Carbamoyl phosphate and its substitutes for the uracil synthesis in origins of life scenarios

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.

A Nonenzymatic Analog of Pyrimidine Nucleobase Biosynthesis

“…Full optimization details can be found in Table S1. From a mechanistic standpoint, cyanate is known to be produced, along with phosphate, by thermal fragmentation of CAP and is therefore a likely intermediate in the reaction [42, 43] . In line with this proposal, the use of pH 8 phosphate buffer, rather than water adjusted to pH 8, decreased the yield of CAA to 10 % (Table S1, entry 10), which might be explained by a less favorable equilibrium between CAP and cyanate in the presence of excess phosphate.…”

Building the uracil skeleton in primitive ponds at the origins of life: carbamoylation of aspartic acid

“…S4 ) discriminated the signals amongst each set. An extra 13 C signal at 162.0 ppm with no correlation with hydrogen atoms was identified as a carbonate resulting from cyanate partial decomposition 15 , 20 . HSQC and HMBC correlations (Fig.…”

“…To check this idea, a solution of phosphorylating agents (generated from trimetaphosphate ammonolysis at 70 °C for 66 h 15 ) was reacted at 100 °C with L-aspartic acid and ammonium carbamate and analysed by 1 H and 13 C NMR. The targeted reaction—condensation of aspartic acid with carbamate—did not occur, but the initial signals of aspartate were nonetheless profoundly altered.…”

“…Thus, the geochemical continuity hypothesis is only partially vindicated 58 . While the reaction could very well have happened in prebiotic settings, it is likely to have used cyanate rather than the carbamoylating agent of current biochemistry, carbamoyl phosphate, which was probably not easily available 15 .…”

“…In previous work, we showed that carbamoyl phosphate (CP) is rather unstable under prebiotic conditions, but produces two other molecules still containing the energy-rich carbamoyl moiety: cyanate and urea 15 . It is therefore unlikely that CP itself was involved in a prebiotic carbamoylation pathway.…”