Complex sentence: In humans, "beta"-hydroxybutyrate is synthesized in the liver from acetyl-CoA in a reaction catalyzed by the enzyme beta-hydroxybutyrate dehydrogenase and can be used as an energy source by the brain when blood glucose is low.
Split into simpler sentences: In humans, "beta"-hydroxybutyrate is synthesized in the liver from acetyl-CoA. The compound can be used as an energy source by the brain when blood glucose is low.