Primitive metabolic pathways 201

Glycolysis is a process that results in the conversion of a molecule of glucose into two molecules of pyruvate. It is a primitive metabolic pathway since it operates in even the simplest cells and does not require oxygen. The pathway of glycolysis performs five main functions in the cell ... 

A self-replicating polymer would quickly use up available supplies of precursors provided by the relatively slow processes of prebiotic chemistry. Thus, from an early stage in evolution, metabolic pathways would be required to generate precursors efficiently, with the synthesis of precursors presumably catalyzed by ri-bozymes. The extant ribozymes found in nature have a limited repertoire of catalytic functions, and of the ribozymes that may once have existed, no trace is left. To explore the RNA world hypothesis more deeply, we need to know whether RNA has the potential to catalyze the many different reactions needed in a primitive system of metabolic pathways. 

The FeS world is a still incomplete model for building up a primitive metabolism and other pathways to enlarge the repertoire of biogenic compounds. Although limited because of the missing enantiomeric amino acids, any FeS scenario is open-ended... 

The reaction is basically the citric acid cycle run in reverse. Where the Krebs cycle takes complex carbon molecules in the form of sugars and oxidizes them to C02 and water, the reverse cycle takes C02 and water to make carbon compounds. This process is used by some bacteria to synthesize carbon compounds, sometimes using hydrogen or sulfates as electron donors. The reaction is a possible candidate for prebiotic early Earth conditions and so it is of interest in the origin of life. On the early Earth, a primitive form of acetyl-CoA-like thioacetate played the role of the essential coenzyme. It has been found that some of the steps can be catalyzed by minerals. Thus, the FeS world proposes that the reverse citric acid cycle operated nonenzymatically on the primitive Earth. The question is whether it is possible to retrace other ancient metabolic pathways. Combination of the recently found plugged... 

The significance of the evolutionary tree in Fig. 12 lies in the time scale covered. The enzymes compared and related here were already necessary in the most essential metabolic pathways (such as glycolysis) of the most primitive cells. In contrast, phylogenetic trees based on amino acid sequences or fossil data mostly relate to the development of species. 

Abstract. Giant lipid vesicles have been extensively used in recent years as in vitro artificial models for protocells, i.e. primitive cell models or synthetic celllike systems of minimal complexity. Due to their dimensions in the micrometer range, giant vesicles can be designed as micro-sized enzymatic chemical reactors fed by a flux of substrates from the outside and monitored by confocal light microscopy in order to follow the production of fluorescence compounds. In this paper we present a 3D modelling approach to the simulation of giant vesicle where enzymatic reactions take place, and we apply this approach to a case study of a three-enzymes metabolic pathway. 

The development of PA-defident mutants in prokaryotes and primitive eukaryotes has demonstrated that PAs are essential for thdr growth, and has corroborated the PA metabolic pathways in these cells. In higher plants, such mutants have not yet been isolated. Therefore, at the present time, the only possible way to block the synthesis of PAs, and at least partly to deplete cells of their PAs, is by using specific inhibitors of their biosynthetic enzymes. 

Mature red blood cells do not have nuclei, mitochondria, or microsomes therefore red blood cell function is supported through the most primitive and universal pathway. Glucose, the main metabolic substrate of red blood cells, is metabolized via two major pathways the Embden-Meyerhof glycolytic pathway and the hex-ose monophosphate pathway (Fig. 1). Under normal circumstances, about 90% of the glucose entering the red blood cell is metabolized by the glycolytic pathway and 10% by the hexose monophosphate pathway. 

However, it should be pointed out here that not all plants follow the same synthetic pathway for ethylene. Lower plants (liverworts, mosses, ferns, lycopods) do not produce it from methionine, nor from ACC—an alternative ethylene pathway therefore exists (Osborne et al., 1996). In evolutionary terms this is very significant and it remains to be established whether any cells in higher plants still retain this earlier primitive route for ethylene synthesis as part of their metabolic repertoire. 

Using various combinations of these primitives, hypotheses can be formed regarding the evolutionary trajectory that led to extant metabolic systems. Because these primitives allow us to trace multiple pathways to extant metabolism, other assumptions must be made to limit the space of possible evolutionary trajectories. Melendez-Hevia et al. [141] suggest some additional assumptions of evolutionary opportunism . 

All symbiotic hypotheses assumed that the host possessed this primitive and inefficient mechanism of energy generation until the capacity of aerobic respiration was implanted in it by the acquisition of aerobic protomitochondrial symbionts. Those who had opposed this scenario argued that the modern eukaryote is not an anaerobic cell, containing mitochondria, but that an aerobic pathway of the cytoplasm was a primitive feature of the bacteria that evolved into a prokaryote (Raff and Mahler 1972, 1973 Uzzell and Spolsky 1973). They proposed instead that the mitochondria may have evolved from the enclosure of a bacterial plasmid within a metabolically specialized sac. 

In order to draw conclusions about the origin and evolution of central metabolism from a study of these pathways in archaebacteria, eubacteria and eukaryotes, a definitive phytogeny of the organisms involved is required. In particular, a knowledge of which organisms are primitive is essential. 

Studies on the pathways of central metabolism of the archaebacteria take on special significance when it is realised that, from the universal phylogenetic tree in rooted form, Woese et al. [66] have proposed that the domain of the archaebacteria be known as archaea to denote their apparently primitive nature, especially with respect to the eukaryotes. Furthermore, within the archaea, thermophily is regarded as the ancestral phenotype. 

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