The German-British biochemist Sir Hans Adolf Krebs (1900-1981) shared the Nobel Prize in Physiology or Medicine for his discovery of the citric, or tricarboxylic, acid cycle (Krebs cycle).
Hans A. Krebs, the son of Georg Krebs, an otolaryngologist, was born in Hildesheim, Germany, on April 25, 1900. He studied medicine at the universities of Göttingen, Freiburg im Breisgau, Munich, and Berlin, qualified in 1924, and in 1925 graduated as a doctor of medicine in the University of Hamburg. After a year's study of chemistry in Berlin, he was assistant to the biochemist Otto Warburg in Berlin-Dahlem from 1926 to 1930. Krebs then returned to university clinical work, first at Altona and then as assistant at the University Medical Clinic in Freiburg. In June of 1933 the Nazis terminated his appointment, and Sir Frederick Gowland Hopkins invited him to work, with a Rockefeller studentship, at the Biochemical Institute at Cambridge. In 1934 Krebs was appointed demonstrator of biochemistry at the University of Cambridge.
In 1935 Krebs went to the University of Sheffield as a lecturer in pharmacology. In 1938 he was appointed lecturer in biochemistry and director of the newly founded Institute of Biochemistry. In 1945 his appointment was upgraded to a professorship, and he was also director of a research unit of the Medical Research Council already established in his department. In 1954 he was appointed Whitley professor of biochemistry in the University of Oxford, and the Medical Research Council's research unit was transferred there. He was also elected a Fellow of Trinity College, Oxford.
The Ornithine Cycle
To keep organs and tissues alive for biochemical tests, they had been perfused with physiological salines as a substitute for blood. The results were often unsatisfactory. Early in his career Krebs devised the tissue-slice technique. The organ, rapidly removed after the death of the test animal, was cut into thin slices and kept in fresh saline for biochemical testing. He used this technique in his study of the synthesis of urea by the liver.
It was known that urea is produced in a liver undergoing autolysis, and in 1904 it was shown that the autolysis produces the amino acid arginine, which is acted on catalytically by the enzyme arginase to produce urea. In 1932 Krebs found that, when an amino acid is added to liver, ammonia is liberated and is converted approximately quantitatively into urea. All the amino acids tested gave this result except two. When ornithine was added, the urea production was 10 times the expected amount, and arginine also gave an excess yield of urea. He therefore suggested that ornithine reacted with added ammonia and carbon dioxide to form arginine. Under the action of arginase, the arginine was broken down to urea and ornithine. If ammonia was omitted, there was no appreciable formation of urea. Further, ornithine was not observed to disappear while, with added ammonia, the synthesis of urea was in progress. Krebs therefore concluded that the ornithine acted as a catalyst. Many other substances were tested, but the only one that acted like ornithine was citrulline, and he suggested that citrulline formed a stage midway between ornithine and arginine. His ornithine cycle is still regarded as a sound explanation of the synthesis of urea in the body.
The Citric Acid Cycle
Krebs then turned to the intermediary oxidation of carbohydrates. In 1935 Albert von Szent-Györgyi elucidated the sequence of oxidations of the C4-dicarboxylic acids as follows:
succinic acid→fumaric acid→malic acid→maoxaloacetic acid
He also showed that these reactions were at least in part catalytic. This was later proved, but the manner of action remained unknown. In 1936 C. Martius and F. Knoop showed that in biological material citrate yields alphaketoglutarate on oxidation. They further suggested that the intermediate products were cis -aconitic acid, isocitric acid, and oxalosuccinic acid. It was already known that alpha-ketoglutarate forms succinate. In 1937, when Krebs started his work, the following sequence of reactions was therefore known:
citric acid→cis -aconitic acid→iso-citric acid→oxalosuccinic acid→alpha-ketoglutamic acid→succinic acid→fumaric acid→malic acid→oxaloacetic acid
Krebs and W. A. Johnson found that citrate was not only rapidly broken down in muscle but was also readily formed provided that oxaloacetate was added. The assumption was that some of the oxaloacetate was broken down to pyruvate or acetate and that the formation of citrate was due to a combination of the remaining oxaloacetate with pyruvate or acetate. But pyruvate or acetate could be derived from carbohydrate. In 1937 Krebs conceived the whole process as a cycle in which an undefined derivative of pyruvate, resulting from the breakdown of carbohydrate, condensed with oxaloacetate to form citric acid. The citric acid then passed through the changes noted above until oxaloacetic acid was regenerated, and the cycle was repeated. The full cycle is therefore as follows:
citric acid→cis -aconitic acid→iso-citric acid→oxalosuccinic acid→alpha-ketoglutamic acid→succinic acid→fumaric acid→malic acid→oxaloacetic acid+pyruvic acid→citric acid
Since Krebs originally described this cycle, he and others did further work on it. In 1950 Fritz Lipmann showed that the derivative of pyruvic acid that combines with oxaloacetate to form citrate is acetyl-coenzyme A and that this coenzyme is also active at two other points in the cycle. It was shown that acetyl-coenzyme A, in addition to its formation from carbohydrate, is also formed from fatty acids and many amino acids. The Krebs cycle is therefore a most important concept of biochemistry. Krebs shared with Lipmann the Nobel Prize in Physiology or Medicine in 1953.
Among Krebs's other important contributions to biochemistry were his studies of the synthesis of glutamine in brain tissue under the influence of the enzyme glutaminase (1935), the passage of ions across cell membranes (1950), and the effect of primitive intrinsic regulating mechanisms in controlling the metabolism of metazoan cells (1957).
In 1967 Krebs, having reached Oxford's mandatory retirement age of 67, retired from his Oxford chair and from his fellowship. He refused to stop researching, however. He was thereupon appointed a research scientist in the Nuffield Department of Clinical Medicine at Oxford and was elected a Supernumerary Fellow of St. Cross College. He was also appointed a visiting professor at the Royal Free Hospital School of Medicine in the University of London. Krebs died at Oxford in 1981 at the age of 81.
Krebs received many honors in addition to his Nobel Prize. In 1947 he was elected a Fellow of the Royal Society, and he was awarded its Royal (1954) and Copley (1961) Medals. He delivered its Croonian Lecture in 1963. He was a member of many foreign scientific societies, and he held honorary doctorates from 14 universities. He received the Gold Medal of the Royal Society of Medicine in 1965, and he was knighted in 1958.
Further Reading on Sir Hans Adolf Krebs
There is a biography of Krebs in Nobel Lectures, Physiology or Medicine, 1942-1962 (1964), which also contains his Nobel Lecture. The Krebs cycle is discussed in all textbooks of biochemistry, such as A. White, P. Handler, and E. L. Smith, Principles of Biochemistry (3d ed. 1964), and E. Baldwin, Dynamic Aspects of Biochemistry (4th ed. 1963). A thorough two-volume chronicle of Krebs's life and work is Frederic Lawrence Holmes, Hans Krebs: Architect of Intermediary Metabolism (1993).