Manuel Varela: Peter Mitchell: Who was he and what did he have to do with chemiosmotic theory?

Nov 20, 2017 by

An Interview with Manuel Varela: Peter Mitchell:  Who was he and what did he have to do with chemiosmotic theory?

Michael F. Shaughnessy –

1) Peter Mitchell was the sole recipient of the Nobel Prize for Chemistry in 1978. Where was he born and where did he go to school during his early years?

Dr. Peter Dennis Mitchell, discoverer of chemiosmosis, was born in England, in a suburb of London called Mitcham, in Surrey County, in 1920 on the 29th day of September to parents Christopher Gibbs Mitchell and Kate Beatrice Dorothy Taplin.

Young Mitchell attended Streatham Grammar School starting in 1926, and in 1927, he moved to Barrow Hedges School, in Carshalton, a school he was reported to have hated, primarily because of bullying. Eleven-year old Mitchell then went to school in Taunton at Queen’s College in 1931. It is during these years at Queen’s in which Mitchell, while excelling in the sciences and mathematics, also developed an enduring interest in music, learning to play the violin. Unfortunately, Mitchell did poorly in geography and history, and he was disinterested in literature.

His main weakness at Queen’s, however, turned out to be Latin, but not so much because he was not good at it; on the contrary, he was almost gifted in the discipline. He simply failed to enroll in the required course, which proved to be his undoing.

Despite his giftedness in Latin, Mitchell had no time to learn it, and he did poorly on the topic in his scholarship application examination to enter Cambridge. Luckily for Mitchell, the College’s headmaster at Queen’s, Christopher Wiseman, intervened by putting in a good word on Mitchell’s behalf so that he could enter Jesus College, in Cambridge in 1939.

At Jesus College, Mitchell flourished. His interests widened in the arts, philosophy, and music, even learning to play the piano—his mother had given him a baby grand piano for his birthday. He also made lifelong friends, some of whom later played significant roles in his scientific career. A physiology faculty and Nobel Laureate, Prof. Edgar Douglas Adrian, had an influential role in Mitchell’s burgeoning interest in physiology, biochemistry and membranes.

In 1941, Mitchell majored in biochemistry and entered their biochemistry department at Jesus College in Cambridge. One interesting story is that one of his biochemistry professors, Malcolm Dixon, a young faculty at the time, lectured on respiration biochemistry by drawing enzymes as square boxes with arrows pointing to starting chemical groups (donors) in one part of the enzyme box and leading the arrows to the acceptor chemical groups in another part of the enzyme box. Although it was not Dixon’s intent, it gave Mitchell the correct impression that within enzymes, these sorts of chemical groups moved around—an insight that Mitchell was later to exploit in his development of vectorial metabolism and especially in his ingenious elucidation of the mechanism called the Q cycle.

In 1942, Mitchell took his undergraduate biochemistry degree from Jesus College, Cambridge and entered graduate school in the same department at the institution. At first, Mitchell’s graduate advisor was Dr. James Danielli, a membrane biochemist, but World War II interrupted Mitchell’s thesis efforts. Mitchell participated in war-related work in which he developed chemical dyes to trace serum proteins to test serum leakage from cells exposed to the poisonous arsenic-laden gases, so that others could develop antidotes. The works were published and represented Mitchell’s first forays into the scientific journals, two of which were published in the prestigious Nature.

Interestingly, while in graduate school at Cambridge, Mitchell was forever influenced in a most profound way by Dr. David Keilin, professor of biology and director of the Molteno Institute, also housed in Cambridge. According to some accounts, the story has it that at a small party, Mitchell and Keilin were holding a conversation and eating olives, when at some point during their conversation Keilin, without provocation or warning flung an olive across the room—to Mitchell’s (and Keilin’s) great delight! From that point on, Mitchell, too, took on Keilin’s easy-going and relaxed but clearly anti-establishment attitude. I surmise this new outlook on life played a role in Mitchell’s later efforts to establish successfully his own research institute at Glynn Manor at Bodmin.

Returning to his graduate thesis work after the war, in 1945, Mitchell learned that his thesis advisor left for another post, leaving Mitchell to acquire another thesis supervisor. Officially, Dr. Albert Chinball filled the vacancy, but in practice, at Chinball’s request, Dr. Ernest Gale provided the requisite advisory function.

In 1948, Mitchell submitted his Ph.D. thesis write-up for consideration of acceptance by his committee. They rejected it, criticizing it for not being well focused and advising him to revise and resubmit later.

This first thesis had three main chapters to it. The first chapter was philosophical and physiological in nature and dealt with ‘fluctoids’ a term Mitchell derived on his own (he coined new terms throughout his life) which he meant to denote a connection between diffusion (fluctid) of molecules with their static distribution (statid). The second chapter dealt with bacterial cell wall surface molecules; and the third dealt with transport of the amino acid tryptophan into bacteria.

Instead of revising and resubmitting the rejected first thesis, Mitchell, being soundly chagrinned, took on a second altogether brand new project for a new Ph.D. thesis.

This second Ph.D. thesis project dealt with the mechanism of inhibitory action of penicillin upon bacteria. In it, he proposed that the antibiotic penicillin worked to kill bacteria by affecting their DNA synthesis activities. The committee accepted this second more-conventionally focused thesis, and Mitchell received his Ph.D. in biochemistry from Cambridge in 1951.

He even published several papers pertaining to this latter thesis work. Unfortunately, Mitchell’s second Ph.D. thesis idea, accepted for his degree, that penicillin acted upon DNA, was later shown to be incorrect.

Penicillin’s mode of action is at the level of bacterial cell wall synthesis inhibition, rather than at the level of DNA replication, as Mitchell had incorrectly proposed.

2) Mitchell first proposed the chemiosmotic theory in 1961. First of all, what is this theory all about and why is it important?

Mitchell’s theory of chemiosmosis explains an important biological process that occurs during metabolism of foodstuffs to make biological energy. Briefly, Mitchell’s Nobel Prize-winning theory illustrates how one form of biological energy is generated from nutrients and used to produce another form of biological energy, called ATP.

According to Mitchell, the first energy form is the proton gradient, while the second energy form is the more well-known ATP.

In more precise biological terms, the chemiosmotic theory states that the energy of the proton gradient (first energy form) established across a biological membrane during the respiratory chain oxidation-reduction transfer of electrons to oxygen is used as a driving force to make ATP (second energy form).

The chemiosmotic process involves coupling the respiratory chain machinery to the ATP-making machinery, in all living systems. Furthermore, the term oxidative phosphorylation describes how respiration coupled to ATP production leads to oxygen gaining electrons. As such, the ‘oxidative’ part refers to the loss of electrons from metabolic foodstuffs and their intermediates (electrons later are transferred to oxygen), while the ‘phosphorylation’ part refers to the addition of phosphate to ADP to make ATP. The overall process is as follows:

During nutrient utilization by living cells, the electrons stored in food are removed from glucose and other metabolites and then taken to the respiratory chain. As the electrons are shuttled from one respiratory component to another, protons are transported across the membrane. The protons build up on one side of the membrane creating a proton gradient. The energy of the proton gradient is used as the driving force to energize the ATP making process. In this manner, the respiratory chain is coupled to ATP synthesis by the proton gradient.

One example that illustrates the importance of the overall chemiosmotic process is that if one interrupts the respiratory chain, death of the individual will occur in minutes. The interruption process is known as uncoupling; that is, respiration uncoupled from ATP synthesis will signify a quick death, such as that seen with carbon monoxide or carbon dioxide poisoning. Thus, Mitchell’s theory speaks to the heart of all life itself.

3) Apparently, Mitchell developed theories first and then did experiments later.  Where exactly did he work over the course of his lifetime, and what were some of his main discoveries?

You are completely correct about Dr. Mitchell’s propensity to come up with ideas before any supporting experiments were conducted. Mitchell performed most of his research work as a professor at Edinburgh University, in England, and later at his own privately funded research institute called Glynn Research Laboratories, housed at Bodmin, in Cornwall, England.

In addition to the chemiosmotic theory, Mitchell was key to many other discoveries. A few of these discoveries are denoted below.

He envisaged the idea of the so-called ‘vectorial metabolism’ to describe the movement of protons (H+) and hydroxyl ions (OH) across the membrane. The idea was later expanded upon to include the transport of virtually all other biomolecules across the biological membrane.

Further, Mitchell coined the term primary active transport to describe the concomitant processes of a catalytic chemical reaction and the transport of molecules across the membrane. Nowadays, the term primary active transport has been refined to describe the use of ATP energy to perform transport against a solute concentration gradient.

Importantly, Mitchell came up with the term proton-motive force (PMF), which is the energy stored in the proton gradient and which is used to drive the synthesis of ATP. Early investigators in Mitchell’s time did not believe in the PMF, and these contemporaries would publically ridicule him, referring to the PMF as the ‘Peter Mitchell’s Fallacy.’ Mitchell had been correct.

Mitchell coined the term ‘symport’ to denote transport of solute across the membrane as driven by an energetic ion gradient. The process has also been called ‘co-transport’ and has been distinguished from ‘monoport’ (later uniport), which is the transport of one solute across the membrane, and antiport (transport of solute and ion across the membrane in opposite directions).

Mitchell discovered the phenomenon of ‘uncoupling’ the respiratory chain from ATP synthesis using a chemical called 2,4-dinitrophenol, which would function to dissipate the proton gradient. The result was the collapsing of the energy stored in the PMF, which then halted ATP synthesis.

Mitchell had discovered that the energy coupling between respiration and ATP production was mediated by ion gradients. While studying these energy coupling systems, he also found that the membrane itself formed a barrier and did not itself play a role in proton translocation across the membrane. Complexes within the respiratory chain and the ATP synthesis machinery itself, called ATP synthase, instead performed this proton transport across the biological membrane.

Mitchell invoked the term ‘ligand conduction’ to describe proton translocation across the membrane through the respiratory chain complexes and the ATP synthase molecule.

Importantly, Mitchell formulated a clever scheme, called the Q cycle, while experiencing a bout of insomnia during the night of May 20, 1975. The Q cycle explained how protons and electrons move through one of the respiratory chain components, called complex III, or the ubiquinone-cytochrome C oxidoreductase.

Mitchell invoked the term ‘proton well’ to illustrate how the protons that built-up on one side of the membrane would collect within the ATP synthase molecule.

Taken together, these scientific contributions by Mitchell make him a key pioneer, if not one of the founding fathers, of the important field of bioenergetics.

4) Mitchell seemed to study the link of metabolism with membrane transport. Why is this important and why is it relevant?

In a sense, metabolic machinery first works by garnering electrons stored in food molecules, like glucose, fatty acids, nucleic acids and amino acids. These electrons are then concentrated together and taken to the respiratory chain, which resides in the inner membrane of mitochondria from all animals (including all humans), in the thylakoid membrane of chloroplasts from plants, and in the plasma membrane from all bacteria.

Mitchell linked metabolism with membrane transport in three distinct ways. First, he connected the metabolic electrons and their transport along the respiratory chains with proton transport across the various membranes, to produce the necessary proton gradients, a new form of biological energy that’s used to make ATP.

Second, Mitchell connected the reverse translocation of protons across the membrane with the synthesis of ATP. During transport of protons across the membrane down their concentration gradient, i.e., from a high proton concentration on one side to a relatively low proton concentration on the other side of the membrane, ATP is consequently formed.

Third, Mitchell’s work connected nutrient uptake that’s performed by living cells to their own metabolic purposes. All living biological cells allow entry of nutrients into their cytoplasmic spaces to be metabolized for energy production. The energy produced allows the cell to carry out their various biological functions.

These three aspects were shown to be true for animals, plants and bacteria alike. This work is important in that it speaks to the energetics inherent in all living organisms, all of which require energy for life. Thus, without this needed energy, there would essentially be no life.

The ATP molecules made by living beings are necessary to fuel the processes required to make cells, tissues, organs, and to conduct the physiological, biochemical and cellular systems, all of which are important for all life on Earth.

5) Apparently, Mitchell did not like working in higher-ed, so he started his own research institute called “Glynn”. What discoveries came out of Glynn?

The story of Mitchell’s establishment of Glynn Research Laboratories is steeped in legend. Glynn was an old manor house converted into an independent and privately funded research institute, headed by Mitchell.

One legend posits that Mitchell was disillusioned by academic life, quit his tenured professorship at Edinburgh, started a lucrative dairy farm business with his brother, and bought a castle with which to live and conduct research. While this story is great, it is inaccurate.

It is more accurate to say that Mitchell’s health caused him to take an extended leave of absence from Edinburgh. The dairy business came later. Funding for Glynn was provided partly by an inheritance to Mitchell and by benefactors. The ‘castle’ was actually an old dilapidated manor that was infested with fungi and which he had originally meant to restore as a summer home, but later became the Glynn Institute.

His ultimate decision to resign from the university post, however, was indeed influenced by his anti-establishment sentiments (the ‘Keilin effect’), but it was also the result of his constant fight with the academic and research establishment, both of which had staunch opponents to his chemiosmotic theory.

These latter incidents have been referred to as the so-called ‘Ox Phos Wars’ a direct reference to Mitchell’s alternative views regarding how ATP production was energized during oxidative phosphorylation (Ox Phos). Mitchell had correctly proposed that the proton gradient was the energy to make ATP while many in the establishment held that an elusive ‘high energy-bond intermediate metabolite’ was the necessary energy for ATP synthesis—none was ever found. While such a high energy-bond intermediate, ATP, was most certainly true for intermediary metabolism, as rightly proposed by Fritz Lippmann, it was just as equally NOT to be true for the making of ATP.

Mitchell’s biggest discovery at Glynn was the so-called Q cycle. In his Q cycle hypothesis, Mitchell addressed the nature of the movements of protons and electrons through the ubiquinone-cytochrome C oxidoreductase system, known also as complex III, present in the respiratory chain. The Q cycle was a clever and insightful contribution to science. The Q cycle involves four main steps, each of which trace the movement of electrons and protons within Complex III. Mitchell’s Q cycle has a prominent place in all textbooks dealing with nutrition, metabolism, biochemistry, animal and microbial physiology, and energetics.

6) According to many accounts, he was quite a character, and quite a ladies’ man. Do you have any great stories about his amorous exploits?

In university, while a student, accounts state that Mitchell was considered attractive by many an admirer, a flamboyant dresser, and was described as having had a flowing mane of hair to his shoulders, reminiscent of that of a young Beethoven. He seemed to project a lasting presence among potential female devotees.

According to biographers of Mitchell, starting in college and well into his later years, he apparently had a propensity for being overly keen with girlfriends and spouses of his friends and colleagues, alike. Furthermore, many of these significant others of others were apparently equally as taken with Mitchell. Many found him both charming and witty. One of these attractions involved a spouse of a fellow biochemist. Both of them left their own spouses and for a brief while moved in together in a bungalow nearby to Glynn. The affair did not last long (a week), and each returned penitent, back to their respective spouses. The affair had had a lasting strain on the marriage between Mitchell and his then second wife, Helen.

One seemingly amusing anecdote involves one occasion at a wedding. During the gathering Mitchell noticed an attractive female from across the table, whereupon he inquired of her whether they had ever met before, to which she replied that indeed they had; she introduced herself as his first former wife, Eileen.

7) At the end of his life, Mitchell sadly had a number of health and medical problems.  What do we know about these?

Mitchell suffered from recurrent stomach ulcers. At the time, such ulcers were considered the result of stress and type of lifestyle. As alluded to above, one of these ulcers was so serious that Mitchell had been advised by physicians to rest and relax to minimize the stress—remember, he was in the midst of the Ox Phos Wars. The extended leave of absence from Edinburgh due to his ulcers was one of the prime factors that eventually led Mitchell to resign his academic post and start up his work at Glynn. It was not until the late 1970s that one of the true causes of stomach ulcers was found to be a bacterial agent called Helicobacter pylori. Nowadays, such infectious maladies may be effectively treated with antimicrobial agents.

Shortly after the fiasco of the extramarital affair mentioned above in 1977 Mitchell suffered a nervous breakdown. It represented a turning point in Mitchell’s life. He reorganized the administrative structure at Glynn. Interestingly, his creative genius for generating new ideas diminished and gave way to create new and ingenious ways to defend his largely correct earlier ideas from the attacks of his numerous critics.

Sadly, Mitchell, a supreme aficionado of classical music, who played the works of Mozart, Bach, and Beethoven on the piano, lost his hearing. He had developed an ear infection, which required surgery. Unfortunately for Mitchell, the operation was botched, causing him to be deaf for the remainder of his life. It affected him greatly as music had been a refuge.

8) What have I neglected to ask?

Of the various reasons for the overall scientific establishment actively opposing Mitchell’s chemiosmotic theory, one of these being his having gone against the prevailing notion of the high energy bond intermediates, there was another prime reason: his published journal articles were exceedingly difficult to read.

He wrote in relatively long and convoluted sentences, and he developed his own nomenclature. Some of his nomenclature was eventually adopted (e.g., vectorial metabolism and proton-motive force) but many other new words (e.g., proticity, chemicalicity) that he coined failed to endure the test of time.

Ernesto Uribe and Andre Jagendorf conducted one of the first published studies that seemed to turn the tide towards eventual acceptance of Mitchell’s version of oxidative phosphorylation in 1966. In their experiment, Uribe and Jagendorf added an acidic solution to chloroplasts (which harbor all of the necessary components necessary for chemiosmosis to work) to equilibrate the protons on both sides of the membrane. Next, they added an alkaline solution to the equilibrated chloroplasts to establish an artificial proton gradient. Lastly, they added ADP plus radioactive phosphorous-32 (32P) and observed a tremendous burst of radioactive ATP production!

Interestingly, one of his most vocal opponents, Dr. Efraim Racker, had later published key evidence in 1973 that actually supported Mitchell’s chemiosmotic theory. Racker and co-author Walter Stoeckenius tested the prediction based on Mitchell’s idea regarding oxidative phosphorylation that the proton gradient was connected to the respiratory chain to provide energy for ATP synthesis. In their experiment, they mixed tiny membrane vesicles containing a purified light-driven proton transporter from bacteria with purified ATP synthase molecules taken from bovine heart muscle mitochondria. Adding light to turn on the proton pumps they invoked the production of a proton gradient. They watched as the protons went through the ATP synthase, making ATP on the other side of the membrane vesicles!

Many new experiments in the field were subsequently published, and all or most of these supported Mitchell’s version of bioenergetics. Hence, many of the detractors of Mitchell’s chemiosmotic theory eventually came around and actually became some of his strongest proponents. They know who they are.

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