Studies in prebiotic synthesis and the origins of the metabolic pathways.

摘要

It is frequently stated that UV light would cause massive destruction of prebiotic organic compounds because of the absence of an ozone layer. The elevated UV flux of the early sun compounds this problem. This applies to organic compounds of both terrestrial and extraterrestrial origin. Attempts to deal with this problem generally involve atmospheric absorbers. It was shown that prebiotic organic polymers as well as several inorganic compounds likely to have been present in the primitive oceans are sufficient to protect oceanic organic molecules from UV degradation. This aqueous protection is in addition to any atmospheric UV absorbers and should be a ubiquitous planetary phenomenon serving to increase the size of planetary habitable zones.; The majority of enzymatic reactions are mediated by cofactors, suggesting that they must have been available before most modern biosynthetic pathways were established. The prebiotic synthesis of thiamin was thus investigated.; Life may have begun with a self-replicating genetic polymer. This idea has been bolstered by demonstrations that RNA may act as a catalyst, and “molecular fossil” evidence that RNA may have played a much larger role in earlier organisms. There are, however, numerous problems with the prebiotic synthesis of RNA. It has been suggested that a peptide nucleic acid (PNA) may have predated RNA as an early genetic material. The prebiotic synthesis of several PNA molecules was investigated.; The central metabolism of life is remarkably homogeneous. The origin of the biosynthetic pathways has attracted much speculation over the years. The earliest proposal favored a backward development of the pathways, as constituents of the prebiotic soup were exhausted. Later proposals suggested that the pathways were put in place as a result of forward development or the recruitment of non-specific enzymes and subsequent gene duplication. Recently, it was proposed that the pathways were originally constructed at least partially non-enzymatically. At one time there may have existed organisms with few or no coded protein enzymes, and they may have relied a great deal on non-enzymatic metabolism. The biosynthetic pathway for nicotinamide from dihydroxyacetone phosphate and aspartic acid was investigated to see if it could have originated non-enzymatically.