Chemists have highlighted a chemical cycle that relies on simple compounds and could have produced molecules that are essential for the emergence of life and initiate current biochemical cycles, four billion years ago.
Essential for oxygen-consuming organisms, the Krebs cycle, or citric acid cycle, is responsible for transforming carbon molecules to produce energy. How could such a cycle have begun in the very first living organisms?
In the 1990s, the German chemist Günter Wächtershäuser hypothesized that an inverted version of this cycle, which can now be observed in certain bacteria, could have existed several billion years ago. This inverted cycle would have produced the first molecular blocks necessary for the appearance of life. However, the current cycle of citric acid uses biological molecules to function (eight catalytic enzymes), which could not be the case for primitive cycles.
Greg Springsteen, of Furman University, Ramanarayanan Krishnamurthy, of Scripps Institute, and their colleagues highlighted a novel chemical cycle that could have operated under the conditions of Early Earth to produce usable molecules by the living.
By arranging molecules that cause reactions similar to those of current biochemistry under various conditions, the researchers observed cycles producing chemical intermediates identical to those of the current citric acid cycle. The researchers first selected relatively simple prebiotic molecules, the malonate ion (-OOC-CH2-COO-) and the glyoxylate ion (-OOC-COH). As a substitute for the current enzymes that facilitate redox reactions, they used hydrogen peroxide – also called hydrogen peroxide -, a product of the photochemical oxidation of water.
These compounds participate in a four-step chemical cycle producing two molecules of carbon dioxide and a crucial intermediate: oxaloacetate (-OOC-CO-CH2-COO-). This cycle is coupled with a second, also initiated by simple molecules and producing a molecule denoted HKG, and one of the intermediates is also oxaloacetate, thus connecting this cycle with the previous one.
All the interest of this result lies in the large number of intermediate compounds produced, like the oxaloacetate which enters the current Krebs cycle, in the urea cycle and in the synthesis of aspartate, an amino acid.
Chemists have experimentally followed these reactions by magnetic resonance spectrometry and have established that the two cycles (called malonate cycle and HKG cycle) operate at a pH between 7 and 8.5 and a temperature below 50 ° C, compatible conditions with those supposed to reign on the surface of the primitive Earth.
Greg Springsteen and his colleagues conclude that if these protometabolic cycles that do not require biological catalysis by enzymes were active on the early Earth, they could have synthesized the elements necessary to initiate the current biochemical cycles. Joseph Moran, from the Chemical Catalysis Laboratory at the University of Strasbourg, who did not take part in this study, underlines the interest of this work and the many questions they pose about the evolution of metabolism: “Is hydrogen peroxide really a plausible oxidizer on the primitive Earth? Why would life today use the citric acid cycle rather than these malonate / HKG cycles? Did life initially follow this path and then abandon it for the benefit of another, or has it always used it only insignificantly? “. The chemistry of the early Earth remains largely to be discovered.