Abstract
The first step of pyrimidine synthesis along the orotate pathway is studied to test the hypothesis of geochemical continuity of protometabolic pathways at the origins of life. Carbamoyl phosphate (CP) is the first high-energy building block that intervenes in the in vivo synthesis of the uracil ring of UMP. Thus, the likelihood of its occurrence in prebiotic conditions is investigated herein. The evolution of carbamoyl phosphate in water and in ammonia aqueous solutions without enzymes was characterised using ATR-IR, 31P and 13C spectroscopies. Carbamoyl phosphate initially appears stable in water at ambient conditions before transforming to cyanate and carbamate/hydrogenocarbonate species within a matter of hours. Cyanate, less labile than CP, remains a potential carbamoylating agent. In the presence of ammonia, CP decomposition occurs more rapidly and generates urea. We conclude that CP is not a likely prebiotic reagent by itself. Alternatively, cyanate and urea may be more promising substitutes for CP, because they are both “energy-rich” (high free enthalpy molecules in aqueous solutions) and kinetically inert regarding hydrolysis. Energy-rich inorganic molecules such as trimetaphosphate or phosphoramidates were also explored for their suitability as sources of carbamoyl phosphate. Although these species did not generate CP or other carbamoylating agents, they exhibited energy transduction, specifically the formation of high-energy P–N bonds. Future efforts should aim to evaluate the role of carbamoylating agents in aspartate carbamoylation, which is the following reaction in the orotate pathway.
Highlights
The first step of pyrimidine synthesis along the orotate pathway is studied to test the hypothesis of geochemical continuity of protometabolic pathways at the origins of life
A 0.355 M carbamoyl phosphate (CP) solution was prepared in D2O
A minority signal between + 2.74 and + 2.69 ppm (Q0 region) can be assigned to monophosphate ions that could derive from the hydrolysis of Carbamoyl phosphate (CP)
Summary
The first step of pyrimidine synthesis along the orotate pathway is studied to test the hypothesis of geochemical continuity of protometabolic pathways at the origins of life. The idea of an evolution of biosynthetic pathways progressively evolving from prebiotic chemistry dates back to the works of Horowitz in 1 9453 He proposed that molecules which are products of present-day metabolic pathways might have been nutrients, directly harvested from the primordial soup. Wächtershäuser laid the foundations of a systematic approach aiming to predicting surface-based analogues of metabolic p athways[7,8], where mineral surfaces played the role of heterogeneous catalysts which would later be taken over by enzyme catalysts Many of his suggestions still remained untested. Core aspects of metabolism would not need to have been created from scratch, since they would be elaborations of pre-existent abiotic reactions It satisfies the principle of parsimony (Ockham’s razor) and it is experimentally testable, and falsifiable, allowing progress “from conjecture to hypothesis”[14]. Traces of this hybrid world might be found in activated molecules, between which energy transduction mechanisms are carried out
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