|Topic:||The glucose-fatty acid cycle .|
|Details:||The glucose-fatty acid cycle. This cycle represents the interactions between glucose uptake and metabolism and the consequent inhibition of fatty acid oxidation and the effects of fatty acid oxidation on the inhibition of glucose utilization. The reciprocal regulation is most prevalent in skeletal muscle and adipose tissue. When glucose levels are high it is taken into cells via the GLUT4 transporter and phosphorylated by hexokinase. The reactions of glycolysis drive the carbon atoms to pyruvate where they are oxidized to acetyl-CoA. The fate of the acetyl-CoA is complete oxidation in the TCA cycle or return to the cytosol via citrate for conversion back to acetyl-CoA via ATP-citrate lyase (ACLY) and then into into malonyl-CoA and subsequent long-chain fatty acid (LCFA) synthesis. The synthesis of malonyl-CoA is catalyzed by acetyl-CoA carboxylase (ACC) and once produced will inhibit the import of long-chain fatty acyl-CoAs (LC acyl-CoA) into the mitochondria via inhibition of carnitine palmitoyltransferase 1 (CPT-1) in the outer mitochondrial membrane. This effectively blocks the oxidation of fatty acids leading to increased triacylglyceride synthesis (TAG). The equilibrium between malonyl-CoA synthesis and breakdown back to acetyl-CoA is determined by the regulation of ACC and malonyl-CoA decarboxylase (MCD). As long as there is sufficient capacity to divert glucose carbons to TCA cycle oxidation and fatty acid synthesis there will be limited acetyl-CoA mediated inhibition of the pyruvate dehydrogenase complex (PDHc). On the other hand, when fatty acid levels are high they enter the cell via one of several fatty acid transporter complexes [fatty acid translocase (FAT)/CD36 is shown since this transporter has a preference for LCFAs], and are then transported into the mitochondria to be oxidized. The large increase in fatty acid oxidation subsequently inhibits the utilization of glucose. This is the result of increased cytosolic citrate production from acetyl-CoA and the inhibition of phosphofructokinase-1 (PFK1). The increased acetyl-CoA derived from fat oxidation will in turn further inhibit glucose utilization via activation of PDH kinases (PDKs) that will phosphorylate and inhibit the PDHc. Although not shown, PDKs are also activated by increased mitochondrial NADH/NAD+ ratios in response to increased fatty acid β-oxidation. Under conditions where fat oxidation is favored ACC will be inhibited and MCD will be activated ensuring that LCFA that enter the cell will be able to be transported into the mitochondria. MPC1/MPC2 is the inner mitochondrial membrane pyruvate carrier responsible for mitochondrial uptake of pyruvate. SLC25A1 is the mitochondrial inner membrane citrate transporter.|
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