Chemical processes occurring on the primitive Earth about four billion years ago yielded molecules that had the ability to make copies of themselves or replicate. These rudimentary replicating molecules eventually developed into contemporary life that uses both protein and DNA molecules for replication. Since the DNA of contemporary life appears to be too complex to have been chemically made on the primitive Earth, the first replicating systems may have been composed solely of small proteins - called peptides. Peptides are good candidates for the first replicating molecules because they are constructed from very simple building blocks - activated amino acid molecules - which could have been made by chemical processes on the primitive Earth.
To understand how peptides that are necessary for the origin of life could have been synthesized on the primitive Earth four billion years ago, a model chemical process was investigated. This model process has the potential to make peptides from very simple chemical ingredients--formaldehyde, ammonia, and hydrogen sulfide. So far, studies of this process have shown that reaction of formaldehyde, glycolaldehyde (a formaldehyde dimer), and ammonia in the presence of a thiol yields amino acids via activated amino acid thioesters capable of forming peptides. In addition to activated amino acids, the process also generates important biochemical intermediates (such as pyruvate and glyoxylate), and other products that catalyze their own synthesis (such as amino acids, thiols, and imidazoles). The ability of the process to generate catalytic products gives it the potential to be artificially "evolved" to a higher level of chemical activity made possible by the action of its catalytic products.
A peptide catalyst of the model process, polylysine, was confined to a small semipermeable container (a small dialysis unit), suspended in a much larger solution of triose sugar substrate. This reaction system functioned as a catalytic flow reactor. It continually pulled new substrate molecules into the dialysis unit to replace those that had been catalytically converted to product (pyruvaldehyde), as the product molecules diffused out of the dialysis unit back into the surrounding substrate solution.
In some respects, this chemical flow reactor resembles fermentation by microorganisms that take in and catalytically convert sugars to products (ethyl alcohol or lactic acid) that eventually diffuse out of the cell back into the surrounding medium. The pathway for peptide synthesis, from formaldehyde to activated amino acids, is an attractive model of an early stage in the origin of life. The model generates products in a single-reaction vessel from simple substrates that catalyze reactions involved in their own synthesis.
In contemporary life, metabolic pathways transform organic substrates into useful biomolecules - amino acids, lipids, etc. The energy required to drive metabolism comes from the transfer of high-energy electron pairs in organic substrates to lower energy states, in numerous biochemical end products. Organic substrates are capable of donating the greatest number of high-energy electron pairs, and they have the potential to drive the greatest number of carbon group transformations; the optimal biosynthetic substrate would contain the largest possible number of high-energy electron pairs per carbon atom. Viewed this way, the optimal biosubstrate functions like an optimal battery by generating the largest number of high-energy electrons per unit mass of storage material. The biosynthetic ability of a carbon substrate is determined mainly by the number of high-energy electron pairs per carbon atom. Nevertheless, the optimal biosubstrate would also contain any chemical group that strongly facilitates its conversion to a variety of metabolic intermediates of different size and composition. Since the carbonyl group is the only carbon group that strongly facilitates the synthesis of metabolic intermediates of varying size, the optimal biosubstrate would certainly contain one carbonyl group. Based on the foregoing considerations, this study found sugars to be the optimal biosynthetic substrate of life. They contain the largest number of high-energy electrons per carbon atom, and possess one carbonyl group that facilitates their conversion to a variety of biosynthetic intermediates. This conclusion applies to aqueous life throughout the universe, because it is based on invariant aqueous carbon chemistry - primarily the universal reduction potentials of carbon groups.
Point of Contact: A. Weber
(650) 604-3226
aweber@mail.arc.nasa.gov
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