Under conditions where acetyl-CoA concentration is high, the thiolase reaction can be reversed to yield acetoacetyl-CoA (Figure 18.21) in the following reaction:

2 Acetyl-CoA <=> Acetoacetyl-CoA + CoASH

Addition of another acetyl-CoA to acetoacetyl-CoA yields -hydroxy--methylglutaryl-CoA (also called HMG-CoA):

Acetyl-CoA + Acetoacetyl-CoA <=> HMG-CoA

This reaction is catalyzed by HMG-CoA synthase.

In the cytosol, HMG-CoA is an early intermediate in cholesterol biosynthesis. In the mitochondria, however, HMG-CoA lyase acts on HMG-CoA to yield acetoacetate plus acetyl-CoA, as follows:

HMG-CoA <=> Acetoacetate + Acetyl-CoA

Acetoacetate can be reduced to form -hydroxybutyrate by action of the enzyme -hydroxybutyrate dehyrdogenase, as shown below.

Acetoacetate + NADH + H⁺ <=> -Hydroxybutyrate + NAD⁺

Acetoacetate and -hydroxybutyrate are known as ketone bodies and are important in delivering energy to the brain when glucose is not available, such as during starvation.

Acetoacetate can also undergo spontaneous decarboxylation to form acetone.

The process of ketogenesis is also referred to as ketosis. One sign of ketosis is the odor of acetone on the breath. This may occur in some forms of diabetes.