Use of ATP in Biosynthesis

October 29, 2008 by ravi

The input of energy in the form of the hydrolysis of ATP to either ADP and Pi or to adenosine monophosphate (AMP) and pyrophosphate powers the synthesis of biological molecules, including, as we have seen, carbohydrates in photosynthesis, proteins, DNA, RNA, and fatty acids. To delve into the role of ATP in biosynthesis in depth is not possible in this brief article. Aspects of fatty acid biosynthesis, however, reveal interesting principles of the energetics of biosynthetic pathways. Fatty acids are oxidized completely to CO2 and water by ?-oxidation and the citric acid cycle. Acetyl CoA is the end product of ?-oxidation of fatty acids and is the source of carbon for fatty acid biosynthesis. Yet, the pathways for fatty acid degradation and synthesis are so very different that they even occur within different compartments within cells. Fatty acid synthesis takes place in the cytoplasm of animal cells and in the plastids of plant cells, whereas ?-oxidation is located in mitochondria in both animal and plant cells.

Often, the pathway for the synthesis of a compound differs significantly from that for its degradation. Among the reasons that the separation of synthetic and degradative pathways evolved are energetics and regulation. The oxidation of fatty acids to acetyl CoA is very exergonic. It is not feasible on energetic grounds to make fatty acids from acetyl CoA by reversing ?-oxidation. Metabolism of carbohydrates and fats is regulated in mammals by a number of hormones, including insulin, glucagon, and epinephrine (adrenaline). Having separate pathways for the degradation and the biosynthesis makes it possible to turn off one pathway while up-regulating another. For example, glucagon and epinephrine selectively stimulate the breakdown of fats and fatty acids, whereas insulin has the opposite effect. The fine control of fatty acid metabolism that has evolved would clearly not be possible without the existence of separate pathways for biosynthesis and catabolism. CO2 is required for the synthesis of fatty acids. Yet, when fatty acid synthesis is carried out in the presence of radioactive CO2, the fatty acid made is devoid of radioactivity. ATP is used to add CO2 to a precursor, and in a subsequent step in the pathway of fatty acid biosynthesis.

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