Palladium replaces a microbial enzyme to make ADP from phosphite in Dusseldorf lab tests

The size of the demand is what makes the question hard. According to the release, a well-nourished human makes and consumes a body weight of ATP every day, and an E. coli cell makes 10 body weights of it during each division ^[2]. Modern cells meet that demand with ATP synthases. These are protein machines that turn ion gradients into rotary motion and join ADP and phosphate into ATP ^[3]. William Martin heads the Institute of Molecular Evolution at HHU and is the study's senior author ^[14], and he said those machines cannot have come first. "ATP synthases are complex and evolutionarily advanced molecules. They cannot be the starting point of energy conservation at the origin of life," he said ^[4]. Phosphate is a poor starting material as well. It "is poorly soluble in the environment and is a very stable molecule that is generally inert. It does not want to react," Martin said ^[5].

The experiment is well designed because it starts from a reaction living cells already run. Microbes that grow on phosphite convert it to phosphate with an enzyme called AdpA, and in doing so they phosphorylate AMP to ADP ^[6]. "It conserves the energy in phosphite as a high-energy phosphate bond," lead author Manon Schlikker said ^[7]. She then tried swapping the enzyme for vent metals ^[8]. That made the question narrow and checkable: could a metal reproduce one known enzyme's chemistry?

It could. "We tried that combination in the laboratory and found that some hydrothermal vent metals can activate phosphite to phosphate, and some can even phosphorylate molecules, just like ATP," Schlikker said ^[15]. She went on: "Metallic nickel worked, but palladium, which also occurs in vents, worked much, much better" ^[8]. She also said, "There was no literature at all on anaerobic phosphite oxidation using palladium before our work" ^[9]. The paper is in The FEBS Journal ^[1]. The release that accompanies it does not give yields or rates for either metal.

The thing this doesn't tell you is whether the route gets as far as ATP. The product reported is ADP. ATP synthase makes ATP by adding a phosphate to ADP ^[3], so the metal route as described stops one phosphorylation short of the molecule cells actually spend ^[16]. The release calls phosphite and palladium "an inorganic precursor of ATP" ^[1]. Precursor is the right word, as long as it is read to mean an earlier way of making a high-energy phosphate bond.

The setting matters as much as the chemistry. "But to replace enzymes, the metals need to be in their native state: not just some rock, but metals in their shiny, native state, like coins," Martin said. He added that H2-producing vents "naturally deposit the metals in exactly that shiny state" ^[10]. The release says the palladium found in those vents is mostly an alloy with nickel and iron ^[11]. Before treating the vent version of this chemistry as more than plausible, I would want to see how that alloy performs against pure palladium.

Plausible still counts for something. The idea that life began at hydrothermal vents is held by some scientists, Martin's team among them ^[12]. A reaction that works in a flask shows the chemistry was available to early cells. It does not show they used it. For a decade the group has been replacing enzymes with native vent metals such as nickel, iron and cobalt ^[13]. This result adds a phosphorylation step to that record. I think it is a real addition, because it offers a chemical way around the inert-phosphate problem through phosphite, a compound vent bacteria still use today ^[6].