Synthesis and oxidation of ketone bodies: biochemistry of energy supply during fasting

Biological significance of ketone bodies

Under conditions of prolonged starvation, diabetes mellitus, and prolonged intensive physical exercise, fatty acids become the primary energy substrate for most tissues. However, they are hydrophobic molecules and cannot cross the blood-brain barrier, which is why nervous tissue requires an alternative energy source. This function is performed by ketone bodies: acetoacetate, β-hydroxybutyrate, and acetone.

Normally, the blood concentration of ketone bodies is extremely low—about 1.3 mg/dL, whereas the glucose level is 60–100 mg/dL. An increase in the concentration of ketone bodies occurs only during carbohydrate deficiency and active lipolysis.

Conditions that trigger ketogenesis

The hormonal trigger is glucagon, which activates lipolysis in adipocytes via the adenylate cyclase system. As a result, a large amount of fatty acids enters the blood and subsequently the liver. In the hepatocyte, β-oxidation is enhanced, producing acetyl-CoA and NADH.

• The accumulation of NADH and acetyl-CoA inhibits the enzymes of the Krebs cycle.
• A high NADH level shifts the equilibrium of the malate ↔ oxaloacetate reaction toward malate.
• Malate is transported out of the mitochondrion into the cytosol and is used in gluconeogenesis.
• The concentration of mitochondrial oxaloacetate drops, meaning acetyl-CoA cannot enter the TCA cycle and accumulates.

It is the excess of acetyl-CoA in the hepatocyte mitochondria that triggers the biosynthesis of ketone bodies.

Reactions of ketone body synthesis

1. Condensation of two molecules of acetyl-CoA by the action of thiolase to form acetoacetyl-CoA with the release of free coenzyme A.
2. Addition of a third acetyl-CoA molecule involving the regulatory enzyme HMG-CoA synthase to form β-hydroxy-β-methylglutaryl-CoA (HMG-CoA).
3. Cleavage of HMG-CoA by HMG-CoA lyase with the release of acetyl-CoA and the formation of acetoacetate.
4. Reduction of acetoacetate to β-hydroxybutyrate by β-hydroxybutyrate dehydrogenase; the reaction is reversible, and the direction depends on the NADH/NAD⁺ ratio.
5. At high concentrations, acetoacetate can undergo non-enzymatic decarboxylation to form acetone, which is not used as an energy source and is excreted via expired air and urine.

HMG-CoA synthase is inhibited by free coenzyme A. During active influx of fatty acids into the mitochondrion, CoA is consumed for their activation, its concentration drops, and the ketogenesis pathway is disinhibited.

Ketoacidosis and its causes

Acetoacetate and β-hydroxybutyrate are organic acids that readily dissociate to form an anion and an H⁺ proton. A significant increase in their concentration leads to ketonemia, ketonuria, and metabolic ketoacidosis—a characteristic complication of decompensated diabetes mellitus and prolonged starvation. Acidification of the medium shifts the equilibrium of protein dissociation and disrupts the function of enzyme systems.

Oxidation of ketone bodies in peripheral tissues

Ketone bodies are oxidized in all tissues containing mitochondria, except for the liver itself and erythrocytes. The liver lacks the enzyme succinyl-CoA:3-ketoacid CoA-transferase and is therefore unable to utilize its own ketone bodies.

1. β-hydroxybutyrate is oxidized by β-hydroxybutyrate dehydrogenase into acetoacetate, generating NADH.
2. Acetoacetate is activated by succinyl-CoA:3-ketoacid CoA-transferase; the CoA donor is succinyl-CoA, which is converted into succinate in the process and enters the Krebs cycle.
3. The resulting acetoacetyl-CoA is cleaved by thiolase into two molecules of acetyl-CoA.
4. Each molecule of acetyl-CoA is oxidized in the TCA cycle and the coupled ETC.

Energy yield

ATP calculation upon complete oxidation of a single molecule:

• β-hydroxybutyrate: 3 ATP from NADH (dehydrogenase reaction) + 2 × 12 ATP from two acetyl-CoA molecules in the TCA cycle − 1 ATP equivalent (high-energy bond of succinyl-CoA consumed for acetoacetate activation) = 26 ATP.
• Acetoacetate: 24 ATP from two acetyl-CoA molecules − 1 ATP equivalent for activation = 23 ATP.

The activation of acetoacetate via succinyl-CoA is energetically favorable for the cell because the resulting succinate immediately enters the TCA cycle.

Key points

• Ketone bodies serve as an alternative energy carrier for nervous tissue during glucose deficiency.
• Ketogenesis occurs exclusively in hepatocyte mitochondria upon the activation of lipolysis by glucagon.
• The regulatory enzyme of synthesis is HMG-CoA synthase, which is inhibited by free CoA.
• The accumulation of acetyl-CoA is associated with the inhibition of the TCA cycle and the diversion of oxaloacetate into gluconeogenesis.
• Acetone is formed non-enzymatically and is not used as an energy source.
• The liver does not oxidize ketone bodies due to the absence of succinyl-CoA:3-ketoacid CoA-transferase.
• Oxidation of β-hydroxybutyrate yields 26 ATP, and acetoacetate yields 23 ATP.
• An excess of ketone bodies causes ketoacidosis, which is typical for decompensated diabetes mellitus.