Krebs, Sir Hans Adolf (1900–1981), German-British biochemist. Krebs made major contributions to the understanding of metabolic processes. In 1932 he discovered with the German biochemist Kurt Henseleit a series of chemical reactions occurring in mammalian tissue by which ammonia is converted to urea. In 1937 he discovered an essential series of intermediate reactions in the oxidation of foodstuffs: the citric acid cycle. Now known as the Krebs cycle, these reactions have proved to be of vital importance in our understanding of metabolic processes in the cell. In 1953 he was awarded the Nobel Prize for Physiology or Medicine for his researches in metabolic processes.