Dr. Manuel Varela – Who was Hans Krebs? And why did they name a cycle of Metabolism after him?

Jun 12, 2017 by

An Interview with Dr. Manuel Varela – Who was Hans Krebs? And why did they name a cycle of Metabolism after him?

Michael F. Shaughnessy –

1) Professor Varela, there are some outstanding scientists who made amazing discoveries, and there were other scientists who established a foundation for future work. Now, Hans Krebs actually had a cycle of metabolism named after him.   Let’s start by defining some words. When you say the word metabolism, what does it mean to you as a scientist? And does it mean something different to the layman, like myself (who is trying to lose weight)?

I have to say that of all of the amazing scientists whom we’ve been reviewing in this series, Prof. Dr. Sir Hans Krebs is one of my all-time favorites. He was a key figure in establishing the extremely important scientific field known as intermediary metabolism. In fact, it has been argued that Dr. Krebs is the architect of this metabolic domain.

In laymen terms, metabolism often refers to the internal activities inside the human body that regulate body weight and confer a general state of healthiness. Simply put, if one wants to lose weight, one “merely” needs to not overeat during meals, avoid snacking between meals, and (with a physician’s approval) exercise regularly, of course.

Scientifically speaking, the term metabolism refers, in general, to the entirety or the sum total of all of the chemical or biochemical reactions that happen to occur in living systems. All living beings on Earth metabolize. There are basically two sub-categories that fall under the overall umbrella of metabolism.

The first metabolic sub-category is called catabolism. Here, foodstuffs are broken down into smaller molecules, some of which are called building blocks, thus releasing the energy stored in the foods. During catabolism, the foods and their smaller chemical intermediates are oxidized, meaning that their electrons are taken away. Frequently, many of the foods are broken down all the way to water and carbon dioxide molecules.

The second metabolic sub-category is called anabolism. Here, small biochemical building blocks are used to synthesize larger molecules, sometimes called macromolecules, that are needed for living beings in order to live. Anabolic reactions often require energy in order to proceed to completion.

All of the stages and the players which function during metabolism that fall in between the foodstuffs eaten by a living being and the many types of the new molecules synthesized, that is, the enzymes, substrates, products, co-factors, energetic mechanisms, etc., constitute the components of intermediary metabolism. There are two types of biological energy; the first is called ATP, which stands for adenosine triphosphate, and the second is called an ion gradient, but is also called a motive force.

In summary, when foods are catabolized, the electrons are taken to a so-called final electron acceptor, such as oxygen, or an inorganic molecule or an organic molecule. During the electron transport, protons or sodium are pumped across the membrane, building up on one side of the membrane. This motive force is the energy used to make ATP.  The ATP in turn is used to make or biosynthesize molecules needed for life to occur.

2) Where was he born and where did he go to school and how did he first get interested in science (do you know?)

Hans Adolf Krebs was born on the 25th of August, 1900, in the then beautiful northern city of Hildesheim, located in Hanover Province, Germany. His parents, George and Alma Krebs, were Jewish. Krebs had two siblings, Elisabeth (5 years older and nicknamed Lise or Lisa) and Wolfgang (2 years younger and nicknamed Wolf). George Krebs was a physician and Alma Davidson Krebs was a stay-at-home mother. Living in a large country house filled with books and harboring an outside garden, the family had many cultural and intellectual pursuits, including reading, theatre, museums, local concerts, and learning to play music—Krebs played the piano.

Krebs attributes his interest in living beings to his father, who taught the young Krebs about the beauty of the countryside and its associated birds, plants and trees. Krebs was an avid reader and had a deep desire to constantly acquire new knowledge, but his parents had encouraged the analysis of such knowledge, as well.

Krebs attended the Gymnasium (grammar school) called Andreanum between 1910 and 1918. Another famous alumnus of the school was Georg Philip Telemann. Although Krebs had an interest in all subjects, his favorite at the time was History—he was, many years later, to write a series of historical works. In his autobiography Krebs notes that both his parents and teachers had plenty of criticism for him, causing him to have the impression of being both unattractive and unpopular. His father’s criticisms caused him to be uncertain of his putative potential and abilities. Yet, Krebs, admiring how his father helped his patients in a positive manner, was influenced to become a physician, too.

Just prior to his finishing school in 1918 at the Andreanum, Krebs was drafted into the military, as the Great War (World War I) was in full swing, having started in 1914. The military stint, however, surprisingly lasted only two months, at which time Germany had collapsed, and the war was over. In the newspapers, which Krebs regularly read, the German civilian populace had been made evidently unaware about the extensive nature of the precariousness of Germany’s war situation.

Given a fast-track to graduation at the Andreanum because of his military service, Krebs was allowed to take his final exams early. He then entered university at Göttingen in December of 1918.  He stayed only one term. It is in Göttingen during a lecture about inorganic chemistry that Krebs was reported to have said to his friend and colleague Erich Stern, “Oh God, I can never learn this.”

In mid-1919, leaving Göttingen, Krebs moved to Freiburg University, Germany; but shortly after his arrival to the city of Freiburg im Breisgau, his mother died suddenly of influenza. The world was in the midst of the worst flu pandemic it had ever experienced, and an estimated 50 to 100 million had perished during this time.

While at Freiburg, Krebs took courses in bacteriology, anatomy, and physiology, but became interested in research after listening to faculty talk about their various research projects during their lectures. Thus, entering the research laboratory of Prof. Wilhelm von Möllendorff, Krebs conducted studies in the methods of histological (tissue) staining and published his very first paper at 22 years of age—the paper was written by Krebs with very little revision by Möllendorff and immediately submitted to a journal. This was an early demonstration of the scientific acumen possessed by Krebs, as decisively recognized by his research supervisor.

Although Krebs had a very successful start performing research with Dr. Möllendorff, it was, however, Prof. Franz Knoop, and his lectures in 1920 about fatty acid metabolism and its catabolic process of β-oxidation, who first influenced the young Krebs to pursue research in what was to eventually become part of the field of intermediary metabolism. In March of 1921, Krebs passed his pre-clinical or preliminary examination, obtaining the equivalent of the Second M.B., tantamount to a bachelor’s degree in medicine.

Continuing at Freiburg for an additional semester, Krebs studied pathology under the tutelage of Prof. Ludwig Aschoff, a teacher whom Krebs considered inspiring. Next, Krebs went to Munich for clinical studies in medicine, spending a brief semester interlude at Berlin, and back in Munich completing his clinical courses of study and passing his final exams in December of 1923.

In January of 1924, in order to attain a medical licensure, Krebs began the requisite year of clinical hospital work at the Third Medical Clinic, associated with the University of Berlin, in Germany. During these pre-clinical and clinical medicine years, post-war Germany suffered from instability, including unchecked inflation. It is also the same time period in which Krebs solidified his interest in pursuing scientific research, instead of practicing medicine. Thus, he entered the laboratory of Prof. Anneliese Wittgenstein and conducted research on the entry of certain anionic dyes through the blood brain barrier and into the cerebrospinal fluid, eventually publishing the work later, in 1926.

Shortly after the start of 1925, in January, Krebs took his M.D. degree and then joined the laboratory of Prof. Peter Rona, housed in Berlin at the Pathological Institute of the Charité Hospital, where he (Krebs) learned techniques useful in biochemistry. Rona’s laboratory is famous for having trained other prominent biochemists, such as Ernst Chain, Fritz Lipmann, Karl Meyer, and Hans Weber.

In January of 1926, to his great delight, Krebs moved to the Kaiser Wilhelm Institute for Biology to become a research apprentice in the laboratory of the very famous Dr. Otto Warburg. The Kaiser Wilhelm Institute for Biology is well-known for having housed many famous biochemists, such as Fritz Lipmann, Otto Meyerhof, Lise Meitner, Carl Neuberg, Severo Ochoa, to name only a few.

Warburg’s studies in photosynthesis, respiration, and intermediary metabolism were extremely important and influential to the field of biochemistry. Warburg had discovered that tumors metabolize sugars at higher than normal reaction rates even when oxygen is present—so called aerobic glycolysis, becoming part of metabolism called respiration. Hence, the “Warburg effect” became useful for diagnosing and treating cancer.

In Warburg’s laboratory, Krebs first studied an enzyme, called cytochrome oxidase, of the respiratory chain, an area for which Warburg would later be awarded the Nobel in Physiology or Medicine, in 1931. Today, the presence of this respiratory enzyme is tested in basic Microbiology Laboratory courses, the oxidase test; and it is often used to help identify the species of bacteria. Importantly for Krebs, he learned the methods associated with measuring metabolic respiration, such as manometry, used in the day to measure oxygen consumption and carbon dioxide production resulting from respiration during the process of intermediary metabolism.

3) Cycles are obviously important things to know about- in philosophy, in history, but why are cycles so critically important in science, and in this case, the cycle of metabolism?

In metabolism, the cycles become important in re-supplying elements like carbon, nitrogen, oxygen, sulfur, and in recycling intermediates that are formed during nutrient metabolism, such as citrate and other central metabolites, back to the living systems, ranging from the bacteria, the plants, animals, to humans and to every living being in between. Without these elemental and nutrient recycling processes in place, these important metabolic molecules would be locked up into unattainable forms, and thus permanently used up. Thus, living organisms would eventually starve and die, and life on Earth could very well cease to exist.

It is during the late 1920s in Warburg’s laboratory where Krebs learned the then powerful technique, called manometry, which would help him make his first major contribution to the field of intermediary metabolism—the urea cycle, in 1932. Krebs studied the catabolic degradation of amino acids and the excretion of the nitrogen waste in the form of urea. This is actually the very first metabolic cycle to have been discovered, and it was Krebs who discovered it. The urea cycle figures prominently in the metabolism of amino acids, which then are used to produce proteins, and which, in turn, carryout life’s existence. Hence, the urea cycle is an important topic that is still covered in any good course or textbook dealing with Biochemistry.

There is, however, another cycle for which Krebs was to become famous. Although popularly known as the Krebs cycle, Krebs himself actually referred to it as the citric acid cycle, and others have called it the tricarboxylic acid cycle (TCA). In the strictest sense of the word, the cycle per se consists of 10 metabolites and 8 enzymes in many well-studied eukaryotes and with minor variations 11 metabolites and 8 enzymes in prokaryotes. In eukaryotes the Krebs cycle occurs in the inner matrix of the cellular organelle called the mitochondrion. Whereas in prokaryotes, the Krebs cycle occurs in the cytoplasm of the bacterial cell.

During the 1930s, Krebs elucidated his famous nutrient cycle by studying pigeon breast muscle tissue, which was known to carry out metabolism at rapid rates. He added various starting materials to the minced pigeon tissue in the manometer and measured the resulting consumption of oxygen during the ensuing respiration activity inherent in catabolic metabolism. Interestingly, when he artificially supplied the pigeon breast muscle in the manometer with citrate, he observed its depletion and then saw its regeneration! It was then a matter of identifying the intermediates that occurred in between the citrate removal and its reappearance. Adding the requisite intermediates into the manometer along with pigeon muscle, Krebs then showed the re-appearance of the citrate—hence, the cyclic nature of the pathway was established.

It is interesting that Krebs cited the works of Martius and Knoop (his former inspirational teacher) in which they found that oxaloacetate plus pyruvate could make citrate, to argue his (Krebs) case for the cycling aspect of the metabolic pathway.

Unable to identify one of the metabolites in the cycle, however, Krebs simply called it “an unknown substance,” in this famous Nobel winning paper of 1937.  We now know from the work of Fritz Lipmann, who discovered the central metabolic reaction converting acetyl-coenzyme A (AcCoA or AcSCoA) to citrate by combining it with oxaloacetate that filled in the missing link, so to speak. When glycolysis takes glucose that we eat to form pyruvate, it loses carbon dioxide (which we then breathe out) and the remaining acetate combines with coenzyme A to form AcCoA, which then enters the Krebs cycle to form citrate. The unknown substance that Krebs predicted was the AcCoA. Together, Lipmann and Krebs would share the Nobel Prize in Physiology or Medicine in 1953 for this pivotal discovery.

Interestingly, and importantly, additional work conducted in Warburg’s laboratory by Krebs and assisted by a medical student, Kurt Henseleit, the so-called ornithine cycle was discovered, in 1932. Krebs considered himself fortunate to be involved in this discovery for a variety of reasons. First, he chose liver as the tissue to study—it showed high levels of biochemical activity. Second, the ornithine cycle discovery led to another even more important discovery, namely, that of the so-called urea cycle. Third, the discovery of the cycle led to the concept of a metabolic cycle, in general. Fourth, these metabolic cycles were found to be relevant medically and practically in many important diseases. Lastly, Krebs was fortunate that he made the major ornithine discovery prior to the rise in power of Hitler. In this regard, Krebs had officially established himself as a first-rate scientist, and it helped him escape Nazi Germany by accepting an offer from Oxford and later to Sheffield.

Lastly, several metabolic links were discovered between the Krebs cycle and the urea cycle. Collectively called the aspartate-argininosuccinate shunt, the links provided a metabolic conduit between the inner matrix of the mitochondrion, where the Krebs cycle resides, and the cytoplasm, where the urea cycle resides. Anyhow, these two systems are often happily referred to as “Krebs Bicycle!”

4) What relevance does this cycle of metabolism have today? Is it still discussed?

Among the many justifications for its relevance today, I think there are several primary reasons why the Krebs cycle is still discussed in modern times. The first is that the citric acid cycle is a central repository for metabolic intermediates during the consumption of all foods and synthesis of all important compounds that living organisms need. For example, whether the foods consumed by living beings consist of meat (protein), carbohydrates (sugars), nucleic acids, or fats, all such nutrients are eventually taken to the Krebs cycle; likewise, Krebs cycle intermediates are often the starting points for the bio-production of the amino acids, which are then used for making much needed proteins.

One should keep in mind, then, that there is only so much Krebs cycle machinery present in living cells. Hence, if one constantly snacks or overeats during meals, the catabolic intermediates saturate the Krebs cycle machine, forcing the cell to shunt the AcCoA to produce fats, instead.

Another reason Krebs cycle is important is because it provides a means for living beings to collect and concentrate those delicious and nutritious electrons that are stored in foods we eat in order to take them (electrons) to the respiratory chain to transfer them to their final electron acceptors (oxygen, or perhaps other molecules) to generate an energizing proton motive force in order to have energy to make ATP. Hence, without the Krebs cycle, making biological energy, ATP, might be virtually impossible.

The third reason I think the Krebs cycle is discussed today is that its enzymes can make good targets to inhibit during the growth of tumor tissue, like cancer. The diseases involving cancer remain in modern times a significant cause of mortality.

A fourth reason I think Krebs’ cycle is relevant today is because without it there would be no human life—this is not true, however, with many of the microbes, but it certainly is true as far as human life is concerned. With life being such an important entity and, hence, a primary preoccupation, I think it is no wonder that in many scientific textbooks, the cycle of Krebs is quite often given its very own dedicated chapter.

5) What have I neglected to ask about the life and work of this famous scientist?

Altogether, Krebs listed 356 publications in his autobiography, published in 1980, shortly before his death on the 22nd of November in 1981.

Although many biochemists know about this, perhaps it may not be so well known to the layman, but Krebs’ well-written manuscript, in which he provided obviously well thought-out and carefully collected manometric evidence for his cycle, was soundly rejected on June 14, 1937, by the editor of Nature!

It is said that Krebs actually framed the rejection letter and displayed it in his office for the rest of his career. I do know that Krebs faithfully reproduced the rejection letter on page 98 of his autobiography.

It might further interest your readers to know that my postdoctoral supervisor at Harvard, Prof. Dr. Thomas Hastings Wilson, had himself taken his Ph.D. in 1953 in the research laboratory of Krebs, when they were at the University of Sheffield, England. When I was in Dr. Wilson’s laboratory during the late 1990s, he was often nostalgic about Krebs and admired him greatly, and I remember thinking how neat it was to know someone who knew the universally famous Krebs.

I think your readers who enjoy music might also appreciate knowing that Krebs happened to come across a song with a description of his cycle as the lyrics, written by biochemists, and which accompanied an old Australian folk song, “Waltzing Matilda,” but colloquially called instead “Waltz Round the Cycle.” The story is told that Krebs encouraged these lyricists to keep making more of these scientific songs.

In apparently so doing, there was another song about his urea cycle, titled “We’re Here Because Urea.”  I recall with great fondness that my good friend and colleague, Dr. John A. Badwey, during his lectures on nitrogen metabolism in the large Amphitheatre of the Medical Education Center building at Harvard Medical School, would play this urea tune, to the delight of the postdocs, students, and faculty, alike.

“Waltz Round the Cycle”


“We’re Here Because Urea”

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