Hans Adolf Krebs postulated the operation of a series of chemical oxidation and reduction reactions that convert the food humans eat to energy in a form useful to the cells of the body.
The foods humans eat consist largely of carbohydrates, fats, and proteins; each of these classes represents a source of energy and molecules required for growth and repair of tissue. A detailed understanding of the conversion of foods into energy and chemical building blocks is necessary to any attempt to use particular foods to maintain or restore health. The central problem in the early 1930’s was that of describing exactly how this conversion is conducted in the cell.
When Sir Hans Adolf Krebs first became interested in these questions, it had already been established that carbohydrates, or sugars and starches, are converted to carbon dioxide, a gas. As these names imply, the chemical structures involve the carbon atom. It had been known for many years that this single element is the main constituent of all living matter. It is not obvious from the names that many complex molecules, or unique collections of six to thousands of carbon atoms, are converted to a specific substance of a single carbon atom. This idea is important because it suggests that such a process might take place in a series of steps rather than all at once. In fact, it was well known that energy is transformed more efficiently in such a series.
Little was known at that time about the nature of these steps as applied to the carbohydrates, although some progress had been made with the fats and proteins. Krebs had heard of a proposal that combined two acetate fragments, each containing two carbon atoms, to form a four-carbon structure called succinate. Both of these structures are acids that had been shown to involve carbon atoms that had reacted with the element oxygen. These facts and ideas were entirely consistent with respiration, which involves breathing in oxygen and breathing out carbon dioxide. More important, they suggested a research lead that turned out to be vital: the study of acid oxidation.
Krebs appreciated that the most promising approach to the problem of describing the chemistry that takes place in the cell was the study of the rates, or velocity, at which possible intermediate molecules are oxidized. He made an important contribution in the development of a sensitive method of following the changes of pressure in oxygen as the acid was being transformed. In this way, he found that a number of acids are oxidized rapidly enough to play a role in the overall conversion sequence.
His most important discovery concerned the common food substance citric acid, which had long been known to be connected directly with the healthy operation of the body. Not only does citrate—the form of an acid in cells—undergo oxidation rapidly, but it also speeds up, or catalyzes, respiratory chemistry. This effect of a compound causing a greater increase in the rate of a reaction had been demonstrated in 1935 for several acids by Hungarian biochemist Albert Szent-Györgyi.
The key feature of Krebs’s hypothesis is that the process is cyclic in design. It is apparent that each step, or individual change of one molecule into another, must be connected to the next. Biochemists refer to such an arrangement as a pathway. In asserting that the pathway is cyclic, Krebs linked the last chemical reaction to the first as well. The scheme involves some material being constantly fed into the pathway and reacting, or undergoing a chemical change, with one of the products of the last reaction of the sequence.
To be specific, acetate, already proposed by other scientists, enters from the food supply. Reaction takes place with a four-carbon acid, oxaloacetate, which Krebs suggests is a product of the final chemical conversion in his cycle. The product, by a well-known chemical reaction, is citrate.
There is an unexpected characteristic of many pathways that leads to the breakdown of substances: They may require first the synthesis of a more complicated structure, in this case, citrate. Such apparently wasteful indirect steps can be explained by considerations such as efficiency and control of the complex process.
Krebs’s proposal for what he called the tricarboxylic, or citric, acid cycle has come to be known as the Krebs cycle. Citrate contains as part of its structure a carbon atom linked sequentially to an oxygen atom and a hydrogen atom. Such an arrangement is characteristic of the common substance alcohol and may be easily lost, or eliminated, along with another hydrogen atom as a molecule of water; this process is known as dehydration. The product formed, aconitate, can undergo the reverse process of hydration and produce another alcohol, called isocitrate. This sequence is a clear example of apparently unproductive chemistry. Although the chief concern is oxidation, none has taken place in three chemical reactions. These mysteries can be explained by the fact that citrate is very difficult to oxidize, whereas isocitrate undergoes such chemical change easily.
The next step in the Krebs cycle involves oxidation, but in a surprising way. The element oxygen is not directly involved. Such chemistry had been known long before Krebs’s work, and required chemists to look at oxidation in more general terms. The most satisfactory view is that oxidation demands that a substance lose electrons from its structure. Such a definition includes, but is much broader than, reaction with oxygen.
Hans Adolf Krebs.
The very nature of this extended view brings about a fundamental chemical concern of direct interest in terms of the Krebs cycle. When chemists say a substance must lose electrons, they must not be taken too literally. The electrons are transferred actually to some other chemical; that is, they are gained also in a process called reduction. First, it is necessary to look at another basic understanding of the biochemist. In essentially every chemical reaction that makes up the myriad chemical processes called metabolism, a highly specific and efficient catalyst, called an enzyme, is found. For many of these enzymatic reactions to occur, a second component, or coenzyme, is also required. In the Krebs cycle, specific coenzymes are reduced by accepting the electrons lost, as, for example, when isocitrate is oxidized. As isocitrate is oxidized, it becomes a very unstable substance known as a beta-ketoacid. In spontaneously losing a molecule of carbon dioxide, it provides a route to one of the known final products of respiration. This event does not prove that Krebs is right, but it demonstrates that his proposal is in accord with experimental observations.
The subsequent steps in the Krebs cycle are similar in that they involve oxidations and loss of carbon dioxide, coupled with the reduction of coenzymes. The final production of oxaloacetate is the result of an oxidation of malate whose structure involves an alcohol similar to isocitrate. Thus the cycle is prepared to begin again with the arrival of another acetate group.
Many men and women worked untold years to provide evidence that Krebs’s brilliant hypothesis is a valid approximation of the truth.
At the beginning of the nineteenth century, chemists were fascinated by the extraordinary changes that matter undergoes in living organisms. It is from this interest that the designation “organic” was used for the ubiquitous carbon compounds of nature. It was generally believed, however, that these reactions could not be studied outside of living beings.
In the century between the 1830’s and the 1930’s, this “vital force theory” was so completely abandoned that a new science, biochemistry,
The proposal of the citric acid cycle was met with characteristic scientific skepticism, because it was a breathtaking leap into uncharted territory. It was about a decade before the conversion of pyruvate to citrate was demonstrated experimentally. There were severe criticisms directed toward the hypothesis, especially because so much of its experimental justification remained to be found.
In time, most working biochemists came to accept the cycle as a hypothesis, and a huge amount of extremely important experimental work was conducted. Much of the refinement was done by Krebs and his students, but in a review characteristic of his generosity, he gives full credit to many other workers who contributed significantly to the development of his basic scheme, such as William Arthur Johnson and Hans Leo Kornberg.
Of central importance is the fact that Krebs’s original proposal, although greatly enhanced and extended, has stood the test of many studies. For example, although the proposal was originally conceived to explain the oxidation of carbohydrates, it was shown later that all major foodstuffs are readily accommodated by its chemistry. Furthermore, the functioning of the cycle in plant, as well as animal, tissue soon became apparent. About two-thirds of all the oxidation that takes place in plants and animals using carbohydrate, fat, or protein takes place through the Krebs cycle.
The discovery of the Krebs cycle has been called the greatest imaginative and experimental accomplishment of the science of biochemistry. The breadth of application of the cycle is so great that it lies at the very heart of life, a concept that is not understood completely. Its greatest feature is demonstrated in its delicate control of chemical rates of conversion and balanced supply of vital molecular building blocks for the growth and repair of tissue.
In 1970, a volume of essays was dedicated to Krebs on the occasion of his seventieth birthday. In a dedication, his research director asked, “What had he [Krebs] achieved since 1925?” The answer: “He had discovered the essential chemical reactions of energy-transformations in life.”
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Bartley, W., H. L. Kornberg, and J. R. Quayle, eds. Essays in Cell Metabolism: Hans Krebs Dedicatory Volume. New York: Wiley-Interscience, 1970. Collection of highly technical essays also includes vivid and unique personal recollections by Krebs’s former students and colleagues. Essential for a full appreciation of Krebs as a scientist and as a person. -
Igelsrud, Donald E. “How Living Things Obtain Energy: A Simpler Explanation.” American Biology Teacher 51 (February, 1989): 89-93. Well-written presentation for the biology teacher and the nonspecialist, with classroom suggestions that aid understanding. -
Kornberg, H. L. “H. A. Krebs: A Pathway in Metabolism.” In The Metabolic Roles of Citrate, edited by T. W. Goodwin. New York: Academic Press, 1968. Fascinating biographical sketch by one of Krebs’s students and close collaborators. Full of highly personal observations and rare insights. -
Krebs, H. A. “The History of the Tricarboxylic Acid Cycle.” Perspectives in Biology and Medicine 14 (Autumn, 1979): 154-170. Not only describes the origin of Krebs’s ideas and their development but also offers details concerning their success. A revealing analysis of how one scientist finds a solution when others, just as brilliant, overlook it. -
Krebs, H. A., and H. L. Kornberg. Energy Transformations in Living Matter: A Survey. Berlin: Springer-Verlag, 1947. Careful review of the citric acid cycle as a means of changing energy from foods to forms directly useful for the organisms. -
Krebs, H. A., and M. B. V. Roberts. The Citric Acid Cycle: An Historical Analysis. New Rochelle, N.Y.: Audio Learning, 1973. Package consisting of manual, audiocassette, and eleven slides presents discussion between Krebs and an English scientist and illustrates chemical reactions. Invaluable description of the history and goals of biochemistry. -
McMurray, W. C. Essentials of Human Metabolism. 2d ed. New York: Harper & Row, 1983. Places the Krebs cycle within the context of the overall metabolic picture. Treats the regulation of the cycle and the role of the mitochondria in some detail. Includes excellent glossary. -
Piel, Gerard. The Age of Science: What Scientists Learned in the Twentieth Century. New York: Basic Books, 2001. An overview of the scientific achievements of the twentieth century. Chapter 5 includes brief discussion of Krebs’s work. Includes many illustrations and index. -
Quayle, J. R. “Obituary: Sir Hans Krebs, 1900-1981.” Journal of General Microbiology 128 (1982): 2215-2220. Far more than the usual obituary, written by a colleague and admirer. Full of inside detail on Krebs as a person, scientist, and coworker. Provides important information about the development of the entire field of biochemistry. -
Rensberger, Boyce. Life Itself: Exploring the Realm of
the Living Cell. New York: Oxford University Press, 1996. Comprehensive examination of scientific knowledge about human cells and the frontiers of cell research. Includes glossary and index. -
Steiner, Robert F., and Seymour Pomerantz. The Chemistry of Living Systems. New York: D. Van Nostrand, 1981. Well-written, detailed review of the chemistry of the Krebs cycle and its relationship to oxidative phosphorylation. Includes an array of thought-provoking questions and a list of suggested readings at various levels.
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