Pentoses are essential carbohydrates in the metabolism of modern lifeforms, but their availability during early Earth is unclear since these molecules are unstable.
A new study, published in the journal JACS Au and led by the Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology, Japan, reveals a chemical pathway compatible with early Earth conditions and by which C6 aldonates could have acted as a source of pentoses without the need for enzymes. Their findings provide clues about primitive biochemistry and bring us closer to understanding the Origins of Life.
The emergence of life on Earth from simple chemicals is one of the most exciting yet challenging topics in biochemistry and perhaps all of science. Modern lifeforms can transform nutrients into all sorts of compounds through complex chemical networks; what’s more, they can catalyze very specific transformations using enzymes, achieving a very fine control over what molecules are produced.
However, enzymes did not exist before life emerged and became more sophisticated. Thus, it is likely that various nonenzymatic chemical networks existed at an earlier point in Earth’s history, which could convert environmental nutrients into compounds that supported primitive cell-like functions.
The synthesis of pentoses is a prominent example of the above scenario. These simple sugars, containing only five carbon atoms, are the fundamental building blocks of RNA and other molecules that are essential to life as we know it. Scientists have proposed and studied various ways pentoses could have been generated prior to the origin of life, but current theories beg the question: how could pentoses ever accumulate in quantities enough to partake in pre-life reactions if these compounds are extremely short-lived?
To tackle this question, a research team led by Research Scientist Ruiqin Yi from ELSI conducted a study to find an alternative explanation for the origin and sustained supply of pentoses on early Earth. They explored an enzyme-free chemical network in which C6 aldonates, which are stable six-carbon carbohydrates, accumulate from various prebiotic sugar sources and then convert back to pentoses.
The proposed chemical pathway begins with gluconate, a stable C6 aldonate that may have been readily available on early Earth through known prebiotic transformations of basic sugars. The next step is the nonselective oxidation of C6 aldonate into uronate; here, the term ‘nonselective’ means that the oxidation process does not discriminate between the various carbon atoms in the aldonate structure, leaving five possible oxidation outcomes.