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ketogenesis
Ketogenesis is the process by which ketone bodies are produced as a result of fatty acid breakdown.
==Production==
Ketone bodies are produced mainly in the mitochondria of liver cells, and synthesis can occur in response to an unavailability of blood glucose. This is caused by low glucose levels in the blood, after exhaustion of cellular carbohydrate stores, such as glycogen. Synthesis of ketones can also occur due to excessively high levels of blood glucose that are unable to be stored as glycogen in liver and muscle. The production of ketone bodies is then initiated to make available energy that is stored as fatty acids. Fatty acids are enzymatically broken down in β-oxidation to form acetyl-CoA. Under normal conditions, acetyl-CoA is further oxidized by the TCA cycle and mitochondrial electron transport chain to release energy. However, if the amounts of acetyl-CoA generated in fatty-acid β-oxidation challenge the processing capacity of the TCA cycle; i.e. if activity in the TCA cycle is low due to low amounts of intermediates such as oxaloacetate, acetyl-CoA is then used instead in biosynthesis of ketone bodies via acetoacyl-CoA and β-hydroxy-β-methylglutaryl-CoA (HMG-CoA). Deaminated amino acids that are ketogenic, such as leucine, also feed the TCA cycle, forming acetoacetate & ACoA and thereby produce ketones.
Besides its role in the synthesis of ketone bodies, HMG-CoA is also an intermediate in the synthesis of cholesterol, but the steps are compartmentalised. Ketogenesis occurs in the mitochondria, whereas cholesterol synthesis occurs in the cytosol, hence both the processes are independently regulated.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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