Pathways major primitive 201

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. 

The availability of building materials (including amino acids) under hypothesized primitive Earth conditions is a major concern in prebiotic chemistry. Major - closely linked to each other - concerns in the field are which compounds can be formed , how much , and through which mechanisms . When estimating the availability of prebiotic organics, two relevant, parallel pathways are usually pointed out ... 

Among activated forms of amino acids, mixed anhydrides with inorganic phosphate or phosphate esters require a special discussion because they are universally involved in peptide biosynthesis through the ribosomal and non-ribosomal pathways. These mixed anhydrides have stimulated studies in prebiotic chemistry very early in the history of this field. Amino acyl adenylates 18c have been shown to polymerize in solution [159,160] and in the presence of clays [139]. However, their participation as major activated amino acid species to the prebiotic formation of peptides from amino acids is unlikely for at least two reasons. Firstly, amino acid adenylates that have a significant lifetime in aqueous solution become very unstable as soon as either CO2 or bicarbonate is present at millimolar concentration [137]. Lacey and coworkers [161] were therefore conduced to consider that CO2 was absent in the primitive atmosphere for aminoacyl adenylate to have a sufficient lifetime and then to allow for the emergence of the modern process of amino acid activation and of the translation apparatus. But this proposition is unlikely, as shown by the analysis of geological records in favor of CO2 contents in the atmosphere higher than present levels [128]. It is also in contradiction with most studies of the evolution of the atmosphere of telluric planets [30,32], Secondly, there is no prebiotic pathway available for adenylate formation and ATP proved to be inefficient in this reaction [162]. 

In many families of angiosperms, phosphatidylglycerol (9) is the only major product of the prokaryotic pathway. Other chloroplast lipids in these plants are synthesized only by the eukaryotic pathway. In some primitive angiosperm families, both pathways contribute to the s)mthesis of mono-galactosyldiacylglycerol (10) and other lipids (Browse and Somerville, 1991). 

Up until recently it had always been considered that /3-oxidation was confined to mitochondria. Although animal mitochondria do contain all the enzymes necessary and are a major site for )8-oxidation, other subcellular sites, such as the microbodies, are implicated. Peroxisomes or glyoxysomes, together are often referred to as microbodies. They contain a primitive respiratory chain where energy released in the reduction of oxygen is lost as heat. The presence of an active /3-oxidation pathway in microbodies was first detected in glyoxysomes from germinating seeds by de Duve in 1976. Since that time the various enzymes involved have been purified and characterized for microbodies from rat liver. 

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