Scheffler Immo E. Mitochondria

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Mitochondria, Second Edition, by Immo E. Schefﬂ er

HISTORY

Summary

Major landmarks in the history of studies on mitochondria are presented here. Ideas

do not evolve in a vacuum, and insights grow on a rich base of observations and

interpretations of the past. While some may be of interest only to the historian of

science, history often connects the facts as we can present them today, and seeing

facts in the context of history enriches them for us.

A review by Cowdry (1918), quoted by Lehninger (1) , contains more than a

dozen terms referring to structures that we now identify as mitochondria:

blepharoblasts, chondriokonts, chondriomites, chondrioplasts, chondriosomes,

chondriospheres, ﬁ la, fuchsinophilic granules, Korner, Fadenkorper, mitogel,

parabasal bodies, plasmasomes, plastochondria, plastosomes, vermicules, sar-

cosomes, interstitial bodies, bioblasts, and so on. Chondros (Greek), grain

(English), and Korn (German) are descriptions of the morphology of distinct

structures noted inside of cells by microscopists beginning about 1850. Improve-

ments in staining yielded more accurate morphological descriptions, and the

grains were in some tissues seen as threads (Faden in German, mitos in Greek),

hence Fadenkorper, or mitochondria (Benda, ca. 1898). In 1888 (R. A.

Kollicker), by microdissecting some of these granules out of insect muscle and

observing them to swell in water, microscopists reached the conclusion that

the mitochondria had a membrane.

2 HISTORY

With our current knowledge it is astonishing to learn that Altman in around

1890 referred to mitochondria as bioplasts in a book on Elementarorganismen,

in which he proposed that these granules were autonomous, elemental living

units, forming bacteria - like colonies in the cytoplasm of the host cell. A wild,

lucky guess, or an idea ahead of its time?

For a while, in the early part of this century, cytologists were anxious to

provide their geneticist colleagues with a concrete entity to which they could

assign a role in the transmission of genetic information, and mitochondria

were favored for a short time. Another function, challenging their role in

genetics, was also proposed as early as 1912 (Kingsbury); and as retold by

Lehninger, Warburg in 1913 found granular, insoluble subcellular structures

to be associated with respiration. But another 30 – 40 years of intense and

painstaking biochemical analyses were required to lead to the characterization

of mitochondria as the “ powerhouse of the cell. ” Some of the most illustrious

names in the early history of Biochemistry can be found among contributors

of the highlights of the evolution of this concept. The discoveries of cyto-

chromes, iron – porphyrin compounds (heme), ﬂ avin and pyridine nucleotides,

and various dye - reducing dehydrogenases fall into the period between 1920

and the 1930s, and the formulation of the citric acid cycle by Krebs was one

of the crowning achievements of the study of metabolism and respiration in

muscle preparations.

Although ATP was discovered in 1931 (Lohmann), it took 10 more years

to demonstrate its general role beyond muscle, and during this period Warburg

and Meyerhof described what is now referred to as substrate level phosphory-

lation (ATP synthesis coupled to the enzymatic oxidation of compounds such

as glyceraldehyde phosphate), in contrast to oxidative phosphorylation, ﬁ rst

shown by Kalckar to be ﬁ rmly coupled to respiration (1937 – 1941).

This was also a time when biochemists proceeded to grind up tissue, ﬁ lter

it through cheese cloth, perhaps even centrifuge the mixture at some indeter-

minate speed, and then discard everything but the supernatant. Insoluble

particles constituted a nuisance and an insurmountable obstacle in the puriﬁ -

cation of an enzyme, and students were urged not to “ waste clean thoughts on

impure enzymes. ” The study of mitochondria required the isolation and puri-

ﬁ cation of larger quantities of the organelle, and the ﬁ rst attempts in this

direction were doomed by the use of unsuitable buffers and media for cell

suspension and breakage.

Cell Biology metamorphosed out of the older science of Cytology when

two powerful new methodologies were perfected and applied to the study of

biological tissues. Pioneering advances and applications were made in parallel

and synergistically at the same institution during the 1940s: At the Rockefeller

Institute, Claude and his colleagues began to use the centrifuge as a sophisti-

cated analytical tool for the fractionation of subcellular structures (differential

centrifugation), a technique that De Duve (2) would later describe as “ explor-

ing cells with a centrifuge. ” Subcellular particles were fractionated reproduc-

ibly and with increasing resolution to achieve pure fractions. At the same time,

HISTORY 3

careful biochemical characterizations were carried out. Important concepts to

emerge were the recognition of the polydisperse population of particles (size

variation) and the postulate of biochemical homogeneity. Microsomes and

mitochondria were represented by overlapping populations of granules of

different size, but at the large end were almost pure mitochondria and at the

small end were almost pure microsomes (which were later resolved further

into lysosomes and peroxisomes (2) ). The discovery that ∼ 0.3 M sucrose greatly

stabilized mitochondria (Hogeboom, Schneider, Palade, 1948) greatly aided

their isolation from liver in a morphologically intact form.

The criterion of morphological intactness could not have been applied if at

the same time the groups led by Porter and Palade had not pursued the appli-

cation of the electron microscope to the exploration of cells. Particles could

now be viewed and compared in situ and after isolation by differential cen-

trifugation, thereby increasing conﬁ dence that the particle was intact and

therefore most likely completely functional.

Nevertheless, isolating and looking at a particle does not immediately give

many clues about its biological function, although remarkably prescient guesses

and deductions had been made from staining experiments. An approach from

a different direction ﬁ nally led to the full appreciation of the role of mitochon-

dria in respiration. Lehninger (and independently Leloir and Munoz) in the

period 1943 – 1947 had focused on the oxidation of fatty acids in liver homog-

enates and found the activity to be dependent on an insoluble component that

was sensitive to osmotic conditions. The newly established conditions for mito-

chondrial isolation were applied by Kennedy and Lehninger to prove that fatty

acid oxidase activity of the liver was found almost exclusively in mitochondria.

The same investigators then extended the biochemical characterization of

these organelles by demonstrating that (a) the reactions of the citric acid cycle

can be carried out in mitochondria at a rate that can account for most, if not

all, of the activity found in liver cells and (b) such reactions were accompanied

by the synthesis of ATP (oxidative phosphorylation).

Just as a cell was recognized to be much more than a “ bag of enzymes, ” it

soon became clear that many of the enzymes catalyzing the biochemical reac-

tions observed in mitochondria were not simply contained within this organ-

elle in soluble form by the mitochondrial membrane. In fact, the potential

complexity of this organelle became apparent from the early electron micro-

scopic observations that revealed the existence of an inner and an outer

membrane, with the inner membrane often highly folded (termed cristae by

Palade). Topologically, one can therefore distinguish two spaces inside the

mitochondrion: the intermembrane space and the matrix. However, a full

appreciation of the signiﬁ cance of this compartmentalization was probably not

achieved until much later.

The enzymes for fatty acid oxidation and for the citric acid cycle (with the

exception of SDH) were found to be soluble in the mitochondrial matrix. The

enzymes responsible for oxidation of NADH and electron transport to oxygen

were insoluble and localized to the inner membrane (cristae). A more detailed

4 HISTORY

description of their characterization will be deferred to a later chapter, but

even an abbreviated historical introduction should mention two accomplish-

ments of the 1960s. First, the overall arrangement of the components of the

electron transport chain and the ﬂ ow of electrons from dehydrogenases to

ﬂ avoproteins to various nonheme iron – sulfur centers and cytochromes and

ﬁ nally to oxygen, ﬁ rst glimpsed by Keilin in the 1930s, was established by a

combination of spectroscopic studies and the use of speciﬁ c inhibitors such as

rotenone, antimycin, and cyanide in various laboratories, with that of B. Chance

deserving special mention. D. E. Green was another inﬂ uential investigator in

the 1950s and 1960s. While his ideas about an “ elementary particle ” within

mitochondria have not stood up to the test of time, his Institute for Enzyme

Research was the training ground for a number of prominent researchers in

the subsequent decades. A very informative, entertaining, and highly personal-

ized account has been written by one of the pioneers in the ﬁ eld, E. Racker

(3) . Second, the efforts of Hateﬁ and colleagues culminated in the fraction-

ation and characterization of ﬁ ve multisubunit complexes from the inner

membrane, four of which are involved in the respiratory chain, and the ﬁ fth

was identiﬁ ed as the site of the phosphorylation of ADP to ATP (4, 5) . Among

the high points in the past decades in the biochemical and structural analysis

of these complexes is undoubtedly the achievement of high - resolution struc-

tures for complex II (6 – 8) , complex III (9) , complex IV (cytochrome oxidase)

(10) , and complex V (ATP synthase) (11 – 14) by X - ray diffraction and other

biophysical means. A Nobel Prize in Chemistry has been awarded for the elu-

cidation of the structure and function of the F

- ATP synthase to J. Walker of

Cambridge and P. Boyer at UCLA. After the initial breakthroughs, similar

structures from different organisms and at higher resolution have quickly

followed.

The distinction between substrate - level phosphorylation and oxidative

phosphorylation is made the subject of examination questions for thousands

of undergraduate students every year. The former is straightforward to explain

in terms of enzyme kinetics and the coupling of exergonic and endergonic

enzyme - catalyzed reactions. The challenge to explain oxidative phosphoryla-

tion has preoccupied some of the best biochemists for a good part of their

career. An ingenious solution, offered by P. Mitchell (15) , was slow to be

accepted, but it eventually revolutionized our thinking about bioenergetics,

membranes, membrane potentials, active transport, ion pumps, and “ vectorial

metabolism. ” A detailed understanding of the structure of membranes, along

with an understanding of the relationship between lipids and integral mem-

brane proteins, was a prerequisite (16) . The chemiosmotic hypothesis has

found universal acceptance in explaining not only oxidative phosphorylation

in mitochondria, but also aspects of photosynthesis in chloroplasts and light -

driven phosphorylations in bacteria.

While failing to live up to early expectations of mitochondria as the carriers

of all hereditary information, a most important discovery was made in 1963

when one of the ﬁ rst deﬁ nite identiﬁ cations of DNA in mitochondria was

HISTORY 5

made (17) . This discovery had in many ways been anticipated by the discovery

of non - mendelian, cytoplasmic inheritance in yeast by the Ephrussi laboratory

(18) . Ramiﬁ cations of this discovery are wide. It renewed and strengthened

interest in the evolutionary origin of mitochondria. The problem of under-

standing how two genomes, nuclear and mitochondrial genes, interact in the

biogenesis of this organelle is still an acute one. Changes in mitochondrial

DNA sequences are believed to represent a molecular clock on a time scale

that appears particularly suitable for human evolution; and provocative ideas

and speculations have centered around deductions from such sequence

comparisons, with implications for primate and human evolution ( “ the Mito-

chondrial Eve ” ) and for the spread of human populations by migrations.

Controversies still center around the question whether this clock is more

accurate than a sundial (N. Howell). Because of the high degree of polymor-

phisms in human mitochondrial DNA, forensic investigations utilize compari-

sons of mitochondrial DNA sequences from victims and suspects. Where there

is DNA, there must be mutations. An explosion of publications in the last two

decades, triggered by the pioneering work of Wallace and his colleagues (19)

and Holt et al. (20) , has described mutations in mitochondrial DNA which are

directly responsible for human genetic diseases (myopathies and neuropa-

thies), while speculations go even further in relating accumulating defects in

mitochondrial DNA to a variety of ailments accompanying aging and senes-

cence. “ Mitochondrial diseases ” are now recognized to be caused by both

mitochondrial and nuclear mutations.

The ﬁ nal chapter on mitochondria has only started to be written during the

past decade when it was recognized that mitochondria are not merely the

“ powerhouse of the cell, ” but are much more intimately involved in the func-

tion, life, and death of a cell. They supply ATP, but they are also critically

involved in maintaining the cellular redox potential and ionic conditions in

the cytosol (e.g., Ca

). They are the target of numerous cellular signaling

pathways, and in turn they can be triggered to release factors/proteins that

initiate the process of apoptosis. They are the major source of reactive oxygen

species (ROS); and while these can be highly injurious at high concentrations,

they also have a positive regulatory function at lower concentrations. Pro-

teomic studies have emphasized the diverse composition of mitochondria in

different tissues and cell types, and understanding the tissue - speciﬁ c behavior

of mitochondria remains one of the challenges of the future. It is the key to

understanding the many still puzzling features of mitochondrial diseases. A

book published in 2005 entitled Power, Sex, Suicide — Mitochondria and the

Meaning of Life (N. Lane, Oxford University Press) reﬂ ects the hype associ-

ated with mitochondrial studies nowadays. Mitochondria may not help in

explaining the “ meaning of life, ” but understanding them is deﬁ nitely essential

for understanding a healthy life and many pathological conditions. Nowadays

they also occupy a prominent position in discussions on aging.

Thus, mitochondria occupy a central position in our understanding of the

cell, the “ basic unit of life, ” and from the very beginning their study not only

6 HISTORY

has contributed to more details on ATP production, but also has led to fun-

damental insights covering the entire spectrum of biological sciences from

anthropology to biophysics to cell biology, genetics, and physiology. Their

demonstrated role in disease, and their implied role in neurodegenerative

developments in advanced age makes them particularly fashionable now, but

a history of biochemistry is unthinkable without reference to mitochondria

and the monumental discoveries made in the course of their study.

REFERENCES

1 . Lehninger , A. L. ( 1965 ) The Mitochondrion — Molecular Basis of Structure and

Function , W. A. Benjamin , New York .

2 . De Duve , C. ( 1975 ) Science 189 , 186 – 194 .

3 . Racker , E. ( 1976 ) A New Look at Mechanisms in Bioenergetics , Academic Press ,

New York .

4 . Hateﬁ , Y. , Galante , Y. M. , Stiggal , D. L. , and Ragan , C. I. ( 1979 ) Methods Enzymol.

56 , 577 – 602 .

5 . Hateﬁ , Y. ( 1985 ) Annu. Rev. Biochem. 54 , 1015 – 1069 .

6 . Sun , F. , Huo , X. , Zhai , Y. , Wang , A. , Xu , J. , Su , D. , Bartlam , M. , and Rao , Z. ( 2005 ) Cell

121 , 1043 – 1057 .

7 . Yankovskaya , V. , Horseﬁ eld , R. , Tornroth , S. , Luna - Chavez , C. , Miyoshi , H. , Leger , C. ,

Byrne , B. , Cecchini , G. , and Iwata , S. ( 2003 ) Science 299 , 700 – 704 .

8 . Cecchini , G. ( 2003 ) Annu. Rev. Biochem. 72 , 77 – 109 .

9 . Xia

, D. , Yu , C. - A. , Kim , H. , Xia , J. - Z. ,

Kachurin , A. M. , Zhang , L. , Yu , L. , Deisenhofer ,

J. , Yu , C. A. , and Xian , J. Z. ( 1997 ) Science 277 , 60 – 66 .

10. Tsukihara , T. , Aoyama , H. , Yamashita , K. , Tomizaki , T. , Yamaguchi , H. , Shinzawa - Iyoh ,

K. , Nakashima , R. , Yaono , R. , Yoshikawa , S. , Yamashita , E. , and Shinzawa - Itoh , K.

( 1996 ) Science 272 , 1136 – 1144 .

11. Abrahams , J. P. , Leslie , A. G. W. , Lutter , R. , and Walker , J. E. ( 1994 ) Nature 370 ,

621 – 628 .

12. Walker , J. E. , Collinson , I. R. , Van Raaij , M. J. , and Runswick , M. J. ( 1995 ) Methods

Enzymol. 260 , 163 – 190 .

13. Van Raaij , M. J. , Abrahams , J. P. , Leslie , A. G. W. , and Walker , J. E. ( 1996 ) Proc. Natl.

Acad. Sci. USA 93 , 6913 – 6917 .

14. Dickson , V. K. , Silvester , J. A. , Fearnley

, I. M. , Leslie , A. G. , and Walker , J. E. ( 2006 )

EMBO J.

25 , 2911 – 2918 .

15. Mitchell , P. ( 1961 ) Nature 191 , 144 – 148 .

16. Singer , S. J. and Nicolson , G. L. ( 1972 ) Science 175 , 720 – 731 .

17. Nass , S. and Nass , M. M. K. ( 1963 ) J. Cell. Biol. 19 , 593 – 629 .

18. Slonimski , P. P. and Ephrussi , B. ( 1949 ) Ann. Inst. Pasteur 77 , 47 .

19. Wallace , D. C. , Singh , G. , Lott , M. T. , Hodge , J. A. , Schurr , T. G. , Lezza , A. M. S. , Elsas ,

L. J. , II , and Nikoskelainen , E. K. ( 1988 ) Science 242 , 1427 – 1431 .

20. Holt , I. J. , Harding , A. E. , and Morgan Hughes , J. A. ( 1988 ) Nature 331 , 717 – 719 .

Mitochondria, Second Edition, by Immo E. Schefﬂ er

EVOLUTIONARY ORIGIN

OF MITOCHONDRIA

We can ’ t rewind the tape of life and replay it to see what happens next time, alas,

so the only way to answer questions about such huge and experimentally inac-

cessible patterns is to leap boldly into the void with the risky tactic of deliberate

oversimpliﬁ cation .

— From Darwin ’ s Dangerous Idea: Evolution and the Meanings of Life

by Daniel C. Dennett

Summary

Hypotheses proposing a relationship between mitochondria and bacteria have been

around almost since their discovery, but speculations were bolstered considerably

after the detection of DNA in mitochondria. Today the Endosymbiotic Theory of the

origin of mitochondria is widely accepted. As more and more complete mtDNA

sequences from diverse organisms have become available, reﬁ nements in phyloge-

netic relationships have been achieved; and in particular, the identiﬁ cation of the

closest living prokaryotic species has become possible. There is also an emerging

consensus about the monophyletic origin of mitochondria: A singular event in evolu-

tion gave rise to this organelle, rather than multiple independent symbiotic partner-

ships. Nevertheless, a surprising divergence can also be observed between vertebrates,

plants, fungi, protozoa, and so on, with regard to the number of mitochondrial genes

retained, their organization, and their expression.

8 EVOLUTIONARY ORIGIN OF MITOCHONDRIA

Soon after mitochondria were ﬁ rst glimpsed in cells of higher organisms about

120 years ago, the idea that they were somehow related to bacteria was

expressed in a more or less explicit form (Altmann, 1890; Mereschovsky, 1910;

Portier, 1918; Wallin, 1927; Lederberg, 1952; see reference 1 for a complete

bibliography). The idea continued to ﬂ oat about without much further experi-

mental stimulus for several decades, until the discovery of DNA in this organ-

elle gave it a major boost. Then, thoughts about the origin of mitochondria

and chloroplasts culminated in an extremist statement of the Serial Endosym-

biotic Theory, that “ The eukaryotic ‘ cell ’ is a multiple of the prokaryotic ‘ cell ’ ”

(Taylor, 1974), and L. Margulis became a particularly eloquent and forceful

popularizer of ideas on symbiosis in eukaryotic cellular evolution. In 1925 such

ideas had been dismissed as “ too fantastic for present mention in polite bio-

logical society ” (Wilson, 1925). The review of a book by Portier on the subject

by an anonymous reviewer in Nature contained the following: “ . . . Prof. Portier

is good humored enough to quote the paradox that a theory is not of value

unless it can be demonstrated to be false. We have no hesitation in prophesying

that his theory will attain that value . . . ” . Today, while there may still be snick-

ering in “ polite biological society ” about the proposed derivation of eukaryotic

cilia and ﬂ agella from symbiotic spirochetes, the prokaryotic ancestry of

mitochondria is discussed in most textbooks, including those written by the

“ establishment. ” Formally, it is necessary at least to consider two alternative

hypotheses concerning the evolutionary origin of mitochondria. The autoge-

nous origin hypothesis (direct ﬁ liation) proposes that mitochondria arose from

within a single cell by a process of intracellular compartmentalization and

functional specialization. When DNA was discovered in these organelles, the

assumption had to be added that genetic information from a single nuclear

genome became distributed between these two compartments. The second

hypothesis suggests that mitochondria originated from a symbiotic relation-

ship between a proto - eukaryotic cell and a primitive prokaryote capable of

oxidative phosphorylation. Only the second hypothesis is being seriously

considered today.

A critical review of the arguments for and against the endosymbiont hypoth-

esis published by Gray and Doolittle in 1982 (2) made an attempt to deﬁ ne

the kind of data on which proof for the endosymbiont hypothesis must rest,

and it concluded that “ the case for an endosymbiont origin of mitochondria

was more problematic and considerably less certain ” than the case for the

origin of chloroplasts (plastids) from photosynthetic eubacteria (cyanobacte-

ria). A huge volume of additional data began to accumulate during the 1980s

which was periodically reviewed by Gray and others, culminating in an exhaus-

tive and authoritative review and evaluation by Gray in 1992 (3) . The author

came to the conclusion that reevaluating alternate hypotheses was pointless

in view of the massive and compelling evidence in favor of the endosymbiont

hypothesis.

The greatest boost to the hypothesis of the endosymbiotic origin of mito-

chondria came from the discovery of DNA in mitochondria (4) , as well as from

EVOLUTIONARY ORIGIN OF MITOCHONDRIA 9

the generalization that all mitochondria in all eukaryotes contained DNA

(however, see below for exceptions). Another statement, that all eukaryotes

contain mitochondria, is also no longer completely tenable, since eukaryotes

lacking mitochondria have been identiﬁ ed — for example, those placed in the

kingdom of Archezoa (5) . One idea was that they may represent surviving

relatives of the earliest amitochondriate eukaryotes. However, evidence is

increasingly favoring the proposal that these eukaryotes had harbored mito-

chondria at one time, but had lost them in their adaptation to certain environ-

ments (see below). It may in fact be impossible to ﬁ nd a relative of the

proto - eukaryotic host cell. Instead, one plausible scenario envisions the origin

of eukaryotes from the combination of two bacteria, one acting as the host

and the other as the symbiont. In other words, the postulated symbiosis deﬁ nes

the origin of the ﬁ rst eukaryotic cell (6, 7) . Eukaryotic cells can be deﬁ ned as

having a nucleus, with its outer membrane contiguous with the endoplasmic

reticulum, a Golgi complex, various other membrane - enclosed subcellular

organelles, and structures referred to as the cytoskeleton including actin ﬁ la-

ments, intermediate ﬁ laments, and microtubules, in addition to mitochondria.

No prokaryotic organisms are known that have some or all of these particular

structures (8) . However, a search for a “ mitotic apparatus ” in microorganisms

has uncovered proteins that are distant relatives of tubulin and actin. Before

the genetic information in mtDNA was completely analyzed, it was shown that

mitochondrial DNA could be mutated and that such “ cytoplasmic ” mutations

had serious consequences — not surprisingly affecting the capacity of the cells

to carry out oxidative phosphorylation and respiration (9) . Thus, the functional

importance of this genome was proved. These early genetic investigations were

primarily performed with the yeast Saccharomyces cerevisiae , and many others

have followed. Mitochondrial DNA encoded very speciﬁ c and unique proteins,

although it was obvious almost from the start that the mitochondrial genome

was far too small to encode information sufﬁ cient for a free - living organism.

The loss of genetic information from primordial proto - mitochondria was com-

pensated by functions encoded by the nucleus, but at this point one may have

to distinguish between two possibilities. An obvious one is that proto -

mitochondrial genes were transferred to the nucleus, and there is persuasive

evidence for such events to have occurred. It should also be obvious, however,

that symbiosis implied that the bacteria destined to become mitochondria had

to invade a host cell, a proto - eukaryote, which prior to this evolutionary event

was equally capable of an independent existence. The distinction between the

two cells in the simplest view is that the proto - eukaryote already had its DNA

compartmentalized in a nucleus with a nuclear membrane, while the proto -

mitochondrion resembled a bacterium without a nucleus. One can speculate

that the free - living proto - eukaryote had the capacity for anaerobic glycolysis

and fermentation, while the prokaryote contributed the electron transport

system. Both must have been capable of DNA replication, transcription,

protein synthesis, and the biosynthesis of various lipids to form a membrane.

Krebs cycle enzymes may also have been present in both for the purpose of

10 EVOLUTIONARY ORIGIN OF MITOCHONDRIA

interconverting short carbon compounds and amino acids. Therefore, the

initial association between these two cells must have led to a considerable

amount of redundancy of genetic information. The redundant genes in the

mitochondrion could simply be lost, others were transferred to the nucleus,

and only a very small number had to remain in the mitochondrion.

As pointed out by Gray (3) , it is absolutely necessary to start with the pre-

sumed common ancestor of both eukaryotic and prokaryotic cells and then to

consider distinct traits (genes) that evolved speciﬁ cally in one of the groups.

Only such genes constitute a unique and novel contribution to the symbiotic

relationship and distinguish the symbiont from the host. From the latest pro-

teomic studies it has been estimated that only 14 – 16% of the modern mito-

chondrial proteome has an origin that can be traced back to the bacterial

endosymbiont. The large majority of the very diverse mitochondrial proteins

were recruited to this organelle because it assumed many more functions in

addition to ATP production (10) .

It is not the intent here to speculate about the origin of life and the minimal

number of genes required for an independent existence, regardless of the

ultimate energy source. Arguments similar to the above apply to the hypoth-

esized origin of chloroplasts (plastids) from photosynthetic bacteria. Light as

an energy source has been available since the beginning of the earth. Presum-

ably the evolution of oxygenic photosynthesis in prokaryotes (cyanobacteria)

preceded respiration. Respiration requires oxygen, and oxygen did not become

abundant until massive quantities were liberated by photosynthesis. If photo-

synthesis was performed exclusively by prokaryotes during this early period

of life on earth, it is possible to speculate that mitochondria were acquired by

a eukaryote before the acquisition of photosynthetic prokaryotes which sub-

sequently evolved into chloroplasts in plants. Thus, the mitochondria of plants,

animals, and fungi may all have a common ancestor and hence similar proper-

ties. The review by Embley and Martin (8) includes one of the most thorough

and up - to - date discussions with a new tree describing the most plausible phy-

logenetic relationships among the major groups of eukaryotes, including

species harboring hydrogenosomes and mitosomes. The view that the evolu-

tion of cells with mitochondria is causally linked to the increase in atmospheric

oxygen ∼ 2 Gyr ago is now being challenged by new information of anaerobic

eukaryotes using protons (to form H

), or other carbon compounds as the

ultimate electron acceptors (8) . Fascinating discussions with a much broader

perspective on the effect of oxygen on the “ earth ’ s metabolic evolution ” can

be found in the paper by Raymond and Segre (11) and commentary by

Falkowski (12) . An analysis referred to as metabolic network expansion that

makes use of 6836 reactions involving 5057 distinct compounds in 70 different

organisms in the KEGG database leads to the conclusion that the availability

of oxygen in the earth ’ s atmosphere caused a major proliferation of new reac-

tions and metabolic pathways resulting in a very much expanded pool of

metabolites.