Hans Krebs

A surgeon’s son from Hildesheim

Hans Adolf Krebs was born in Hildesheim, in the Prussian province of Hanover, on 25 August 1900, the middle of three children. His father Georg was an ear, nose and throat surgeon; his mother Alma was born Davidson. The family was Jewish. In September 1918, six months before he would have left the Gymnasium Andreanum, he was conscripted into the army; he sat an emergency school-leaving examination, passed it so well that he suspected the examiners of being “unduly lenient and sympathetic”, and the war ended two months later.

He followed his father into medicine, starting at Göttingen in December 1918 and moving on to Freiburg, Munich and Berlin. A first paper, on a tissue-staining technique, appeared in 1923. After a year at the Third Medical Clinic of the University of Berlin he took his MD at Hamburg in 1925, then spent a year studying chemistry in Berlin. By then he had stopped planning to be a practising doctor and meant to be a medical researcher in biochemistry. He never quite let go of the physician’s question: at the 1953 Nobel banquet he said that, having started in medicine, “I cannot help having the problems of medicine at the back of my mind.”

Oxygen in a flask: Warburg’s assistant

In 1926 Krebs became an assistant to Otto Warburg at the Kaiser Wilhelm Institute for Biology in Berlin-Dahlem, on 4,800 marks a year, and stayed four years. The method he learned there was Warburg’s: a thin slice of tissue sits in salt solution in a closed flask joined to a pressure gauge, and as the tissue takes up oxygen the pressure falls. The readings, taken over time, give the rate of respiration, and you can add a substance or an inhibitor and watch the rate change. He left with sixteen publications and a debt he stated plainly in his Nobel lecture: Warburg was the teacher “from whom I have learnt more than from any other single teacher”.

The relationship was a working one, not a cosy one. Warburg urged him to move on in 1930. Krebs’s papers hold his application of 10 May 1928 for a laboratory directorship at Mannheim, with Warburg’s letter of recommendation, and four typescript notes of January–May 1929 that record the remarks of “W.”. The two kept in contact afterwards: Warburg wrote the dedicatory preface to the volume of essays presented to Krebs on his seventieth birthday in 1970, and Krebs later wrote a biography of him.

When he left, Krebs chose a problem that Warburg’s method could not yet reach. The breakdown of sugar without oxygen, fermentation, had been mapped step by step in cell-free extracts by the schools of Meyerhof, Embden, Parnas, von Euler, Warburg and the Coris. Burning sugar with oxygen, the main energy source of higher organisms, resisted. In his lecture’s account, the oxidative reactions “could not be obtained in cell-free extracts” and decayed fast once tissue was minced or ground, because breaking the cell’s structure exposed its coenzymes to enzymes that destroyed them. Anyone who wanted the route had to work with slices or crude mince and infer the steps from what changed when something was added or blocked. That became his way of working.

Ornithine, citrulline and the first cycle

After Warburg, Krebs went back to hospital medicine: first at the municipal hospital at Altona under Professor L. Lichtwitz, then, from 1 April 1931, at the Medical Clinic of the University of Freiburg under Professor S. J. Thannhauser. At Freiburg he was responsible for about forty patients and free to run experiments of his own.

The question was how the liver turns the ammonia released from amino acids into urea. A hint was already decades old: in 1904 Kossel and Dakin had shown that the enzyme arginase splits arginine into ornithine and urea. Working with Kurt Henseleit, Krebs took the slice method to liver. In the standard account of their 1932 papers, ornithine and citrulline acted as catalysts in the making of urea from ammonia and carbon dioxide: small amounts kept the process going without being used up, which means the ornithine is handed on and regenerated, turn after turn. It was the first metabolic cycle to be identified. The results appeared in 1932 as two short notes in Klinische Wochenschrift and a long paper in Hoppe-Seyler’s Zeitschrift für physiologische Chemie, and the salt solution they worked out together is still called Krebs–Henseleit buffer.

Recognition was quick. The catalogue of his papers lists invitations to lecture from Otto Meyerhof and Max Planck, Carl Neuberg’s recommendation of him for a chair at Münster, an enquiry from Carl Cori about a post in St Louis (made first to Warburg), and his admission in 1932 as a university teacher of internal medicine at Freiburg. Frederick Gowland Hopkins referred to Krebs’s work in his presidential address to the Royal Society, printed in Nature on 10 December 1932; Georg von Hevesy gave Krebs a copy of the issue because of it, and Krebs kept it with a note saying how he came by it.

Notice of termination, Freiburg

In April 1933, within three months of Hitler becoming chancellor, the dean of the Freiburg medical faculty notified Krebs that he was on compulsory leave, and then that his university appointment was ended. The legal instrument was the Law for the Restoration of the Professional Civil Service, and the ground was his Jewish ancestry. His papers include a student manifesto of that spring, ‘Wider den undeutschen Geist’ (Against the un-German spirit), with a translation, and a warning of final termination dated 15 July 1933 and signed by the university’s rector, the philosopher Martin Heidegger. The Nobel Foundation’s biography dates the end of the appointment to June 1933, another account gives 1 July; the surviving paperwork runs from April to mid-July.

Help arrived from several directions at once, and the file shows how quickly people moved. In April Albert Szent-Györgyi wrote to encourage him to approach Frederick Gowland Hopkins at Cambridge; Warburg and Franz Knoop both offered help; on 13 May Krebs wrote to the Rockefeller Foundation. Hopkins invited him to the School of Biochemistry at Cambridge, where he held a Rockefeller studentship, and by 16 July Krebs was writing that everyone there was making life easy for him. By his own later account he brought out “sixteen wooden boxes and several suitcases” of apparatus, notebooks and papers, and a list of the equipment survives in his files. By one account the equipment included the Warburg manometer on which his later discoveries depended.

Cambridge was meant as a way station. He became Demonstrator in Biochemistry in 1934. In March of that year Judah Magnes, rector of the Hebrew University, invited him to head a division of cell chemistry and metabolism in a proposed cancer research institute in Jerusalem. Krebs weighed it for months. His long letter of 22 December 1934, setting out his worries about the finances and above all the management of the university, is marked ‘nicht abgeschickt’ (not sent); a shorter one went on 27 December, declining. He still visited Palestine with David Nachmansohn in March–April 1936 and was corresponding with the cancer-research trustees as late as the summer of 1937. In 1935 Sheffield offered him a lectureship in pharmacology (appointment letter, 21 October 1935), and he went. Hopkins wrote on 12 January 1936 offering him a place in his own laboratory; Krebs’s draft reply is in the file. He stayed at Sheffield for nineteen years.

Minced pigeon muscle and a returned letter

At Sheffield Krebs went back to the problem he had picked on leaving Warburg: how do cells burn sugar? Some of the raw material was older than he was. Between 1906 and 1920 Thunberg had tested the oxidation of more than sixty organic substances in animal tissue, mostly muscle, and found that salts of several acids (lactate, succinate, fumarate, malate, citrate, glutamate) burned fast. They stayed isolated observations because nobody could link them to the main oxidation of carbohydrate. In 1935 Albert Szent-Györgyi, in Szeged, found that pigeon breast muscle, the bird’s main flight muscle and so a particularly powerful one, kept its oxidative capacity after being minced in a Latapie mill and suspended in water, and that part of the effect of the four-carbon acids (succinic, fumaric, malic, oxaloacetic) was catalytic. Stare and Baumann proved it in December 1936: a tiny amount of such an acid raised respiration by a multiple of the oxygen needed to burn the acid itself, and the acid could be found again afterwards. Neither group could say why. They supposed the acids were hydrogen carriers between foodstuff and cytochrome.

The logic of the experiment fits on an envelope. Put minced pigeon muscle in a manometer flask and record its oxygen uptake. Add a little succinate. If it were simply fuel, the extra oxygen would match what burning it takes. If the extra oxygen is many times larger, the acid is a catalyst, and something it does lets the muscle burn something else. Early in 1937 Krebs found at Sheffield that citrate, a six-carbon acid with three carboxyl groups, catalysed in the same way. In March Martius and Knoop reported what tissue does to citrate: it converts it to α-ketoglutarate, with cis-aconitate and isocitrate as the likely stages. They worked on extracts of liver and cucumber seeds.

From March to June 1937 Krebs and William Arthur Johnson pushed citrate through muscle and watched what piled up. The reaction Martius and Knoop had seen in liver ran fast in muscle too, fast enough to be part of the main respiratory process. Arsenite, which preferentially blocks the oxidation of ketonic acids, made α-ketoglutarate accumulate, as did a very high concentration of citrate. Malonate is a near look-alike of succinate that competitively blocks succinic dehydrogenase, the enzyme that handles it (J. H. Quastel’s discovery); with malonate present, succinate became a major product of citrate oxidation. Block the step after an intermediate and the intermediate piles up. Then came the observation Krebs singled out as of major significance: muscle could also make citrate, if given oxaloacetate. Some oxaloacetate broke down to pyruvate or acetate, and the rest combined with that to give citrate. Put together, the pieces closed into a ring: pyruvate or a derivative of it joins oxaloacetate to make citrate; a chain through cis-aconitate, isocitrate, α-ketoglutarate, succinate, fumarate and malate oxidises the equivalent of one acetic acid and regenerates the oxaloacetate needed to start again.

That explains the puzzle of catalysis. Every acid in the ring is remade as the ring turns, so a pinch of any of them can turn over a lot of fuel, and the oxygen used is a multiple of what the pinch itself would need. Krebs proposed the name ‘citric acid cycle’ in 1937, after one characteristic acid, since no term covered all the di- and tricarboxylic acids involved. His 1937 scheme joins pyruvate or a derivative to oxaloacetate; the identification of that derivative as acetyl coenzyme A came around 1950, from the laboratories of Lipmann, Ochoa and Lynen.

William Arthur Johnson is on the key papers: two Biochemical Journal papers of 1937 (the first, ‘Metabolism of ketonic acids in animal tissues’, is the one Krebs’s Nobel lecture cites for the March–June observations), the Enzymologia paper, and a 1938 paper with E. Salvin on the formation of citric and α-ketoglutaric acids in the mammalian body, which his lecture cites for malonate given to intact animals. Little else about him survives in the sources behind this page, beyond a letter to Krebs of 6 June 1978 that Krebs kept. The prize went to Krebs, but his Nobel lecture captions the original scheme ‘Krebs and Johnson, 1937’.

Krebs sent a short account to Nature on 10 June 1937. Four days later the editor returned it, saying that he already had “sufficient letters to fill correspondence columns for seven or eight weeks” and suggesting another periodical. Krebs sent a longer paper, ‘The Role of Citric Acid in Intermediate Metabolism in Animal Tissues’, to the Dutch journal Enzymologia on 27 June; by his own note it was published within three months. He did not forget the episode. His papers hold a carbon of a letter to Nature of 7 April 1967 about it, and correspondence with its editors in October–November 1953 in which he mentions his ‘experience of 1937 and … the difficulty (sometimes) of assessing the merits of a paper’. Nothing in the editor’s stated reason is a verdict on the science; it was space.

A department, a marriage and a cycle with loose ends

Krebs had met Margaret Cicely Fieldhouse after arriving in Sheffield, and they married on 22 March 1938. They had three children, Paul (born 1939), Helen (1942) and John (1945), the last of whom became the Oxford ornithologist John Krebs, later Lord Krebs. Krebs would call his Sheffield years “19 happy years”. Not everyone in the city was welcoming: in 1938 an anti-refugee leaflet headed ‘An APPEAL for … BRITONS’ circulated there, and Krebs kept a copy; he was also corresponding with the German consulate in Liverpool about his passport in 1937–38. He became a British subject in 1939.

The department grew with him. In 1938 he took charge of the new Department of Biochemistry at Sheffield, and by 1945 he was professor and director of a Medical Research Council research unit; from 1943 he also ran the Sorby Research Institute, and the catalogue of his papers records wartime research on nutrition. He was elected a Fellow of the Royal Society in 1947.

The cycle was still being filled in, partly by others. Krebs’s Nobel lecture lists what was added after 1937: oxalosuccinate, established in 1948 by Ochoa and independently by Lynen; acetyl coenzyme A, the form in which the two-carbon unit enters, identified through the work of Lipmann, Stern and Ochoa, and Lynen; and succinyl coenzyme A, from the schools of Ochoa and Green. The lecture also records that the cycle takes in several amino acids and, from 1943, the products of fat breakdown, so that foodstuffs of every kind are burned through one common final pathway, which releases about two-thirds of the energy from food. And it records where the picture did not fit: in baker’s yeast, labelled acetate did not appear in the cycle’s dicarboxylic acids, and yeast and E. coli were left as ‘an open problem’.

Stockholm: two halves of one pathway

The 1953 Nobel Prize in Physiology or Medicine was split: half to Krebs, then at Sheffield, “for his discovery of the citric acid cycle”, and half to Fritz Lipmann for the discovery of coenzyme A. They were distinct discoveries. A Swedish journalist had told Krebs in 1952 that he was a candidate, a year early as it turned out. In November the dean of the Freiburg medical faculty, the faculty that had dismissed him, sent him a testimonial about him written by E. Rehn in December 1932; the catalogue of his papers does not say what it contained.

In the presentation speech, Professor E. Hammarsten of the Karolinska Institute told the audience that “in the beginning Krebs was quite alone with his idea”, that it was “criticized by many”, and that “none of his original ideas had to be revised”; Lipmann’s coenzyme A, he said, was “the last notch of a combination lock” falling into place. The last claim is generous. Krebs’s own lecture, given on 11 December, lists the three intermediates added after 1937 and the organisms where the cycle’s role was still unproven.

At the banquet the day before, Krebs said that studying how foodstuffs yield energy in living cells did not lead to knowledge expected to give immediate practical benefit, and, retelling an unauthenticated Faraday story, that he could not promise a hard-pressed Minister of Finance that it would ever help the exchequer “in the way it has helped my own”. His wife and three children were with him in Stockholm.

The lecture is a better history than any summary. It starts from the obstacle (oxidation would not survive in cell-free extracts), gives Thunberg, Szent-Györgyi, Stare and Baumann, and Martius and Knoop their steps before reaching Sheffield, and ends with an inference: the same cycle appears in organisms from bacteria to mammals, which suggests, Krebs wrote, “that life, in its present forms, has arisen only once”.

Oxford, and a bypass for acetate

In 1954 Krebs left Sheffield for the Whitley Chair of Biochemistry at Oxford, taking his MRC unit and much of his team, and became a Fellow of Trinity College. The Royal Medal came that year, a knighthood in 1958 and the Copley Medal in 1961.

Hans Kornberg joined him there, by a long road. Born in Herford in 1928 to a Jewish family, he was sent out of Germany in 1939 by his father and stepmother, who stayed and were later murdered in the Holocaust. After school in Yorkshire he became a junior laboratory technician for Krebs at Sheffield, who encouraged him to study; he took a chemistry degree in 1949 and a PhD in 1953, spent two years at Yale and the Public Health Research Institute in the United States, and then returned to Krebs at Oxford.

The problem was acetate. The citric acid cycle burns two-carbon units to carbon dioxide, so an organism growing on acetate alone, as some bacteria will, still has to find a way to make new cell material from it. Kornberg and Krebs followed carbon-14-labelled acetate through bacteria grown on it and, in a Nature paper of May 1957, ‘Synthesis of cell constituents from C2-units by a modified tricarboxylic acid cycle’, described a bypass. The enzyme isocitrate lyase splits isocitrate into succinate and glyoxylate; malate synthase joins glyoxylate to another acetate unit to make malate. Two two-carbon units go in, a four-carbon acid comes out, and the two steps in which the ordinary cycle loses carbon dioxide are skipped. The route is now called the glyoxylate cycle and is found in plants, bacteria, protists and fungi.

Credit is spread. Kornberg’s own 2000 reference list includes Smith and Gunsalus’s ‘isocitratase’ (1954), Wong and Ajl on the conversion of acetate and glyoxylate to malate (1956), and a separate Kornberg and Madsen note on the ‘glyoxylate bypass’ in Biochimica et Biophysica Acta (1957); the joint Nature paper states the modified cycle. Kornberg later titled his centenary article on Krebs ‘Krebs and his trinity of cycles’: the urea cycle, the citric acid cycle and this one. In the same year Krebs and Kornberg published a book, Energy Transformations in Living Matter.