The term “ketone bodies” refers primarily to two compounds: acetoacetate and β‐hydroxy‐butyrate, which are formed from acetyl‐CoA when the supply of TCA‐cycle intermediates is low, such as in periods of prolonged fasting. They can substitute for glucose in skeletal muscle, and, to some extent, in the brain. The first step in ketone body formation is the condensation of two molecules of acetyl‐CoA in a reverse of the thiolase reaction.
The product, acetoacetyl‐CoA, accepts another acetyl group from acetyl‐CoA to form β‐hydroxy‐β‐hydroxymethylglutaryl‐CoA (HMG‐CoA). HMG‐CoA has several purposes: It serves as the initial compound for cholesterol synthesis or it can be cleaved to acetoacetate and acetyl‐CoA. Acetoacetate can be reduced to β‐hydroxybutyrate or can be exported directly to the bloodstream. Acetoacetate and β‐hydroxybutyrate circulate in the blood to provide energy to the tissues.
Acetoacetate can also spontaneously decarboxylate to form acetone:
Although acetone is a very minor product of normal metabolism, diabetics whose disease is not well‐managed often have high levels of ketone bodies in their circulation. The acetone that is formed from decarboxylation of acetoacetate is excreted through the lungs, causing characteristic “acetone breath.”