Ketolysis (Ketone Body Utilization)

1. 1

   D-β-hydroxybutyrate + NAD+ → acetoacetate + NADH + H+

   D-beta-hydroxybutyrate dehydrogenase (BDH1) 1.1.1.30 ST-0087 Irreversible/directional NAD+ NADH
   In D-beta-hydroxybutyrate NAD+

   Out Acetoacetate NADH H+
   › Notes

   Mitochondrial D-β-hydroxybutyrate dehydrogenase oxidizes D-β-hydroxybutyrate to acetoacetate. This produces one NADH, and the reaction direction in extrahepatic tissue is favored by the local mitochondrial redox state.

2. 2

   Acetoacetate + succinyl-CoA → acetoacetyl-CoA + succinate

   Succinyl-CoA:3-ketoacid CoA transferase (SCOT / OXCT1) 2.8.3.5 ST-0088 Irreversible/directional
   In Acetoacetate Succinyl-CoA

   Out Acetoacetyl-CoA Succinate
   › Notes

   Succinyl-CoA:3-ketoacid CoA transferase (SCOT; 3-oxoacid CoA-transferase, OXCT1) activates acetoacetate by transferring CoA from succinyl-CoA. This is the committed, functionally rate-determining step of ketone-body utilization; it bypasses succinyl-CoA synthetase and therefore sacrifices the GTP that would otherwise be generated in the TCA cycle. Liver lacks SCOT, preventing futile cycling.

3. 3

   Acetoacetyl-CoA + CoA-SH → 2 acetyl-CoA

   Mitochondrial thiolase (acetyl-CoA acetyltransferase, T2/ACAT1) 2.3.1.9 ST-0089 Irreversible/directional CoA-SH
   In Acetoacetyl-CoA CoA-SH

   Out Acetyl-CoA
   › Notes

   Mitochondrial thiolase cleaves acetoacetyl-CoA. The two acetyl-CoA molecules then enter the TCA cycle.

4. 4

   2 acetyl-CoA → 4 CO2 through two TCA-cycle turns

   TCA-cycle enzyme set (downstream oxidation) ST-0090 Irreversible/directional
   In Acetyl-CoA
   › Notes

   Citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinyl-CoA synthetase, succinate dehydrogenase, malate dehydrogenase, and associated reactions oxidize the acetyl groups. This downstream oxidation yields reducing equivalents and GTP, but the SCOT reaction has displaced one GTP equivalent per acetoacetate activated.

Showing all 4 steps.

Overview

Ketolysis converts circulating acetoacetate or D-β-hydroxybutyrate into acetyl-CoA for oxidation in extrahepatic mitochondria. It is an important fuel pathway in cardiac muscle, oxidative skeletal muscle, renal cortex, and the brain after adaptation to prolonged fasting. The liver exports ketone bodies but cannot use them, preserving its role as a producer.

Cellular location

Ketolysis occurs in the mitochondrial matrix of extrahepatic tissues. It is absent from hepatocytes because they lack SCOT and absent from erythrocytes because they lack mitochondria. The brain increasingly uses ketones during prolonged fasting, although it still requires some glucose.

Net energetics

Oxidation of acetoacetate yields two acetyl-CoA (about 20 ATP by modern P/O accounting) but forfeits one GTP because SCOT consumes succinyl-CoA; its net yield is therefore approximately 19 ATP per acetoacetate. Oxidation of D-β-hydroxybutyrate yields the same 19 ATP plus approximately 2.5 ATP from the NADH formed in step 1, or approximately 21.5 ATP per D-β-hydroxybutyrate. Older ATP conventions give higher integer values; the key distinction is the loss of one GTP equivalent during acetoacetate activation.

Clinical significance

Ketolysis permits extrahepatic tissues to conserve glucose and, in the brain during prolonged fasting, substantially reduces dependence on amino-acid-derived gluconeogenesis. SCOT deficiency causes episodic ketoacidosis because ketone bodies cannot be efficiently consumed despite being produced; episodes often follow fasting or infection. Unlike DKA, which reflects excessive production in insulin deficiency, impaired ketolysis represents a defect in peripheral disposal. The absence of SCOT in liver is physiologically essential because hepatic consumption would waste the ketones it produces.