Carbon unstable molecules synthesis

Fig. 10.7 RNA synthesis in vesicles. Membrane permeability can be regulated by choosing the correct chain length of the fatty acids in the phospholipids. Short chains (a) make the bilayer so unstable that even large molecules such as proteases can enter the vesicle interior and damage the polymerase. Carbon chains which are too long (b) prevent the entry of substrate molecules such as ADR RNA polymerisation in the vesicle occurs only with C14 fatty acids (c)...

As shown in Figure 26-9, the first product, acetoacetyl-CoA, condenses in the mitochondria with a third molecule of acetyl-CoA to yield HMG-CoA. This pool of HMG-CoA is distinct from that in the cytosol that is an intermediate in cholesterol synthesis. The HMG-CoA produced in the mitochondria is then cleaved enzymatically to yield acetoacetate and acetyl-CoA. Some of the acetoacetate formed in liver cells is usually reduced to p-hydroxybutyrate. Because acetoacetate is unstable, a further portion decomposes to form carbon dioxide and acetone, the third ketone body found in... 

Finally, oxaloacetate is simultaneously decarboxylated and phosphorylated by phosphoenolpyruvate carboxykinase in the cytosol. The CO2 that was added to pyruvate by pyruvate carboxylase comes off in this step. Recall that, in glycolysis, the presence of a phosphoryl group traps the unstable enol isomer of pyruvate as phosphoenolpyruvate (Section 16.1.7). In gluconeogenesis, the formation of the unstable enol is driven by decarboxylation—the oxidation of the carboxylic acid to CO2—and trapped by the addition of a phosphate to carbon 2 from GTP. The two-step pathway for the formation of phosphoenolpyruvate from pyruvate has a AG° of + 0.2 kcal mol ( + 0.13 kj moP ) in contrast with +7.5 kcal mol ( + 31 kj mol ) for the reaction catalyzed by pyruvate kinase. The much more favorable AG° for the two-step pathway results from the use of a molecule of ATP to add a molecule of CO2 in the carboxylation step that can be removed to power the formation of phosphoenolpyruvate in the decarboxylation step. Decarboxylations often drive reactions otherwise highly endergonic. This metabolic motif is used in the citric acid cycle (Section IS.x.x), the pentose phosphate pathway (Section 17.x.x), and fatty acid synthesis (Section 22.x.x). 

Living systems rely on kinetic stability for their existence. They are all thermodynamically unstable that is, they would burn up to form carbon dioxide and water if the system reaches thermodynamic equilibrium. In other words, life processes depend upon the ability to restrict these thermodynamic tendencies by controlled kinetics to produce energy as needed. A key feature of living systems is thus the use of catalysts for the controlled release of that energy. Examples of such catalysts are enzymes that control the synthesis and degradation of biologically important molecules. 

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