The son of a physician, Hans Krebs attended several German universities before receiving his medical degree from the University of Hamburg in 1925. Although he set up practice as an ear, nose and throat specialist (his father's occupation), he soon realized he preferred doing research and, a year later, became an assistant to the noted biochemist Otto Warburg at the Kaiser Wilhelm Institute in Berlin, Germany. While there, Krebs became interested in amino acids, the building blocks of protein. He particularly wanted to know more about the then-unknown process by which the body, under certain circumstances, breaks down the amino acids instead of using them for constructive purposes.
In the course of several years of research, Krebs discovered that when amino acids were broken down (or degraded), their nitrogen atoms were the first to be stripped away. After this deamination process, the nitrogen atoms were excreted from the body in the form of urea, a major component in urine. By 1932, Krebs was able to describe several of the basic steps in urea formation and to discuss what happened to the remainder of the amino acids. His "urea cycle" won Krebs some fame but by then the Nazi movement was becoming more powerful and, like almost all of Germany's Jewish scientists, Krebs decided to leave the country.
In 1933, Krebs went to England, where he studied for a while at Cambridge University, working under Frederick Gowland Hopkins. In 1935, he joined the faculty of Sheffield University, where he remained until 1954, then moved on to Oxford University, finally retiring in 1967. While at Sheffield, Krebs concentrated much of his attention on carbohydrate metabolism and discovered the process for which he is best known: the citric acid cycle (also called the tricarboxylic acid cycle or, more simply, the Krebs cycle). Several other biochemists--in particular, Otto Meyerhof and Carl and Gerty Cori --had already shown that glycogen, the carbohydrate stored in the liver and muscles, was broken down to lactic acid by a process that required no oxygen and that released very little energy. Krebs was interested in discovering how the lactic acid was then broken down into carbon dioxide and water--and was now somehow able to release a comparatively great deal of energy.
From the work of Albert Szent-Györgyi, Krebs knew that several four- carbon compounds were able to increase the consumption of oxygen by cellular tissues. Krebs then located two six-carbon acids with similar oxygen-increasing powers, (one of them the citric acid that gave the cycle its original name). All these various compounds, Krebs felt, must be involved in the chain that led from carbohydrates to carbon dioxide, water and energy. And, by performing countless studies--most of them on pigeon breast muscles--by 1937, he was able to work out most of the long, complicated chain.
The chain is, in fact, a cycle--a series of chemical changes that begins when lactic acid (a three-carbon compound) is broken down into a mysterious two-carbon compound (determined by Fritz Lipmann to be acetyl coenzyme A). The two-carbon compound, when combined with the four-carbon oxaloacetic acid, becomes the six-carbon citric acid. The citric acid then undergoes a complicated series of chemical changes, during which hydrogen atoms occasionally break off (and combine with atmospheric oxygen to produce energy), and two-carbon fragments either attach themselves to the chain to regenerate certain compounds or break off (to be broken down into carbon dioxide and water, liberating more energy in the process).
The Krebs cycle, it soon became clear, not only helped explain the metabolic pathways followed by carbohydrates but by fats and proteins as well. The cycle, in fact, appears to play a fundamental role in virtually all cell metabolism and Krebs' discovery is therefore considered a major contribution to biochemistry. For his work, Krebs shared the 1953 Nobel Prize in physiology or medicine with Lipmann. He was also knighted by Queen Elizabeth II (1926-) in 1958.
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