FREE ESSAY ON CHEMIOSMOTIC HYPOTHESIS

CHEMIOSMOTIC HYPOTHESIS

Peter Mitchell (1920 - 1992) : Chemiosmotic Hypothesis
Peter Mitchell's 1961 paper introducing the chemiosmotic hypothesis started a revolution
which has echoed beyond bioenergetics to all biology, and shaped our understanding of the
fundamental mechanisms of biological energy conservation, ion and metabolite transport,
bacterial motility, organelle structure and biosynthesis, membrane structure and
function, homeostasis, the evolution of the eukaryote cell, and indeed every aspect of
life in which these processes play a role. The Nobel Prize for Chemistry in 1978, awarded
to Peter Mitchell as the sole recipient, recognized his predominant contribution towards
establishing the validity of the chemiosmotic hypothesis, and ipso facto, the long
struggle to convince an initially hostile establishment. 
The seeds of the chemiosmotic hypothesis, which lay in Peter's attempts to understand
bacterial transport and homeostasis, were pollinated by the earlier ideas of H.
Lundergard, Robert Robertson, and Robert Davies and A.G. Ogston, on the coupling of
electron transport and ATP synthesis to proton gradients. Mitchell's 1961 paper outlined
the hypothesis in the form of several postulates which could be subjected to test. In
retrospect, it was a great strength of this first paper that Peter did not go into too
much detail; the ideas were new and strange, and were introduced to a field dominated by
a few major laboratories with their own different ideas about how the coupling between
electron transport and phosphorylation occurred. It is interesting to look back and
remember how sparse the clues were on which the hypothesis was based. At the time, the
chemical hypothesis, based on analogy with Ephraim Racker's mechanism of substrate level
phosphorylation linked to triose phosphate oxidation, seemed secure. A few niggling
difficulties were apparent. Why did so many different reagents act as uncouplers? Why
were the enzymes of oxidative phosphorylation associated with the mitochondrial membrane?
Why did coupling seem so dependent on the maintenance of structure? How did mitochondria
maintain their osmotic balance? How did substrates get in and out? But these must have
seemed second-order problems to the main protagonists. It was these niggles that
Mitchell's hypothesis addressed. 
I first met Peter in 1962 when he visited Brian Chappell in Cambridge to talk
mitochondriology. I was in my second year of Ph.D. research, and becoming familiar with
the field. Brian had, at the start of my apprenticeship, set me to work in the library,
with Peter's 1961 paper as a starting point. I must confess that I had little idea at the
time of the importance of the paper; I didn't know enough, either of the background
bioenergetics or the physical chemistry, to understand what the issues were. But by the
time of Peter's visit, I had become involved in the work on mitochondrial ion transport
initiated by Brian in collaboration with Guy Greville, and Brian had become interested in
mechanisms. Peter arrived in an elegant if ancient Bentley convertible, and wrapped us in
a corduroy enthusiasm. He was in trouble with his hypothesis, because three labs claimed
to have disproved it by isolating the intermediates expected from the chemical
hypothesis. Peter was undaunted, and engaged in a mischievous discussion of the data and
its validity. The challenge of the upstart chemiosmotic hypothesis to the prevailing
chemical view of mechanism was to become a running battle, in which Peter engaged the
establishment single-handed for several years before the first of a growing band of
brothers (and sisters) joined him in the fray. The early work from Andre Jagendorf's lab
on H+-uptake and pH-jump driven ATP synthesis by chloroplasts, the parallel work on ion
and metabolite transport in mitochondria from Chappell's lab, the work on ionophores and
uncouplers by Bert Pressman, and by Brian Chappell and myself, the development of
artificial membrane systems by Alec Bangham and by Paul Mueller, and Mitchell's own work
with Jennifer Moyle on proton measurements following O2 pulses, had demonstrated before
1965 the activities expected from the hypothesis, but it was to be ten years before the
established leaders in the field were coaxed into a grudging acceptance of the
hypothesis. 
The bones of the chemiosmotic hypothesis were fleshed out by Mitchell in subsequent
publications, most notably the two slim volumes published by Glynn Research Ltd. in 1966
and 1968, known affectionately in the laboratory as the Little Grey Books of Chairman M.
Mitchell's views were discussed in detail in an important review, A Scrutiny of the
Chemiosmotic Hypothesis by Guy Greville, published in 1969, which established the
seriousness of the challenge. The field was evolving rapidly, and to those of us on the
chemiosmotic side, the body of evidence favoring that point of view looked overwhelming.
The hypothesis found early favor among the photosynthetic community, perhaps because of
the elegance of the early demonstrations from Jagendorf's lab, the explanation of amine
uncoupling, the utility of the electrochromic membrane voltmeters, perhaps also because
of the more physico-chemical bent of the field. The eventual acceptance by the
biochemical community came with the demonstration of reconstituted proton pumping
activities for the isolated and purified enzymes of respiratory and photosynthetic chains
in liposomes, mainly from Racker's group, and the demonstration of coupled
phosphorylation in the chimeric bacteriorhodopsin-ATP-ase liposome system by Walter
Stoeckenius and Racker. Another important element was the growing physico-chemical
sophistication of the bioenergetics community, especially among the younger research
workers. 
Readers of Photosynthesis Research will need no guide to the present status of
chemiosmosis. The ideas Peter Mitchell introduced, which seemed so rare at the time, are
now the common currency of all our discussions. The field has gone on to explore the
deeper ramifications, from molecular mechanism at one end, through the
compartmentalization of the eukaryote cell and metabolic integration, to evolution at the
other. Although the chemiosmotic hypothesis was Peter's most important contribution, he
continued to introduce new ideas, including the Q-cycle hypothesis, which has dominated
discussion of the mechanism of electron transfer and proton pumping in the quinol
oxidizing complexes since 1975, and now seems well established as the basic mechanism. I
found myself initially on the opposite side of the Q-cycle controversy. Of course, there
seemed to me perfectly good reasons for thinking that the Q-cycle as then formulated was
wrong, and Peter was always attentive in listening to them. In trying to account for our
objections (based on observation of electron transfer kinetics in photosynthetic
bacteria), he quite early pointed out that the role of the Rieske iron-sulfur center
might be crucial (Don't you think the electron might be getting hung up on the Rieske?).
Our own results subsequently showed this to be the case, and led us to a modified Q-cycle
mechanism which was among the models discussed by Peter in his 1976 review. 
Although Peter won most of his battles, he suffered a few defeats. The long controversy
about the proton-pumping activity of cytochrome oxidase involved some fairly heated
debates before it finally went to Marten Wikstrom; and it looks as if the mechanism of
ATP synthesis through the F1.F0 ATP-ase is more along the lines envisaged by Paul Boyer
than through Peter's earlier proposals. In both these cases, with the benefit of
hindsight it looks as if Peter underrated the role of the protein and the subtlety of
evolution in designing molecular mechanism. It was part of Peter's charm that, no matter
how strongly he held his views, his stance was based on sound principles and experimental
results, was always well argued, fair, and devoid of malice. When convinced, he conceded
graciously; if his own views prevailed, he was happy to recognize the contributions of
his opponents, and his unfailing habit of giving credit where credit was due allowed for
an easy reconciliation. 
Peter's contributions have been formally recognized through the many honors, prizes and
degrees conferred on him over the years. He was a Fellow of the Royal Society, a Foreign
Associate of the National Academy of Sciences, Honorary Fellow of the Royal Society of
Edinburgh, Fellow of Jesus College, Cambridge (his alma mater), a Foreign Associate of
the Academie des Sciences Francaise, and an Honorary member of the Society of General
Microbiology, and the Japanese Biochemical Society. He received honorary doctorates from
the Technical University, Berlin, the Universities of Exeter, Chicago, Liverpool,
Bristol, Edinburgh, Hull, East Anglia, Cambridge and York. Among other honors and prizes
awarded were the CIBA Medal and Prize of the Biochemical Society in 1973, the Warren
Triennial Prize (jointly) from the Trustees of the Massachusetts General Hospital in
1974, the Freedman Foundation award of the New York Academy of Sciences in 1974, the
Feldberg Foundation Prize in 1976, the Rosenberg Award of Brandeis University in 1977,
the Lipmann Lecturer, Gessellschaft fur Biologische Chemie, 1977, the Medal of the
Federation of European Biochemical Societies in 1978, Nobel Laureate in Chemistry in
1978, the Copley Medal of the Royal Society in 1981, and the Medal of Honor of the Athens
Municipal Council in 1982. 
The dry facts of Peter Mitchell's life do him scant justice, and although he was at ease
with his fame, I am sure he would not wish to be remembered simply in terms of the many
prizes and honorary degrees heaped on him. Peter listed among his leisure interests (and
here I quote from the International Who's Who), family life, home building, the creation
of wealth and amenity, the restoration of buildings of architectural and historical
interest, music, thinking, understanding, inventing, making, sailing. I can picture him
filling out the questionnaire which elicited this list. There would have been a wry
amusement in the task of defining himself, and a certain self-deprecation, but Peter
would have tackled the job with characteristic honesty, diligence and intelligence. Glynn
House and Glynn Research Ltd. (later the Glynn Research Foundation), were the happy
outcome of a spell in hospital in the early 1960's. On the recommendation of his doctor,
Peter was looking for a vacation home in the South where he could recuperate. The estate
agent showed him the burnt-out shell of a country mansion, and Peter, more in jest than
earnest, said he would give ?x,000 for the lot. He was surprised when, a few weeks later,
the man called him in Edinburgh and said It's yours. Using his private resources, Peter
had the building remodelled, with the west wing as a residence, and the east wing and
adjoining areas as research laboratories, library, seminar room, workshop, etc., to
accommodate a small research group. 
Over the years, Peter and Helen welcomed many friends and colleagues to the now
beautifully restored Glynn House, and were unfailingly gracious and hospitable.
Friendships were important to Peter. He enjoyed conversation, and treated topics both
high and low with a mixture of deep seriousness and impish humor. Discussions were a test
bed for his latest ideas, and he relished the pursuit of odd angles and new perspectives.
He held the view that science progresses though open discussion, and abhorred the notion
that ideas or information should be closeted away, hidden from the competition. Peter's
approach to science was based on philosophical principles; he was interested not only in
the science, but in the mechanism of scientific discovery. He was fascinated by the
nature of creativity, the practice of science as a social system, the validation of
scientific truth,- indeed, the whole process of science in action. He was much affected
by Popper and his ideas about the scientific method, and Popper's influence can be seen
in Peter's insistence that hypotheses should be framed in the context of experimental
tests. He regarded experimental results as of prime importance, and was as much
interested in the intriguing observation as in the author's interpretation. He believed
strongly that science advances through the contributions of individuals, and that each
individual is responsible for selection or discrimination with regard to any piece of
information. He thought that much of the effectiveness of a successful scientist lay in
the adequacy of this filtration process. This view was captured in a nice remark he once
made to me, that The trouble with most scientists is not that they don't have good
memories, but that they don't have good forgeteries. Although in private he was not
reluctant to criticize, he was generous and helpful in his more public interactions, and
treated with respect the opinions of others, especially younger research workers coming
into the field. 
In the wider context of his social and political views, Hayek was an early influence, and
Peter would emphasize the role of the individual, and freedom of economic and political
expression. Much of his thinking in the last 15 years was directed towards human and
social problems, especially towards identifying mechanisms for conflict resolution. In
this context, he saw the bioenergetics community as a microcosm and a vehicle for
experiment, and the Round Table Discussion meeting he organized at Glynn, was at least
partly motivated by this interest. Although he had little time for socialism, he was a
very human person, aware of his own foibles and vanities, and found through this a
sympathy with the common human lot. His belief in the individual was tempered by a
recognition that in a rational order, rights are earned and exercised in the context of
the responsibilities each owes to society. He held to a set of standards, those of the
gentleman, which many would see as archaic, and these and his talents raised him above
the fray. His inspiration, humor, friendship, and the high standards of scholarship and
behavior he brought to our field will be sorely missed. 
Obituary, Photosynthesis Research, Antony Crofts, June 29th. 1992 
-  Mitchell, P. (1961) Nature (London) 191, 144-148. 
-  Mitchell, P. (1966) Chemiosmotic Coupling in Oxidative and Photosynthetic
Phosphorylation, Glynn Research, Bodmin. 
-  Mitchell, P. (1968) Chemiosmotic Coupling and Energy Transduction, Glynn Research,
Bodmin. 
-  Greville, G. (1969) Curr. Top. Bioenerg. 3, 1-78. 
-  Mitchell, P. (1976) J. Theor. Biol., 62, 327-367