Energy production from carbohydrates begins with the conversion of glucose into pyruvate (glycolysis), which produces some energy. More energy can be realized after conversion to acetate (acetyl coenzyme A) which is followed, in oxidative metabolism, by the citric acid cycle. In addition to serving for the degradation of nutrient molecules, the citric acid cycle also serves as a major source of precursors for biosynthesis. For example, many amino acids are derived from oxaloacetate and α-ketoglutarate. Acetyl CoA itself is a major biosynthetic precursor for the formation of lipids. A significant portion of acetyl CoA produced subsequent to the decarboxylization of pyruvate will be directed toward biosynthetic pathways leading to lipids. In fact, fatty acids with even numbers of carbons derive all their carbons from acetyl CoA. A thioester is higher energy than an ordinary ester, which is key to the high acetyl group transfer potential of acetyl CoA. An ordinary ester is more stable because the single bond to oxygen can assume some double bond character.