Question

β-hydroxybutyric acid is a ketone body. Explain with suitable equations, how it is formed in the body and under what conditions.

Answer 1

Ketone bodies are three water-soluble compounds that are produced as by-products when fatty acids are broken down for energy in the liver and kidney. Two of the three are used as a source of energy in the heart and brain while the third is a waste product excreted from the body. In the brain, they are a vital source of energy during fasting. Although termed "bodies", they are dissolved substances, not particles. The three endogenous ketone bodies are acetone, acetoacetic acid, and beta-hydroxybutyric acid,although beta-hydroxybutyric acid is not technically a ketone but a carboxylic acid. Other ketone bodies such as beta-ketopentanoate and beta-hydroxypentanoate may be created as a result of the metabolism of synthetic triglycerides such as triheptanoin.

production

beta-hydroxybutyrate is formed from acetoacetate.β-Hydroxybutyric acid, also known as 3-hydroxybutyric acid, is an organic compound and a beta hydroxy acid with the formula CH3CH(OH)CH2CO2H; its conjugate base is beta-hydroxybutyrate, also known as 3-hydroxybutyrate. β-Hydroxybutyric acid is a chiral compound having two enantiomers, D-β-hydroxybutyric acid and L-β-hydroxybutyric acid. Its oxidized and polymeric derivatives occur widely in nature.
In humans, D-3-hydroxybutyrate is synthesized in the liver from acetoacetate, the first ketone produced in the fasting state. The biosynthesis is catalyzed by the enzyme beta-hydroxybutyrate dehydrogenase.

beta-hydroxybutyrate's concentration, as with other ketone bodies, is raised in ketosis.This elevated beta-hydroxybutyrate level is naturally expected, as beta-hydroxybutyrate is formed from acetoacetate. The compound can be used as an energy source by the brain when blood glucose is low. Diabetic patients can have their ketone levels tested via urine or blood to indicate diabetic ketoacidosis. In alcoholic ketoacidosis, this ketone body is produced in greatest concentration. Ketogenesis occurs if oxaloacetate in the liver cells is depleted, a circumstance created by reduced carbohydrate intake (through diet or starvation), prolonged, excessive alcohol consumption, and/or insulin deficiency. Because oxaloacetate is crucial for entry of Acetyl-CoA into the TCA cycle, the rapid production of Acetyl-CoA from fatty acid oxidation in the absence of ample oxaloacetate overwhelms the decreased capacity of the TCA cycle, and the resultant excess of Acetyl-CoA is shunted towards ketone body production.