Fritz Lipmann was a German-born American biochemist who isolated and partially explained (1947) the molecular structure of coenzyme A, derived from the B vitamin pantothenic acid. For this and other work on metabolism, he shared the 1953 Nobel Prize for physiology for medicine with Hans Krebs, who had discovered the citric acid cycle (Krebs cycle). In 1950 Lipmann demonstrated the formation of citric acid from oxaloacetate and acetate, and found that this process required coenzyme A. During his career, Lipmann also investigated the way in which a cell acquires energy and the key role played in that process by adenosine triphosphate (ATP).