Eur. J. Biochem ?OR, 203- 209 (1992)
/c\ FEBS 1992
The laws of cell energetics
Vlddimir P. SKULACHEV
Department of Bioencrgctics, A N. Bclozcrsky Institute of Physico-Chemical Biology, Moscow State University, Russia
(Received March 27. 1992)
~
EJB 92 0429
Rccent progress in membrane bioenergetics studies has resulted in the important discovery that
Na' can effectively substitute for H + as the energy coupliiig ion. This means that living cells can
possess three convertible energy currencies, i. e. ATP, protonic and sodium potentials. Analysis of
interrelations of these componcnts in various types of living cells allows bioenergetic laws of universal
applicability to be inferred.
In 1941 Lipmann [I] formulated a rule that the energy of
external sources is first converted to ATP and only then it is
utilized to support useful work. This point of view was an
alternative to earlier bioenergetic ideas considering, for example, muscle contraction as a process intimately couplcd
with fermentation of glucose to lactate.
During the following two decades, Lipmann's postulate
was supported by many pieces of evidence so that the concept
of ATP as a convcrtible biological energy currency was generally accepted and included in textbooks. However, some exceptions were also described indicating that coupling of energy-releasing and energy-consuming processes in the cell
sometimes occurs with neither ATP nor other hgh-energy
chemical compounds involved.
For instance, Adler and coworkers [2] showed in 1974 that
the respiration-dependent motility of Escherichia coli could be
obscrvcd even when the intracellular ATP level was lowered
to a ncgligible value. The authors proposed that 'a non-phosphorylatcd intermediate of oxidative phosphorylation', rather
than ATP, is the driving force for bacterial motility. By that
time it was already clcar to me that such an intermediate,
originally suggested by Slater [3], is no more than protonic
potential (transmembrane electrochcmical H + potential difference, LIP,+).
The concept of protonic potential was introduced in 1961
by Mitchell [4]. In his chemiosmotic theory of oxidative and
photosynthetic phosphorylation, it was postulated that the
light or respiration energy is transduced to dpH+which is then
utilized to form ATP from ADP and Pi. Moreover, Mitchcll
suggested that not only ATP formation but also some other
energy-consuming processes in mitochondria and bacteria (reverse electron transfer in the respiratory chain, transhydroDrdicutiorz. This revicw is dedicated to the fond memory of Pcter
M itcheil.
Correspondenw to V. P. Skulaclicv, Department of Bioencrgctics,
A. N . Belozcrsky Institute of Physico-Chcmical Biology, Moscow
Slate University, Moscow. Russia 119899
Ahhreviutions. dpH+and
electrochemical potential difference o f H + and Na', respectively: ApH and ApNa, difference in
concentrations of H i and N a + , rcspectively ; P-pyruvate.
phosphoenolpyruvate.
genase reaction, uphill Ca2 uptake) can be directly supported
by ADH+ [4-151. Extending this hypothesis to bacterial motility, we assumed that the flagellar motor of bacteria is
energized by dji,,. [16, 171. Later this assumption was confirmed in our and two other laboratories which succeeded in
demonstrating motility supported by an artificially imposed
dplr+[IS-21]. These and some other studies on bacteria,
mitochondria and chloroplasts clearly showed that dPH+can
serve as the driving force for all the main types of membranelinked work, i.e. chemical (ATP synthesis, reverse electron
transfcr), 'osmotic' (uphill transport of solutes) and mechanical (motility). This allowed me to formulate the principlc that,
besides ATP, there is one more, membrane-linked, form of
convertible biological energy currency, namely AD,,+ [18]. In
the eighties, such a concept was accepted by the bioenergetic
community (for reviews, see [22 - 251).
Again, further investigations revealed some exceptions
from the above-nicntioned scheme of bioenergetics. It was
found that under certain circumstances neither ATP nor
A P H + mediate utilization of the external energy sources. In
particular, this seemed to be the case when the membranelinked energy transductions occur under low dFH+ conditions,
e.g. in the presence of protonophores or in alkaline medium.
Careful investigation of exceptions showed that, at least in
some of the situations, Na' can substitute for H + as the
coupling ion.
It was Mitchell who considered for the first time a possible
bioenergetic role of Na' transport in bacteria. In 1968 he
mentioned [6] that Na' may be pumped from the bacterial
cell by means of the N a + / H f antiporter utilizing the transmembrane pH difference (ApH) to form a difference in Na'
concentrations (ApNa). This is equivalent to an increase in
the cytoplasmic pH buffer capacity and may stabilize the ApH
component of AJ&+. In 1974 the N a + / H + antiporter activity
was discovered by West and Mitchell in E. coli [26] and later
it was revealed in many other bacterial species (for review, see
~71).
In 1978 1 proposed that cooperation between the
electrophoretic K' influx and the dpH-driven N a + / H + antiport is used by bacteria to form K'/Na+ gradients which may
serve as a buffer of the total A D H + [28]. This concept was
+
204
confirmed by experiments of our group and of other laboratories [22, 27, 29, 301.
Further studies demonstrated that thc role of Na' in bacterial energetics is not confined to dpII+buffering. In 1980,
Dimroth described the first primary Na+ pump of bacterial
origin, i. e. the Na' -motive oxaloacetate decarboxylase of
Klehsiella pneumoniae [31]. in 1981 - 1982, Tokuda and
Unemoto discovered the Na+-motive NADH - quinone reductase in Vibrio ulginolyticus r32, 331. The enzyme proved
to be quite different from the H+-motive reductase, being
composed of three subunits which contained FMN, FAD and
no non-heme iron [34- 361. Recently the Na+-motive terminal
oxidase has been described by our group in Bucillus FTU and
E. coli [37 - 411. In E. coli, the a' and o oxidases were found to
be involved respectively in Na' and H + pumping [41]. In B.
FTU the aa3 oxidase was shown to be H+-motive whereas an
alternative oxidase, tentatively of the o type, appeared to be
Na+-motive [38 -401. The o-type Na+-motive oxidase was
reported to be inherent in Vitreoscilla [42: 431.
Among other bacterial primary N a + pumps, one should
mention Na '-motive methylmalonyl-CoA decarboxylase
[44- 461, glutaconyl-CoA decarboxylase [47 - 511, ATPase
[52 -641 and the Na+-motive span of the methanogenesislinked electron transfer chain [65].
The transmembrane electrochemical Na' potential difference, (ADNat), when formed by the respiratory chain, was
shown to be used to synthesize ATP [66,67], to import metabolitcs and some other solutes by means of the Na+/solute
symporters [6S] or to rotate the flagellum by the N a + motor
[69- 711. In anaerobic Propionigenium modesturn possessing
the Na+-motive inethylmalonyl-CoA decarboxylase, Na+driven ATP synthase was described [46, 72 -781. This enzyme
proved to be of the F,F,-type and similar to bacterial H+-ATP
synthases in its subunit composition, amino acid sequence and
inhibitor pattern. Being incorporated into proteoliposome,
the P. modestum Na+-ATP synthase was shown to hydrolyze
ATP in an Na+-motive fashion. In the absence of Na', it
pumped H' [78].
In the last few years, the sodium cycle composed of
AjiNa+ generators (Na+pumps) and ADNa+ consumers (Na 'ATP synthase, Na' /solute symporters, Na.t motor) was found
in some quite different bacterial taxa, namely in Vibrio, Bacillus, Escherichia, Salmonella, Propionigenium, Alcaligenes, Alteromonas, Flavobacterium, Klehsiella, Veilonella, Acidaminococcu.~, Streptococcus, Peplospeteytococcus, Ctosiridium,
Fusobucterium, Vitreoscilla, Pseudomonas, Mycoplasma,
Acholeplusmu, in some methanogenic and acetogenic bacteria
(for reviews, see [22, 36,46,79 - 811). The Na ' cycle is known
to be operative also in the animal cell plasma membrane where
ApNa+is formed by the Na'/K+-ATPase and is utilized by
Na' /solute symporters (reviewed in [22]). Summarizing these
and related observations, I concluded that the sodium potential, likc ATP and the protonic potential, should be regarded
as a convertible energy currency of the living cell [82] (for
details, see also [83, 841).
The discovery of the coupling role of ApNa+ seems to
complete the general picture of biological energy transductions. This allows one to formulate bioenergetic laws which
may be of universal applicability in the present state of our
knowledge [84]. Three general principles of bioenergetics will
be considered below.
The first law
The. living cell avoids direct utilization of external energy
sources in the performance of useful work. It transforms energy
of these sources to a convertible energy currency, 1.e. ATP,
A&+, or A & , t , which is then spent to support various types of
energ~-consumin~processes.
In other words, the cell prefcrs to
deal with energy in a money-type circulation rather than with
barter.
in fact, this law represents the modern version of
Lipmann's rule, assuming that not only ATP but also A & +
and ADNa + can couple energy-releasing and energy-consuming
processes. In the great majority of bioenergetic processes, such
a coupling is carried out with the use of one of the three abovementioned energy currencies. I t does not mean, however, that
other components cannot, in principle, be used as couplers.
For example, phosphoenolpyruvate (P-pyruvate), an intermediate of glycolysis, can be utilized by the membrane-linked
phosphotransferase system to phosphorylate extracellular glucose and transport it to the cytoplasm of the bacterial cell.
This process occurs without involvement of ATP, ADI1+ or
A p N a + . Nevertheless, cases of this type are very rare and
exemplify one more biological principle, i.e. living nature is
never dogmatic. it looks quite reasonable that P-pyruvate
mediates glucose import since its energy is used not only to
phosphorylate the carbohydrate molecule but also to create
the gradient of this carbohydrate between extracellular and
intracellular spaces. This gradient should be higher with Ppyruvate than with ATP as the phosphorylating reagent because of the higher energy charge of the former. Therefore
the traditional ATP-dependent (hexokinase) mechanism of
glucose phosphorylation looks less efficient than the Ppyruvate dependent one if we take into account the size of the
resulting sugar gradient. On the other hand, it is hardly possible to consider P-pyruvate as a fourth convertible energy
currency since glycolysis is the only mechanism of its formation and the number of systems utilizing it is very small.
On the same grounds, GTP cannot pretend to he a convertible currency since among all the aerobic energy-producing
processes there is only one (2-oxoglutarate decarboxylation)
which forms GTP with neither ATP nor A j i H - (ACNa+) being
involved. Similarly, acetyl-CoA or fatty acyl-CoA cannot be
regarded as a currency since they are utilized in a very limited
number of synthetic processes. The same is true for utilization
of NADH and NADPH to drive reducing syntheses. All these
energized components are never used, for example, to support
mechanical work or uphill transport of solutes.
The second law
Any living cell ulways possesses at least two energy currencies, one water-soluble (ATP) and the other membrane-linked
(A&+ andlord&+). Continuingwiththeanalogybetweencell
bioenergetics and everyday life, this law states that the cell
always has some currency in cash and some in cheques.
Mitochondria, chloroplasts, and those respiring and
photosynthetic bacteria that employ the H + cycle to form
ATP always have at their disposal ATP and A & i . If the
external Na' concentration is not too low, bacteria also create
some APNa+which is formed by the N a + / H +antiporter at the
expense of A D H + . In this case, ADNa+ serves as a A j i H t buffer:
A P N a + is formed when AD,,+ is high and it is utilized when
A P H + lowers.
On the other hand, some bacteria, as well as the animal
cell plasma membrane, were shown to use the Na' cycle, not
the H + cycle, as the primary mechanism of the membranelinked energy coupling. In these cases, ADNa+ serves as the
membrane-linked energy currency.
205
The principal bioenergetic patterns described in various
types of the living cells are pictured in Fig. 1. Fig. 1A illustrates the situation in respiring or photosynthetic (purple,
green and halophilic) bacteria using H + pumps to energize
the cytoplasmic membrane. The main function of AFH + is to
form ATP by H '-ATP synthase. Moreover, ApH+may support
(a) reverse electron transfer occurring in the direction from
more positive to more negative redox potentials, (b) uphill
transports of metabolites, ions, etc. by means of H +/solute
symporters or H +/solute antiporters (these processes are provisionally called 'osmotic work') and (c) rotation of the H +
motor of the bacterial flagellum. Among antiporters, those
exchanging Na' for H+should be mentioned. They are found
in all the 'protonic' (i. e. employing H + cycle) bacteria living
at high or moderate Na' concentrations. At the same time,
the N a + / H +antiporter seems to be absent from strictly fresh
water bacteria [22].
AjiNa+formed by the N a + / H + antiporter at the expense
of ApH+can be then utilized (a) to support uphill import of
solutes via Na+/solute symporters or (b) to stabilize the
ApH+level (buffer function of ApNa+).
Besides
the
membrane-linked
(respiratory
or
photosynthetic) phosphorylations, the bacteria in question
usually possess system(s) of substrate-level phosphorylations
occurring in the aqueous phase of the cell. Such phosphorylations are included in the reaction sequences of glycolysis and other fermentation processes as well as in oxidative
decarboxylation of 0x0 acids (e. g. 2-oxoglutarate).
ATP formed by any of the above-mentioned mechanisms
is then used to support biosyntheses (chemical work) or uphill
transport of some substances (e.g. K + import by K t -ATPase
[85,861).
In Fig. 1 B, one can see the bioenergetic pattern of bacteria
employing Na+-motive respiration or decarboxylation to
charge the membrane. The principal difference of the scheme
from that in Fig. 1 A is the mechanism of membrane energization. In Fig. 1B, N a + pumps substitute for H + pumps.
dpNa+generated by the Na pumps can be used to support
chemical, osmotic, and mechanical types of work which are
carried out by Na+-driven ATP synthase, Na+/solute
symporters, and the N a t motor, respectively. As usual, the
ATP formed is then utilized to drive various energy-consuming
processes.
Sometimes the reason for substituting H + by Na'seems
obvious, i. e. adaptation to low A jiH+ conditions. For example,
Semeykina in our group recently showed that Na+-motive
terminal oxidase activity appeared in three different cases to
result in situations where operation of the H + cycle in Bacillus
FTU proved impossible, i. e. (a) formation of reverse pH gradient (alkaline outer pH), (b) increase in the membrane H +
permeability by a protonophore, and (c) specific inhibition of
H+-motive terminal oxidase by low cyanide concentration
~71.
In this context, a recent observation of Tsuchiya and
coworker [67] may be noted. They found that in alkali-tolerant Vibrio parahaemolyticus, possessing the Na' -motive
NADH - quinone reductase, the Na+-ATP synthase activity
was not revealed when glucose was present in the medium and
glycolysis was operative as an alternative A pH+ -independent
mechanism of ATP formation.
There are bacteria which employ (a) glycolytic or other
substrate-level phosphorylations to produce ATP and (b) H
ATPase to energize the membrane (Fig. 1C). As an example,
Streptococcuqj~ecaliscan be considercd. Here A n H + is formed
only secondarily, due to the hydrolysis of glycolytic ATP by
+
+
-
H+-ATPase [88]. Interestingly, S. Juecalis induces N a + ATPase when formation or maintenance of dpH+appears to
be impossible. It was found that this was the case in an H + ATPase-deficient mutant [89] as well as in protonophore-containing [89, 901 or alkaline [54] growth medium. An N a + ATPase-deficient mutant failed to grow at high pH [91]. Thus,
S. fuecalis growing under low dpH+ conditions exemplifies a
glycolytic bacterium employing Na +-ATPase to charge the
membrane (Fig. 1 D). A similar energy transduction pattern
seems to be inherent also in Exiguobucterium aurantiacum,
Acholeplusma laidlawii, Mycoplasma mycoides and Mycoplasma gallisep t icum.
A very complicated scheme of bioenergetic events is inherent in eucaryotic cells. In the cells of green tissues of plants,
there are four energy-coupling membranes (Fig. 1 E). In the
light, energy is accumulated by a A jiH+ -generating photoredox
chain (reaction 1) which also reduces NADP' by electrons
removed from H 2 0(not shown). These processes are localized
in the thylakoid membrane of chloroplasts. In the same membrane, there is only one AFH+-consumingenzyme, namely H + ATPase synthase, which produces ATP. Another H'-ATP
synthase operates in the inner membrane of mitochondria. It
utilizes A&, + generated by the respiratory chain. ATP can also
be formed by glycolysis.
In the outer cell membrane, H -ATPase was found which
hydrolyzes ATP and forms the APH+ required to import substrates by H ' /substrate syrnporters and export sucrose by
H+/sucrose antiporter. N a + / H + antiporter forms dpNa+,
which probably serves as a A j i H t buffer.
One more H ' -ATPase is localized in the membrane surrounding the vacuole (tonoplast). The d p H +formed is used
by corresponding porters to create gradients of some substances between the cytosol and vacuole.
To describe the energy transductions in the plant tissues
containing no chlorophyll (as well as in fungal cells), a similar
scheme can be used but thylakoid-linked processes should be
omitted.
Energetics of cyanobacteria may be presented by the
Fig. 1 E scheme modified in such a way that (a) mitochondria
are absent and (b) respiratory d p H +generators are localized
in the thylakoid membrane together with photosynthetic ones
and also in the cytoplasmic membrane.
Fig. 1 F shows energy transductions in the animal cell.
ApH+is produced in mitochondria and is used to synthesize
ATP or to maintain unequal distribution of some solutes between the mitochondria1 matrix and the cytosol. ATP can also
be supplied by glycolysis. In the outer cell membrane, N a + /
K+-ATPase is localized. This enzyme forms not only the Na+
and K + gradients but also transmembrane electric potential
difference since it catalyzes the ATP-dependent electrogenic
3 Na+/2 K + antiport. A P N a + is then used by Na'lsolute
symporters of the outer cell membrane.
Thus, the mitochondrial membrane employs the H + cycle
and the animal cell outer membrane the Na+ cycle. The H +
cycle is also inherent in the membranes of secretory granules,
endosomes and lysosomes which, like plant and fungal
tonoplasts, contain H+-ATPase, H +/solute symporters and
H /solute antiporters (reviewed in [22, 841).
Summarizing this section, I should stress that the most
typical situation consists in the coexistence of all three convertible currencies in the cell, i.e. ATP, A j i H + , and A D N a + . In some
cases, both A i i H i and A p N a + are generated across one and the
same membrane (the bacterial membrane or the outer cell
membrane in plants and fungi). In others, there are membranes specializing in 'protonic' encrgetics (mitochondria,
+
206
A
cytoplosmlc
/membrane
1
glycolytic
(4
substrates -%TP-work
B
chemical,
{osmotic
Nd
membrane
C
F
Nof
*I \
glycolytic (1)
substrotes-ATP
/
\
membrane
'cherricol,
osmotic
(2)
I.
--.work
D
NO+.
C
Fig. 1. Main pathways of energy transductions in living cells. (A) Respiratory and photosynthetic bacteria cmploying the H + cycle. (1)
Respiratory or photosynthetic redox chains (in halobacteria. bacteriorhodopsin) pump H from the cell at the expensc of lighc or respiratory
energy. (2) 11' moves downhill from the medium to the cell interior via membrane proteins performing different types of uscful work, i.e.
'osmotic' (uphill import of solutes by H+/solute symportcrs). mechanical (rotation of flagellum) or chemical (ATI' synthesis or reverse transfer
of electrons). (3) ATP is synthesized by glycolytic and other substrate-level phosphorylations. (4) ATP is utilized to perform chemical
(biosyntheses) or 'osmotic' work. (5) N a + is extruded from the cell by Na+,'H ' antiporter. (6) N a + moves downhill to the cell via Na'polute
symporters ('osmotic' work) or Na+-driven flagellar motor (mechanical work). (B) Respiring or decarboxylating bacteria employing thc Na'
cycle. (1)N a + is pumped from the cell by Na*-motive respiratory chain enzymes or decarboxylases. (2) Na+ comes back downhill, performing
thereby 'osmotic' and mcchanical work or synthesizing ATP. (3) ATP is used to support chemical or 'osmotic' work. ( C ) Anacrobic bacteria
employing (a) glycolysis (or other substrate-level phosphorylations) as the only energy source and (b) H+-ATPase as the mechanism of
membrane energization. (1 ) Glycolytic ATP lormation. (2) ATP supports performance of the chemical or 'osmotic' work. (3) H+-ATPase
pumps f I + from thc cell at the expense of ATP energy. (4) H + returns to the cell, performing 'osmotic' work. ( 5 ) Na' is exported via Na'i
H + antiporter. (6) N a + influx drives 'osmotic' work. (D) As C but Na+-ATPase substitutes for H+-ATPase. (E) The plant cell. ( I ) Photoredox
chain pumps H f to t h e thylakoid interior. (2) H+-ATP synthase forms ATP which is coupled to the downhill H + e l ' h x from thylakoid. (3)
H + is pumped from the mitochondria1 matrix space by thc respiratory chain. (4)Downhill H + influx to matrix is coupled t o ATP synthesis
or to performance of 'osmotic' work (e.g., uptake of solutes by mitochondria via H'jsolute symporters). (5) ATP is formed by glycolysis. (6)
ATP i s utilizcd to perform chemical, 'osmotic', and mechanical work. (7) ATP is hydrolyzed by the plasma membranc H+-ATPase which
pumps H + from thc cell. (8) Downhill H+movement supports 'osmotic' work of the outer ccll membrane. (9) Na' is pumped from the cell
by N a + / H + antiporter.. (10) H + is pumped t o vacuole by the tonoplast H+-ATPase. (1 1) Downhill H+efflux from vacuole supports 'osmotic'
work. (F) The animal cell. (1) Respiratory chain pumps H + from mitochondria. (2) H + comes back performing chemical work (ATP synthesis)
or osmotic work (uphill transport o f metabolites). ( 3 ) ATP is formed by glycolysis. (4) Chemical, 'osmotic', and mcchanical work is driven by
ATP hydrolysis. (5) N a + is extruded rrom the ccll by Na '/K'-ATPase. (6) Na' comes into the cell via Na'isolute symporters of thc ouler
ccll membrane. (7) H + is pumped to the secretory granules, lysosomes, ctc. by H+-ATPase of vacuolar type. (8) The H + efflux from these
vesicles supports 'osmotic' work.
+
207
ATP
Energy sources
t
Ascaris
Rhizobium
(light; respiratory substrates.02)
/
\
Nitrobacter
Fig. 2. The respiratory chain-linked electron transfer in different types
of cells. Typical mitochondria or, e. g., Paracoccus clmitrjficuns employ
all the three energy coupling sites in the respiratory chain whereas in
borne other systcms, thc first (ascaride mitochondria), the second
(Khizobiurn),or the third (Nirmhacler) energy coupling sites are used.
t
Energy sources
(glycolytic substrates)
chloroplasts, tonoplasts, secretory granules) and some
specializing in 'sodium' energetics (animal cell plasma membrane).
In certain bacteria, however, the number of energy currencies is reduced to two. Besides ATP, they possess A j i H - or,
alternatively, ApK.,+.The former (purely 'protonic') energy
pattern was described in our group when we studied a fresh
water cyanobacterium Phormidium z ~ n c ~ n a ~ from
u m Lake
Baikal. In this lake, the total salt concentration is lower than
0.5 m M . It was found that an artificially imposed N a + gradient fails to energize the cytoplasmic membrane, a fact which
distinguishes this bacterium from E. coli, Vibrio harve-vi and
Halobacterium halobium studied in the same experiments [92].
On the other hand, Propionigenium modestum seems to
exemplify a purely 'sodium' cell. Here the Na+-motive
methylmalonyl-CoA decarboxylase serves as the only mechanism of membrane energization and the ApNa+formed is the
only driving force for the production of ATP [46, 721. This
bacterium employs the Na' cycle because of the absence of
4PHt generators rather than due to other conditions
unfavorable for the H ' cycle. ( P . modesturn is a marine bacterium growing at neutral pH.) This is in contrast to Bacilltis
FTU and E. coli which switch from H + to N a + energetics
when dpH1cannot be maintained due to an increase of the
pH of the growth medium or to the addition of a protonophore
[40, 871.
The third law
All the energy requirements of the living cell can be satisfied
if ut least one of' three convertible energy currencies is produced
at the upense of external energy sources. This law can be
paraphrased as it does not matter how an income is received,
in cash or in cheques, as long as they are interconvertible. The
cell is always competent in the ATP tf ADHi and/or ATP tf
ADNa+ interconversions due to reversibility of H+-ATP and
Na+-ATP synthases.
Studies of organisms occupying biological niches gave the
best example to confirm the validity of the third law. For
instance, the above-mentioned anaerobic P. modestum. possessing neither respiration, nor photosynthesis, nor glycolysis,
nor other substrate-type phosphorylations, pays for all its
energy expenses by 4DNa+ which is produced by a single reaction (methylmalonyl-CoA decarboxylation involved in succinate to propionate conversion). Such a conversion is, in fact,
the only process in P. modesturn, resulting in energy conscrvation. The dPNa+formed may, in principle, be used directly
Fig. 3. Interconvcrsions of three biological energy currencies. ATPAp., , and ATP-dPNa-transduction are catalyzed by reversible H + ATP and Na+-ATPsynthases, respectively. AjiH I can be equilibrated
with Apha+via Na '/H+ antiporter
by membrane-linked d pNa+ consumers or indirectly, being
first utilized to form ATP by Nat-ATP synthase [46]. Apparently, P . modestum adapted to a niche where succinate is
always available as the end product of fermentation of a
substrate by a succinogenic bacterium coexisting with P. modes turn.
For anaerobic bacteria, it is more typical, however, to
produce ATP by glycolysis and then to perform ATP-driven
work or to generate ApH+(or dpNa+)by H ' (Na')-ATPase
(see above, Fig. 1C and D). Moreover, some anaerobes employ initial or middle spans of the respiratory chain to form
ApH+when reducing electron acceptors other than 0 2 .
Fumaratc-reducing mitochondria of Ascaridae living
under hypoxic conditions in the intestine, as well as fumaratereducing anaerobic bacteria like Wolinella succinogenes, employ the first energy coupling site of the respiratory chain to
produce A j i H + .In the former case this process is catalyzed by
the H 'motive NADH - rhodoquinone reductase, in the latter by the H ' -motive formate - menaquinone reductase (reviewed in [22]).
Succinate formed from fumarate may be used as an oxidation substrate of the nitrate-reducing system and this process may be coupled to the AjiIl+ generation as was shown in
Rhizobium juponicum [93]. Apparently, a ADH + generator corresponding to the second energy coupling site (CoQH2cytochrome c reductase) is involved in this case.
Nitrite, the product of nitrate reduction, may serve as a
substrate for Nitrobacter, which can oxidize nitrite by O2 and
form ApH+by the terminal span of the respiratory chain (the
third energy coupling site, reviewed in [94]). A similar mechanism was described in Thiobacillus ferrooxidans, which uses
Fe2 ' oxidation by O 2 as its only energy-yielding process [22,
951.
Thus each of the three respiratory energy coupling sites is
shown to be used as the sole mechanism of energy production
in some organisms (Fig. 2).
It is, however, obvious that such 'monoenergetic' systems
illustrate extreme cases. Far more often the cell possesses
several mechanisms to obtain convertible energy currencies.
In particular, the respiratory chain usually contains three or
two energy coupling sites. Moreover, respiration and photo-
synthesis are supplemented by glycolysis which forms ATP
with no doHLHt
(doNa+)
involved.
In Fig. 3, interconversions of d o H +doNa+
,
and ATP are
shown. When the A g , , or ApNa+formation is the primary
event, H+-ATP or Na+-ATP synthases produce ATP. When
it is ATP that is primarily formed, these or other enzymes
operate as Hf(Naf)-ATPases. dPNai may or may not be,
equilibrated with d o H +via an N a + / H +antiporter. The equilibration does not occur if A g N a+ serves as the primary energy
currency under conditions where a high Agr3+cannot be
maintained. In such a case, ATP-dpH+energy transduction is
also absent.
Thus, analysis of available information concerning cell
bioenergetics indicates that the three laws formulated in this
paper can be applied to the great majority of energy transduction events in living cells.
REFERENCES
1. Lipmann, F. (1941) Adv. Enzymol. 1, 99-107.
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