Beta oxidation: Difference between revisions
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Propionyl-CoA is first carboxylated using a bicarbonate ion into D-stereoisomer of methylmalonyl-CoA, in a reaction that involves a biotin co-factor, ATP, and the enzyme [[propionyl-CoA carboxylase]]. The bicarbonate ion's carbon is added to the middle carbon of propionyl-CoA, forming a D-methylmalonyl-CoA. However, the D conformation is enzymatically converted into the L conformation by methylmalonyl-CoA epimerase, then it undergoes intramolecular rearrangement, which is catalyzed by methylmalonyl-CoA mutase (requiring B12 as a coenzyme) to form <scene name='43/430893/Cv/9'>succinyl-CoA</scene>. The succinyl-CoA formed can then enter the [[Citric Acid Cycle]]. | Propionyl-CoA is first carboxylated using a bicarbonate ion into D-stereoisomer of methylmalonyl-CoA, in a reaction that involves a biotin co-factor, ATP, and the enzyme [[propionyl-CoA carboxylase]]. The bicarbonate ion's carbon is added to the middle carbon of propionyl-CoA, forming a D-methylmalonyl-CoA. However, the D conformation is enzymatically converted into the L conformation by methylmalonyl-CoA epimerase, then it undergoes intramolecular rearrangement, which is catalyzed by methylmalonyl-CoA mutase (requiring B12 as a coenzyme) to form <scene name='43/430893/Cv/9'>succinyl-CoA</scene>. The succinyl-CoA formed can then enter the [[Citric Acid Cycle]]. | ||
However, whereas acetyl-CoA enters the citric acid cycle by condensing with an existing molecule of oxaloacetate, succinyl-CoA enters the cycle as a principal in its own right. Thus the succinate just adds to the population of circulating molecules in the cycle and undergoes no net metabolization while in it. When this infusion of citric acid cycle intermediates exceeds cataplerotic demand (such as for aspartate or glutamate synthesis), some of them can be extracted to the gluconeogenesis pathway, in the liver and kidneys, through [[phosphoenolpyruvate carboxykinase]], and converted to free glucose. | |||
''Unsaturated fatty acids'' | |||
β-Oxidation of unsaturated fatty acids poses a problem since the location of a cis bond can prevent the formation of a trans-Δ2 bond. These situations are handled by an additional two enzymes, [[Enoyl-CoA hydratase|Enoyl CoA isomerase]] or 2,4 Dienoyl CoA reductase. | |||
''Peroxisomal beta-oxidation'' | |||
Fatty acid oxidation also occurs in peroxisomes when the fatty acid chains are too long to be handled by the mitochondria. The same enzymes are used in peroxisomes as in the mitochondrial matrix, and acetyl-CoA is generated. It is believed that very long chain (greater than C-22) fatty acids, branched fatty acids, some [[prostaglandins]] and [[leukotrienes]] undergo initial oxidation in peroxisomes until <scene name='95/951266/Cv/3'>octanoyl-CoA</scene> is formed, at which point it undergoes mitochondrial oxidation. | |||
</StructureSection> | </StructureSection> | ||
== References == | == References == | ||
<references/> | <references/> | ||