosphate.
By combining these results with molecular dynamics modelling, the authors propose a mechanistic explanation of the specificity of the ADP/AcP/iron reaction, by hypothesizing that the small diameter and the high charge density of the iron ion, combined with the conformation of the intermediate formed when the three come together, provide a “just right” geometry that allows the phosphate of AcP to switch partners, forming ATP.
“Our results suggest that AcP is the most plausible precursor to ATP as a biological phosphorylator,” says Lane, “and that the emergence of ATP as the universal energy currency of the cell was not not the result of a “frozen accident”, but the unique interactions of ADP and AcP arose. Over time, with the emergence of suitable catalysts, ATP could eventually replace AcP by as a ubiquitous phosphate donor and promotes the polymerization of amino acids and nucleotides to form RNA, DNA and proteins.
Lead author Silvana Pinna adds, “ATP is so central to metabolism that I thought it would be possible to form it from ADP under prebiotic conditions. 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 that the reaction is so selective – in the metal ion, the phosphate donor and the substrate – with molecules that life still uses. The fact that this happens best in water in soft, life-compatible conditions is really quite important to the origin of life.”
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