Central metabolism evolution

A nearly universal set of several hundred small molecules is found in living cells the interconversions of these molecules in the central metabolic pathways have been conserved in evolution. 

In the course of early evolution, the metabolism becomes more and more integrated and centralized, so much so that a chemical conversion of highly integrated constituents tends to weaken the metabolism. The more central the constituent, the more severe is this effect of metabolic decay by chemical conversions, which leads us to yet another strategy of metabolic evolution the strategy of dual feedback. 

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. 

It is an obvious, but important, statement to make therefore, that central metabolism is crucial to all activities within a cell. Moreover, because they are central and so important, the pathways are found in all living organisms - the precise details will vary between organisms but the basic patterns span the majority of all species. Therefore, one finds that related organisms have the same or closely similar central metabolic pathways and a comparative study can yield invaluable information on the evolution of organisms and their metabolism. 

As to be expected from a peptide that has been highly conserved during evolution, NPY has many effects, e.g. in the central and peripheral nervous system, in the cardiovascular, metabolic and reproductive system. Central effects include a potent stimulation of food intake and appetite control [2], anxiolytic effects, anti-seizure activity and various forms of neuroendocrine modulation. In the central and peripheral nervous system NPY receptors (mostly Y2 subtype) mediate prejunctional inhibition of neurotransmitter release. In the periphery NPY is a potent direct vasoconstrictor, and it potentiates vasoconstriction by other agents (mostly via Yi receptors) despite reductions of renal blood flow, NPY enhances diuresis and natriuresis. NPY can inhibit pancreatic insulin release and inhibit lipolysis in adipocytes. It also can regulate gut motility and gastrointestinal and renal epithelial secretion. 

Structural motifs become especially important in defining protein families and superfamilies. Improved classification and comparison systems for proteins lead inevitably to the elucidation of new functional relationships. Given the central role of proteins in living systems, these structural comparisons can help illuminate every aspect of biochemistry, from the evolution of individual proteins to the evolutionary history of complete metabolic pathways. 

It is a commonly held belief that RNA preceded DNA in the early evolution of living systems. If this is the case then the first DNA polymerases must have been capable of transferring sequence information from RNA to DNA. Enzymes of this sort are called reverse transcriptases because they do the reverse of common transcriptases (see chapter 28). Reverse transcriptases no longer play the central role in genetic information transfer, but they are still found in all species and function in a number of capacities in both cellular and viral metabolism. 

The major dietary lipids for humans are animal and plant triacylglycerols, sterols, and membrane phospholipids. The process of lipid metabolism fashions and degrades the lipid stores and produces the structural and functional lipids characteristic of individual tissues. For example, the evolution of a highly organized nervous system has depended on the natural selection of specific enzymes to synthesize and degrade (turn over) the lipids of the brain and central nervous system. 

How did the citric acid cycle come into being Although definitive answers are elusive, it is nevertheless instructive to speculate how this complicated central hub of metabolism developed. We can perhaps begin to comprehend how evolution might work at the level of biochemical pathways. 

The general arguments about the antiquity of cofactors apply to PLP. The nonenzymatic synthesis of pyridoxal under prebiotic conditions is considered possible, whereas the presence of a 5 phosphate group could hint to an ancestral attachment of the cofactor to RNA molecules. " Furthermore, there are specific grounds to assume that PLP arrived on the evolutionary scene before the emergence of proteins. In fact, in current metabolism, PLP-dependent enzymes play a central role in the synthesis and interconversion of amino acids, and thus they are closely related to protein biosynthesis. In an early phase of biotic evolution, free PLP could have played many of the roles now fulfilled by PLP-dependent enzymes, since the cofactor by itself can catalyze (albeit at a low rate) reactions such as amino acid transaminations, racemizations, decarboxylations, and eliminations. " This suggests that the appearance of PLP may have preceded (and somehow eased) the transition from primitive RNA-based life forms to more modern organisms dependent on proteins. 

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