A mechanism has been identified that could explain how chemical reactions essential to life could have preceded its appearance, providing a stepping stone to the first cells.
As much as we have learned about the way natural selection can produce ever more complex species from the simplest self replicating units, big questions remain. Particularly significant is the question of how the organic compounds that make up RNA could have emerged from the oceans of the early Earth.
Our bodies, and those of other living things, use metabolic enzymes to cut and paste elements and compounds to produce the crucial letters out of which the book of life is written. These enzymes are in turn produced by RNA. However, this creates the original chicken and egg problem – where did the RNA come from, given its own complexity?
However, a Cambridge University team have revealed in Molecular Systems Biology that these same reactions can also be triggered by the chemicals believed to have existed in the Archean sea that existed 4-2.5 billion years ago.
“Our results demonstrate that the conditions and molecules found in the Earth’s ancient oceans assisted and accelerated the interconversion of metabolites that in modern organisms make up glycolysis and the pentose-phosphate pathways, two of the essential and most centrally placed reaction cascades of metabolism,” says Dr Markus Ralser, who heads the team that made the discovery.
“In our reconstructed version of the ancient Archean ocean, these metabolic reactions were particularly sensitive to the presence of ferrous iron that helped catalyze many of the chemical reactions that we observed,” Ralser continues. While in many contexts the word ferrous means all iron, in chemistry it refers to iron compounds with an oxidation number of 2+. Geoscientists contributing to the research concluded from studies of the earliest sediments that ferrous iron was common in the oceans before photosynthesis introduced free oxygen to the air and water.
The discovery came about when Ralser asked one of his students to do quality control of the medium their lab used to culture cells. The student ran some of the medium through a mass spectrometer as a short cut and picked up signs of pyruvate (CH3COCOO−), a base that is key to essential metabolic pathways. As its chemical formula suggests pyruvate is not a simple compound, so it was surprising to find it appearing without the enzymes that within cells produce it from glucose. The glycolysis pathway is a ten step process, each requiring enzyme catalysis.
Ralser's team repeated the experiments to see how the pyruvate got there and found metals were standing in for the enzymes as catalysts. "People have said that these pathways look so complex they couldn't form by environmental chemistry alone," Ralser told New Scientist. “This is the first experiment showing that it is possible to create metabolic networks in the absence of RNA."
The next question was whether the processes could have occurred under the conditions that applied in the first few hundred million years of the Earth's existence. Colleagues at the Cambridge Earth Sciences department were already working on identifying the chemicals in the ocean almost 4 billion years ago.
By adding sugar phosphates to the best estimates of such chemical composition and heating the mixture to 50°-90°C, such as one might find around a hydrothermal vent, for 5 hours Ralser says he hoped to achieve one or two of the important reactions within cells. Instead, “In the presence of iron and other compounds found in the oceanic sediments, 29 metabolic-like chemical reactions were observed, including those that produce some of the essential chemicals of metabolism, for example precursors of the building blocks of proteins or RNA,” says Ralser. “These results indicate that the basic architecture of the modern metabolic network could have originated from the chemical and physical constraints that existed on the prebiotic Earth.”
“We could reconstruct two metabolic pathways almost entirely,” Ralser notes. He believes the fact that certain stages were not observed may indicate his team didn't try the exact conditions needed, since not all reactions were optimized at the same temperatures and chemical combinations. Alternatively the missing reactions may not have been required for the first appearance of life, instead evolving later. The glycolysis or pentose phosphate pathways Ralser largely replicated produce not only the sugars in DNA and RNA, but the molecules that make up fats and proteins and ATP, the coenzyme that transfers energy within cells.
Another interesting feature of the result is the reactions that didn't happen. The paper notes that 182 reactions were theoretically possible for the sugar phosphates used. The 29 that were observed, “Strongly overlap with the enzyme‐catalyzed reactions of the non‐oxidative pentose phosphate pathway and glycolysis.” The authors speculate that it is precisely because these are the reactions that could occur in the Archean ocean that they have become essential to the processes of life, while others have not.
The paper adds, “How and whether the first enzymes adopted the reaction mechanisms of the metal‐catalyzed reactions presented here remains an equivocal question.” However, they point to one reaction that is catalyzed by enzymes in animals and plants, but ferrous iron in bacteria. They suggest all life initially used iron as a catalyst, but once free oxygen depleted its soluble presence in the oceans there were advantages in not relying too heavily on such a diminishing resource.
While a major step, Ralser's work falls short of explaining how life emerged. In order for these pathways to begin the catalysts had to have source molecules to work on, and there is no evidence that these existed at the time, nor a clear idea where they would have come from if they did. There is also a question of what would have concentrated the metabolites to the extent required, given that they would have been exceptionally dilute on a planet-wide basis.