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Chapter 16 Biochemistry6e

Chapter 16 Biochemistry6e - Chapter 16 Biochemistry...

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Chapter 16 Biochemistry Glycolysis and Gluconeogenesis-6th Edition by Berg, Tymoczko and Stryer Glycolysis is the metabolic process whereby one molecule of glucose is converted into two molecules of pyruvic acid with the concomitant synthesis of two molecules of ATP. The process and reactions of glycolysis are anaerobic in that no molecular oxygen (O 2 ) is required. If the pyruvic acid produced by glycolysis is metabolized further via anaerobic reactions into products such as lactic acid or ethanol and CO 2 , the process is termed a fermentation. If, however, the pyruvate is metabolized using O 2 , then a respiration has occurred and the synthesis of a large number of ATP and of CO 2 will result. Glucose can be synthesized from non-carbohydrate precursors, such as lactic acid and pyruvic acid, in a process termed gluconeogenesis . While a number of the enzymes and intermediates are common to glycolysis and gluconeogenesis, the two pathways are not simply the reverse of one another. Notably, the very exergonic, essentially (i.e., metabolically) irreversible reactions of glycolysis are omitted by gluconeogenesis (Overview Slide) . Historically, the discovery of glycolysis was an important milestone. The Buchner brothers noted that yeast cells that were broken open, and hence were not living cells, could convert sucrose (glucose) into alcohol and thus that a living cell need not be present for its enzymes to perform the reactions of metabolism. The significance of this finding was that it allowed the study of biochemistry to proceed as the idea of vitalism to perform metabolism was disproved. In short, the Buchners showed that metabolism is really cellular chemistry. Why is so much of metabolism centered on glucose? Glucose can be formed from formaldehye, one of the presumed components of the ancient, abiotic world. In addition, glucose has a low tendency to spontaneously glycosylate proteins as it preferentially exists in the ring structure, and hence would be favored as a substrate that does not interfere with the proteins of the cell. The ultimate fate of glucose depends on whether or not O 2 is present and if the organism can use it to metabolize the end product of glycolysis, i.e., pyruvate (Show Figure 16.1) . This is because in the absence of O 2 , glucose is fermented to products such as lactic acid or ethanol and CO 2 while if O 2 is
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present, all of the carbons of glucose can be converted into CO 2 . If glucose metabolism via glycolysis results in pyruvate, but the presence of O 2 determines whether fermentation or respiration will occur, why would organisms choose to ferment rather than respire? For those that can choose between respiration and fermentation, respiration is preferred because of the much larger yield of ATP. Why then is glycolysis retained at all? The reason is that glycolysis can proceed without O 2 and hence organisms that are in anaerobic environments will still have a mechanism to make ATP even if O 2 is absent.
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Chapter 16 Biochemistry6e - Chapter 16 Biochemistry...

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