Ketone Bodies Are Formed in the Mitochondria from Acetyl Coenzyme A

Not all mitochondrial acetyl CoA must enter the Krebs cycle. In fact, when fatty acids are rapidly entering the mitochon­dria, there is much more acetyl CoA available than necessary for Krebs cycle activity. It is this excess acetyl CoA, originating from fatty acids, from which the ketone bodies are synthesized (see Figure 32-16). Ketone bodies are able to leave the mito­chondria freely.

Ketone bodies affect fuel homeostasis in peripheral tissues, where they may serve as a substitute for glucose. In this way, they conserve available glucose and reduce the need for gluco­neogenesis.