On the Role of Water as a Catalyst in Prebiotic Chemistry

On the Origin of Sugar Handedness: Facts, Hypotheses and Missing Links-A Review

By paraphrasing one of Kipling's most amazing short stories (How the Leopard Got His Spots), this article could be entitled "How Sugars Became Homochiral". Obviously, we have no answer to this still unsolved mystery, and this perspective simply brings recent models, experiments and hypotheses into the homochiral homogeneity of sugars on earth. We shall revisit the past and current understanding of sugar chirality in the context of prebiotic chemistry, with attention to recent developments and insights. Different scenarios and pathways will be discussed, from the widely known formose-type processes to less familiar ones, often viewed as unorthodox chemical routes. In particular, problems associated with the spontaneous generation of enantiomeric imbalances and the transfer of chirality will be tackled. As carbohydrates are essential components of all cellular systems, astrochemical and terrestrial observations suggest that saccharides originated from environmentally available feedstocks. Such substances would have been capable of sustaining autotrophic and heterotrophic mechanisms integrating nutrients, metabolism and the genome after compartmentalization. Recent findings likewise indicate that sugars' enantiomeric bias may have emerged by a transfer of chirality mechanisms, rather than by deracemization of sugar backbones, yet providing an evolutionary advantage that fueled the cellular machinery.

Water and Life: The Medium is the Message

Water, the most abundant compound on the surface of the Earth and probably in the universe, is the medium of biology, but is much more than that. Water is the most frequent actor in the chemistry of metabolism. Our quantitation here reveals that water accounts for 99.4% of metabolites in Escherichia coli by molar concentration. Between a third and a half of known biochemical reactions involve consumption or production of water. We calculated the chemical flux of water and observed that in the life of a cell, a given water molecule frequently and repeatedly serves as a reaction substrate, intermediate, cofactor, and product. Our results show that as an E. coli cell replicates in the presence of molecular oxygen, an average in vivo water molecule is chemically transformed or is mechanistically involved in catalysis ~ 3.7 times. We conclude that, for biological water, there is no distinction between medium and chemical participant. Chemical transformations of water provide a basis for understanding not only extant biochemistry, but the origins of life. Because the chemistry of water dominates metabolism and also drives biological synthesis and degradation, it seems likely that metabolism co-evolved with biopolymers, which helps to reconcile polymer-first versus metabolism-first theories for the origins of life.