Mitochondria ketone body synthesis

Acetyl CoA undergoes similar reactions in the mitochondrion, where HMG CoA is used for ketone body synthesis. 

Acetoacetate is another vehicle for transporting acetyl groups into the cytoplasm. This molecule, one of the end products of ketone body synthesis, is free to diffuse from the mitochondrion. When in the cytoplasm it can be activated to acetoacetyl-CoA by an ATP-dependent acetoacetyl-CoA synthetase. Edmonds group has shown that the activity of this enzyme parallels the rate of cholesterologenesis in the Uvers of animals given a variety of dietary regimes [11]. Their data also indicate that this pathway furnishes as much as 10% of the carbon required for cholesterol biosynthesis. 

In the mitochondrion it participates in ketone body synthesis, while as a cytoplasmic enzyme it functions in cholesterol biogenesis [22],... 

Ketogenesis The production of ketone bodies by the liver in response to increased P-oxidation with a decreased rate of the Krebs cycle as a resnlt of shnttling acids from the mitochondrion for the synthesis of glncose via glnconeogenesis. 

In muscle, most of the fatty acids undergoing beta oxidation are completely oxidized to C02 and water. In liver, however, there is another major fate for fatty acids this is the formation of ketone bodies, namely acetoacetate and b-hydroxybutyrate. The fatty acids must be transported into the mitochondrion for normal beta oxidation. This may be a limiting factor for beta oxidation in many tissues and ketone-body formation in the liver. The extramitochondrial fatty-acyl portion of fatty-acyl CoA can be transferred across the outer mitochondrial membrane to carnitine by carnitine palmitoyltransferase I (CPTI). This enzyme is located on the inner side of the outer mitochondrial membrane. The acylcarnitine is now located in mitochondrial intermembrane space. The fatty-acid portion of acylcarnitine is then transported across the inner mitochondrial membrane to coenzyme A to form fatty-acyl CoA in the mitochondrial matrix. This translocation is catalyzed by carnitine palmitoyltransferase II (CPTII Fig. 14.1), located on the inner side of the inner membrane. This later translocation is also facilitated by camitine-acylcamitine translocase, located in the inner mitochondrial membrane. The CPTI is inhibited by malonyl CoA, an intermediate of fatty-acid synthesis (see Chapter 15). This inhibition occurs in all tissues that oxidize fatty acids. The level of malonyl CoA varies among tissues and with various nutritional and hormonal conditions. The sensitivity of CPTI to malonyl CoA also varies among tissues and with nutritional and hormonal conditions, even within a given tissue. Thus, fatty-acid oxidation may be controlled by the activity and relative inhibition of CPTI. 

Two acetyl CoAs can combine to form acetoacetyl CoA by the reverse of b-ketothiolase. The acetoacetyl CoA then combines with another acetyl CoA to make hydroxymethyl glutaryl CoA (HMG CoA) by the enzyme hydroxymethyl glutaryl CoA synthase. The HMG CoA in the mitochondrion can be cleaved by HMG CoA lyase in the mitochondrion to form acetoacetate and acetyl CoA. In this conversion, the formation of acetoacetyl CoA from two acetyl CoAs releases a free CoA and formation of HMG CoA from acetyl CoA and acetoacetyl CoA also releases a free coenzyme A. Thus, the release of free coenzyme A allows beta oxidation to continue with the production of acetoacetate. During diabetes and starvation, almost 90% of carbon from a fatty acid such as oleate can be accounted for in the form of ketone bodies during experiments with perfused livers. At this time, it would be worth noting that this process occurs in the mitochondrion later it will be seen that HMG CoA in the cytosol is a major precursor for cholesterol synthesis. 

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