The citric acid cycle, which is also known as the tricarboxylic acid cycle (TCA cycle) or the Krebs cycle, is a series of enzyme-catalyzed chemical reactions of key importance in all living cells that use oxygen as part of cellular respiration. In eukaryotes, the citric acid cycle occurs in the mitochondrial matrix. The TCA cycle begins with acetyl-CoA transferring its two-carbon acetyl group to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate). The citrate then goes through a series of chemical transformations, losing first one, then a second carboxyl group as CO2. The carbons lost as CO2 originate from what was oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetyl-CoA-donated carbons as CO2 requires several turns of the citric acid cycle. However, because of the role of the citric acid cycle in anabolism, they may not be lost since many TCA cycle intermediates are also used as precursors for the biosynthesis of other molecules. Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced. At the end of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle continues.