Download Magnesium and cell energetics in plants under anoxia

Biochem. J. (2011) 437, 373–379 (Printed in Great Britain)
373
doi:10.1042/BJ20110213
REVIEW ARTICLE
Magnesium and cell energetics in plants under anoxia
Abir U. IGAMBERDIEV*1 and Leszek A. KLECZKOWSKI†
*Department of Biology, Memorial University of Newfoundland, St John’s, NL, Canada, A1B 3X9, and †Department of Plant Physiology, Umeå Plant Science Centre, University of Umeå,
901 87 Umeå, Sweden
Stress conditions (e.g. anoxia) frequently result in a decrease
of [ATP] and in an increase of [ADP] and [AMP], with a
concomitant increase of [Mg2+ ] and other cations, e.g. Ca2 + .
The elevation of [Mg2+ ] is linked to the shift in the apparent
equilibrium of adenylate kinase. As a result, enzymes that
use Mg2+ as a cofactor are activated, Ca2 + activates calciumdependent signalling pathways, and PPi can serve as an alternative
energy source in its active form of MgPPi or Mg2 PPi . Under anoxic
conditions in plants, an important source of PPi may come as a
result of combined reactions of PK (pyruvate kinase) and PPDK
(pyruvate, phosphate dikinase). The PPi formed in the PPDK/PK
cycle ignites glycolysis in conditions of low [ATP] by involving
PPi -dependent reactions. This saves ATP and makes metabolism
under stress conditions more energy efficient.
INTRODUCTION
accumulation affects the equilibrium of the biosynthetic reactions,
and it has to be quickly removed [8]. PPase is inhibited by lower
pH values (characteristic for many stresses) [9], making PPi a
valuable player in plant bioenergetics under stress, especially in
the cytosolic compartment. The plant cytosol has been proposed
previously as an environment where PPi can serve as an alternative
to ATP as the energy donor [10].
In the present review, we analyse the operation of plant
metabolism that, upon stress-induced elevation of [Mg2+ ], uses
PPi as energy currency, and we discuss physiological conditions
that are favourable for this operation. The use of PPi during
early stages of glycolysis makes this process independent of ATP.
Similarly, PPi can replace ATP for efficient pumping of protons
into the vacuole (preventing acidosis). Our and other [11–14]
analyses of sources of PPi in stressed cells point to possibly critical
roles of two enzymes, PPDK (pyruvate, phosphate dikinase) and
PK (pyruvate kinase), involved in net PPi production under stress.
Stress factors severely affect the energy state and ATP levels
in living cells. Changes in ATP content and hence the cellular
energy charge status result either from limitations/inhibition of
ATP synthesis or from an increased ATP consumption. ATP
deprivation is especially evident under low oxygen stress, but also
during chilling, drought, phosphate starvation or in meristematic
tissues [1]. The first consequence of anoxia is a decrease and
then stopping of aerobic respiration [2], hence ATP synthesis
via oxidative phosphorylation is inhibited. Other means of ATP
synthesis, i.e. substrate phosphorylation (resulting mainly from
lactic and alcoholic fermentation) [3] and alternative respiration
using nitrite, have a limited capacity for ATP synthesis [4].
An important consequence of the decrease in [ATP] is a
release of divalent cations such as Mg2+ , Ca2+ and Mn2+ [5].
Although the total concentration of Mg2+ in plant tissues is
in the order of 10 mM [6], a significant amount of Mg2+ is
bound to different metabolites, in particular nucleoside tri- and
di-phosphates and, under normal physiological conditions, the
steady cytosolic concentration of free Mg2+ can be as low as 0.2–
0.4 mM [7]. Generally, nucleoside triphosphates bind Mg2+ very
tightly and those complexes function as cofactors and energyrich compounds. Nucleoside diphosphates bind Mg2+ less firmly,
whereas nucleoside monophosphates bind Mg2+ very weakly
and usually participate in metabolism as Mg2+ -free forms. A
decrease in pH levels observed under stress and, in particular,
in hypoxic conditions, shifts chemical equilibria towards release
of Mg2+ , further increasing its concentration. In these conditions,
the species that normally only loosely bind Mg2+ , such as PPi ,
turn into Mg2+ -complexed physiologically active forms.
Under the conditions of ATP deficiency, plants can use PPi as
another energy currency. This molecule is a byproduct of several
biosynthetic processes, and its utilization is usually linked to a
PPase (inorganic pyrophosphatase) activity. This is because PPi
Key words: adenylate equilibrium, adenylate kinase (AK),
free magnesium, pyruvate, phosphate dikinase (PPDK),
pyrophosphate (PPi ), thermodynamic buffering.
Mg2+ AND ADENYLATES UNDER ANOXIA
A fall in total [ATP] leads to an increase in [Mg2+ ] [7]. This may
compensate for low ATP production in such a way that many
enzymes limited by low [Mg2+ ] become activated and thus cause
an increase in corresponding metabolic fluxes. The release of
Mg2+ also results in displacement of apparent equilibria (K app ) of
certain enzymes. Most importantly, increased [Mg2+ ] leads to a
displacement in the K app of AK (adenylate kinase) [7,15]. Also,
an increase of [Mg2+ ] from 0 to 1 mM results in the displacement
of the K app of aconitase, and the citrate/isocitrate ratio rises from 9
to 21 [16]. This is unfavourable for the TCA (tricarboxylic acid)
cycle turnover and stimulates efflux of citrate from mitochondria
[17]. Yet another important consequence of increased [Mg2+ ]
relevant to the present review is the activation of PPi -dependent
glycolysis and the vacuolar proton-pumping PPase (see below).
Abbreviations used: AK, adenylate kinase; GAP, glyceraldehyde 3-phosphate; IMS, intermembrane space; PEP, phosphoenolpyruvate; PFK,
phosphofructokinase; PFP, PPi -dependent phosphofructokinase; PK, pyruvate kinase; PPDK, pyruvate, phosphate dikinase; PPase, inorganic
pyrophosphatase; SuSy, sucrose synthase; TCA, tricarboxylic acid; UGPase, UDP-glucose pyrophosphorylase.
1
To whom correspondence should be addressed (email [email protected]).
c The Authors Journal compilation c 2011 Biochemical Society
374
A. U. Igamberdiev and L. A. Kleczkowski
AK equilibrates pools of free and Mg2+ -bound adenylates
[18] and establishes a link between [Mg2+ ] and ratios of free
and Mg2+ -bound adenylates, as well as the inner membrane
potentials of mitochondria and chloroplasts [5,7]. The AKgoverned link between contents of adenylates and [Mg2+ ] allows
the calculation of [Mg2+ ] from the K app of AK, which varies from
approximately 0.3 to 1.5, depending on [Mg2+ ] [7]. Under anoxia
in germinating lettuce seeds, concentrations of all three adenylates
(ATP, ADP and AMP) fall within the AK equilibrium and there
is a clear decrease of AK mass action ratio (in particular at 2 kPa
oxygen) [19]. This corresponds to an increase of [Mg2+ ], from
low submillimolar levels (0.2–0.4 mM) to millimolar (∼1 mM),
accompanied by a huge increase of [AMP], a decrease of [ATP]
and small or no changes in [ADP] [19]. Even more striking
increases of [AMP] under oxygen deficiency were observed in
maize [3,20] and Carex [21], and the overall adenylate contents
were well beyond the K app of AK. Although the magnitude of
[ATP] decrease and [ADP] increase was relatively modest (up to
2–3-fold decrease and increase for ATP and ADP respectively),
the hypoxia-induced AMP content increased by 10-, if not 100fold, when compared with oxygen-sufficient conditions [3]. With
nitrate in the medium (compared with chloride), this increase is
even higher. Thus in those cases, low oxygen conditions were
likely to bring about AK-independent mechanism(s) of AMP
production. Simple enzymatic hydrolysis of ADP, e.g. by apyrase
[22], would be, in this case, wasteful from the point of view of
cell energetics. A mechanism which consumes ADP and generates
alternative energy currency will be analysed later in the present
review.
CALCIUM SIGNALLING AS A FEEDBACK OF CHANGES IN [Mg2+ ]
DURING ANOXIA
An important consequence of the AK-governed adenylate
equilibrium in the IMS (intermembrane space) in mitochondria is
its effect on signalling networks. It has been shown that Mg2+ is
an allosteric activator of Ca2 + binding to calmodulin [23], with
the latter activating target enzymes in response to submicromolar
increase in [Ca2 + ]. Changes in internal [Ca2 + ] can result from
changes in [Mg2+ ] at a millimolar range [24]. When ATP levels
decrease under hypoxic conditions and [Mg2+ ] increases, Ca2 + is
released from mitochondria to the cytosol, where its concentration
raises above 0.1 μM [25]. On the other hand, as it was established
for animal tissues, Mg2+ is a strong inhibitor of Ca2 + uptake
into mitochondria [26]. Thus the increase of [Mg2+ ] in the
IMS of mitochondria under intensive respiration can lead to
accumulation of Ca2 + in the IMS. This subsequently leads to activation of Ca2 + -dependent enzymes, including NADPH and
NADH dehydrogenases in the inner mitochondrial membrane
facing the IMS [27] and NAD kinase in the IMS [28]. Thus the
AK equilibrium controls Ca2 + homoeostasis, a primary event in
the signalling network, which subsequently has profound effects
on metabolic events.
The elevation of [Ca2 + ] upon decrease of [ATP] also occurs
because Ca2 + tightly binds to ATP, and the lowering of [ATP]
releases Ca2 + in a similar way as Mg2+ . The concentration
of Ca2 + , similar to Mg2+ , can also be controlled by the AKgoverned adenylate equilibrium [29], with calcium chelated by
the adenine nucleotides to nearly the same extent as magnesium
[30]. As a consequence, the ratio of Catotal /Ca2 + is roughly equal
to that of Mgtotal /Mg2+ [29]. This also means that an increase of
[Mg2+ ] from 0.1 to 1 mM leads to an increase of [Ca2 + ] from
0.1 to 1 μM, assuming that the total concentration of calcium
is 1000-fold lower than magnesium. Under hypoxic conditions,
c The Authors Journal compilation c 2011 Biochemical Society
Ca2 + , besides its known participation in the activation of the
GABA (γ -aminobutyric acid) shunt via activation of glutamate
decarboxylase [31], can participate in the regulation of vacuolar
H + -PPase [32], in activation of external mitochondrial NAD(P)H
dehydrogenases which are also more active at lower pH values
(using NADH and NADPH) [33] and in lower pH-stimulated
activation of NAD-kinase [34]. Thus, in calcium signalling, an
energy deficit becomes an important feedback of Mg2+ release
and pH decrease during hypoxic stress.
Mg2+ AND PPi UNDER ANOXIA
PPi is generated in several biosynthetic reactions such as
activation of sugars (e.g. synthesis of UDP-glucose and ADPglucose), activation of amino acids and fatty acids, and during
elongation reactions for synthesis of proteins and nucleic acids.
In these reactions, the removal of PPi by active PPase drives
the equilibrium in the direction of synthesis [35]. On the other
hand, Baltscheffsky [36] recognized that PPi generated during
biosynthesis is not always wasted but in certain conditions can be
used as energy source. For instance, in developing cereal seeds,
PPi produced in the cytosol during starch biosynthesis can drive
sucrose conversion into glucose 1-phosphate [37]. The usefulness
of PPi as an energy source depends on the energy charge of the cell,
[Mg2+ ] and the actual location of PPi synthesis. PPi is ineffective
at high energy charge values, when ATP is the major energy
source, and the role of PPi can only be apparent under low [ATP]
conditions, which leads to an increase of [Mg2+ ].
Most PPi -dependent enzymes use a MgPPi complex as a
substrate rather than free PPi [38]. The enzymes become
more active upon elevation of [Mg2+ ], and in hypoxia-tolerant
plants, the expression of genes for those enzymes is usually
strongly increased [12]. For instance, PFP [PPi-dependent PFK
(phosphofructokinase)] needs higher [Mg2+ ] than the MgATPdependent PFK, owing to the much lower affinity of Mg2+ for
the complexation with PPi than with ATP (K MgPPi = 1.2 mM − 1
compared with K MgATP = 73 mM − 1 ) [39,40]. Free PPi probably
acts as an inhibitor when the MgPPi complex serves as a substrate
[32]. Figure 1(A) shows relative amounts of free PPi , MgPPi
and Mg2 PPi at different [Mg2+ ]. The specific form of the MgPPi
complex serving as substrate depends on a given enzyme. For
instance, some aminoacyl-tRNA synthetases use MgPPi and
others prefer Mg2 PPi in pyrophosphorolysis [41]. On the other
hand, proton-pumping H + -PPase, localized in the tonoplast of
vacuole [42,43], uses Mg2 PPi rather than MgPPi . This means
that for exhibiting efficient activity, this enzyme needs a further
increase of [Mg2+ ]. The activation process is complex, and
it involves competitive inhibition by Ca2 + , which is released
from vacuoles and mitochondria upon low [ATP] conditions [44]
and non-competitive inhibition by the actual substrate Mg2 PPi .
Free PPi and Mg2+ act as competitive inhibitor and activator
respectively [32]. Thus, for the H + -PPase to effectively pump
protons into the vacuole, the [Mg2+ ] should be in the millimolar
range. Proton pumping by the H + -PPase may be important to
prevent cytoplasmic acidosis; the pump is mainly inactive in
non-stressed cells (when H + -ATPase is operating) [45,46] and
is activated under anoxia [47].
The ratio between free PPi , MgPPi and Mg2 PPi is under
pH control [40]. At lower pH values, which are particularly
characteristic of hypoxic stress, the build-up of MgPPi and Mg2 PPi
can be achieved at lower [Mg2+ ]. The maximal fraction of the
most abundant form, MgHP2 O7 − , is observed at pH 6–6.5 [48].
According to the analysis conducted by Davies et al. [35], the
pH-dependence of the ratio of standard energies of hydrolysis of
Magnesium and cell energetics
375
Figure 2 Equilibrium of adenylates by AK in conjunction with PK and PPDK
reactions
The coupling of AK, PK and PPDK leads to production of large quantities of AMP, which is
considered as one of the major hypoxic metabolites. MgATP appears as an intermediate in the
synthesis of MgPPi .
Figure 1
Basic characteristics of PPi hydrolysis
(A) Relative abundance of free and Mg-bound PPi species at different concentrations of
[Mg2+ ]. (B) pH-dependence of the ratio of standard energies of hydrolysis of PPi and ATP.
(C) [Mg2+ ]-dependence of the ratio of standard energies of hydrolysis of PPi and ATP at different
pH values. The parameters for (A) were extracted from [39].
PPi and ATP shows that the energy of PPi hydrolysis becomes
comparable with the energy of ATP hydrolysis when the pH falls
below 7 (Figure 1B). At the hypoxic cytosolic pH range (pH 6.5–
6.8) [49], the energy of PPi hydrolysis does not decrease with the
increase of [Mg2+ ], and at pH 6.4 even increases when the [Mg2+ ]
increases from submillimolar to millimolar values (Figure 1C).
In plants PPases are presumably exclusively located in the
vacuole and plastids, and the cytosolic PPi pool is not hydrolysed
[50], although a possibility of PPases in the cytosol has been
reported [51]. These PPases are less active at pH values below
7 and are inhibited by Ca2 + accumulating under low [O2 ] [9].
PPi concentrations are in the range 0.04–0.8 mM, and PPi can be
used as an energy donor, instead of ATP, in a variety of situations
[52]. PPi generates equilibrium fluxes to bypass and buffer ATPdependent metabolism [53]. Although the ratios of nucleotides
such as ATP/ADP and UTP/UDP change in a similar manner,
[PPi ] changes independently, thus highlighting its importance as
an autonomous energy donor [54]. When the bacterial PPase was
expressed in potato plants, this led to decreased vitality under O2
deficiency [55].
PK/PPDK SUBSTRATE CYCLE
In animal tissues, AK equilibrium is linked to creatine kinase
equilibrium and equilibration of adenylates is shifted from AK
equilibrium values to higher [ATP] and lower [ADP] supported
by creatine kinase activity [29,56]. On the other hand, in anoxic
plant tissues, AK equilibrium is probably linked to PK/PPDK
equilibration of adenylates. This contributes to higher [AMP]
and [PPi ] under oxygen stress (Figure 2). Both PK and PPDK,
as well as AK, are induced under low [O2 ] [13,14,57] and can
displace the AK equilibrium towards AMP and PPi production.
It is worth mentioning that, during photosynthesis in C4 plants,
AK has an additional task of recycling the AMP produced by
the PPDK reaction [58]. In C4 plants, the PPDK/AK cycle
sustains the formation of PEP (phosphoenolpyruvate), the primary
carboxylation substrate during C4 photosynthesis.
The expression of the gene coding for PPDK is exceptionally
high under low [O2 ] conditions (at least a 100-fold induction)
[13,14]. This seems difficult to explain from the point of view
of switching from PK to PPDK, because PK is neither limited
nor down-regulated. The only way to explain the importance of
PPDK in anaerobic glycolysis is its participation in the substrate
cycle where pyruvate is used in a reverse reaction to form PEP and
AMP [12]. The total balance of the coupled PK/PPDK reactions
is: ADP + Pi = AMP + PPi , the latter serving as an energy source
under anoxia. Thus, under low [O2 ], the PK/PPDK substrate cycle
(Figure 3A) becomes the major non-ATP source of energy [12].
The ATP is used there only as a catalyst (it is produced by PK and
consumed by PPDK). The anoxia-induced increase in [AMP]
[3,20] results, in particular, in stimulation of AMP-activated
protein kinase, which is involved in regulation of many cellular
processes [59], and the [PPi ] increase [60,61] ignites glycolysis at
low [ATP] (see below). On the other hand, [ADP] is not strongly
increased in anoxic conditions [3,19,20]. This is consistent with
the role of PK/PPDK reactions in PPi formation from ADP and
Pi when ATP formation from ADP and Pi is ceased, at least at the
level of oxidative phosphorylation.
The adenylate/Pi /PPi levels regulate PPDK activity through
a unique mechanism in which a kinase/phosphatase uses
ADP/forms AMP during phosphorylation (formation of the
inactive form of PPDK) and uses Pi /forms PPi during
dephosphorylation (formation of the active form of PPDK)
(Figure 3B). Both PPDK activation and inactivation require Mg2+
[62], supporting a view that the true substrates are MgADP
for phosphorylation/inactivation (suppressed by pyruvate) and
MgPi for dephosphorylation/activation (suppressed by MgADP)
[63,64]. This unusual mechanism is not directly controlled by
energy charge (or ATP/ADP ratio) [63], but rather by the
MgADP/MgPi ratio, which is displaced to a higher [MgPi ]
only upon a significant release of Mg2+ . The apparent stability
constant for MgPi is 0.5 mM − 1 , which is less than for MgPPi
(1.2 mM − 1 ) and for MgADP (4 mM − 1 ) [40], which means
that the availability of Mg2+ determines the direction of the
c The Authors Journal compilation c 2011 Biochemical Society
376
A. U. Igamberdiev and L. A. Kleczkowski
achieved by anaplerotic reactions catalysed by PEP carboxykinase
(PEP feeding) and NAD-malic enzyme (pyruvate feeding). Martin
et al. [70] have shown that, under anoxia, PEP carboxykinase
may be subject to a mechanism of post-translational control that
selectively inhibits the carboxylating, but not the decarboxylating,
activity. Expression of the PEP carboxykinase gene in anaerobic
rice was increased by 100-fold, whereas that of PEP carboxylase
decreased [13]. Thus the direction of metabolism in anaerobic
conditions is from oxaloacetate to PEP, opposite to the C4 cycle
in C4 plants. The NAD-malic enzyme activity is promoted by
low pH values and relative insensitivity to higher [NADH], the
conditions common to anaerobic tissues.
The PK/PPDK substrate cycle can be a primary engine for
PPi biosynthesis under anoxia. Other PPi -producing enzymes
are related mostly to biosynthetic reactions and are unlikely
to be connected to PPi formation under anoxia. This includes
the futile cycle of starch formation/breakdown [13,71] and the
unconfirmed putative direct synthesis of PPi by the mitochondrial
ATP synthase [72,73]. The latter is unlikely to be physiologically
important because it cannot be more efficient compared with
ATP synthesis in conditions of low [O2 ] or other limitations
of ATP production. It is also possible that mitochondria
contain a unique H + -PPase which is capable of coupling H +
gradient and PPi hydrolysis or synthesis [74]; however, its
capacity cannot be high when mitochondrial electron transport is
suppressed.
GLYCOLYSIS OPERATING WITH PPi
Figure 3 PK/PPDK substrate cycle (A) and mechanism of regulation of
PPDK (B)
RP, PPDK regulatory protein.
PPDK phosphorylation/dephosphorylation reaction. The use of
MgADP for phosphorylation and MgPi for dephosphorylation
would indicate that phosphorylation occurs at submillimolar
[Mg2+ ], whereas dephosphorylation (reactivation) occurs at
millimolar [Mg2+ ] and is observed when [O2 ] falls to very low
values.
The PK/PPDK cycle is composed of one essentially irreversible
reaction (PK) and one reversible reaction (PPDK). This feature is
essential for providing a homoeostatic flux control [65,66] when
the equilibrium reaction builds up and then buffers and tunes the
concentration of a substrate for a non-limiting non-equilibrium
reaction. The homoeostatic flux control enzymes (such as PPDK)
become important when the flux is increased, such as glycolysis
at low [O2 ]. The reactions exerting homoeostatic flux control are
always present within metabolic cycles; in the Calvin cycle these
are catalysed by NADP-GAP (where GAP is glyceraldehyde 3phosphate) dehydrogenase, transketolase and aldolase [64,65], in
the TCA cycle these are catalysed by malate dehydrogenase [67],
aconitase, fumarase [68] and NADP-isocitrate dehydrogenase
[17].
The importance of PPDK is also evident for developing
and germinating seeds which commonly exhibit anaerobic
metabolism [69]. A relative increase of abundance of the
glycolytic enzymes compared with TCA enzymes in maize
endosperm is consistent with the demonstration of anoxic
conditions during starch accumulation in the endosperm. Under
anoxia, provision of substrates for the PK/PPDK reactions can be
c The Authors Journal compilation c 2011 Biochemical Society
In non-photosynthetic tissues, e.g. roots, sucrose is the major
imported source of carbon, and its oxidation is the major source of energy there. Sucrose hydrolysis can be catalysed by
invertase and/or SuSy (sucrose synthase). Low [O2 ] conditions
are generally limiting to invertase, but not to SuSy [75]. In fact,
for Arabidopsis, the only firmly established requirement for SuSy
is under anoxic conditions (flooded roots) [76]. The ‘coupling’ of
SuSy and UGPase (UDP-glucose pyrophosphorylase) reactions
would result in UTP and glucose 1-phosphate formation from
sucrose, UDP and PPi [77]. In general terms, the PPi formed
from ADP and Pi through the PK/PPDK reactions (Figure 3)
feeds UGPase and PFP reactions, thus saving ATP.
For glycolysis that operates under O2 -limiting conditions
(Figure 4), the overall reaction is the following:
Sucrose + 3PPi + 2Pi + 8ADP + 4NAD + → 4Pyr
+ 8ATP + 4NADH
where Pyr is pyruvate. In this PPi -driven glycolysis, instead of four
ATP built upon the oxidation of one sucrose, as many as eight ATP
are generated at the expense of three PPi . To make these three PPi ,
three turns of the PK/PPDK cycle are necessary, in addition to four
conversions of PEP into pyruvate corresponding to the ‘normal’
glycolysis. This also means that, for operation of such glycolysis,
the PK reaction should operate 1.75-fold faster to provide ATP
for the PPDK reaction. Indeed, in anaerobic rice, one PK gene
(Os09g22410) is up-regulated 8-fold and another (Os01g16960)
4-fold [13].
When PPi is created and its level is high, the PPDK equilibrium
can be displaced towards ATP formation and PPDK will work in
the direction of pyruvate synthesis from PEP, but using AMP and
Magnesium and cell energetics
377
The induction of PPi -dependent cytosolic bypasses may help
plants to survive certain stresses by circumventing ATP-limited
reactions and actually conserve ATP [78]. A possible excess of
PPi could be used by the PPi -dependent proton pumps of the
tonoplast [32], contributing to the avoidance of the cytoplasmic
acidosis induced by O2 deprivation. This represents a strategy
for biochemical adaptation to anoxia aiming at cell survival
by switching to low energy consumption and providing salvage
maintenance of major metabolic pathways.
CONCLUSION
For the equilibration of adenylates under low [O2 ] conditions, one
needs to consider both the AK reaction (mainly in IMSs where it
promotes release of magnesium, calcium and other metal ions),
and the PK/PPDK substrate cycle which forms AMP and PPi
from ADP and Pi . The combined action of AK and the PK/PPDK
substrate cycle results in the maintenance of high [Mg2+ ], [MgPPi ]
and [AMP] under conditions of low [ATP] and moderate [ADP],
with PPi serving as an alternative energy currency. Thus the
release of Mg2+ under low [O2 ] has major consequences for cell
energetics, making it less ATP-dependent and more efficient under
conditions of low energy supply.
FUNDING
The work of our laboratories is supported by the Natural Sciences and Engineering
Research Council of Canada (to A.U.I.) and by the Swedish Research Council (to L.A.K.).
Figure 4
Operation of glycolysis from sucrose to ethanol with ATP and PPi
The PPi can be utilized in three steps (formation of glucose 1-phosphate, fructose
1,6-bisphosphate and pyruvate) and can be formed in two substrate cycles (PFK/PFP and
PK/PPDK), using ATP in the first (which is unlikely in anoxia) and ADP in the second
(which may be the major source of PPi ). Enzymes in the pathway: 1, SuSy; 2, UGPase;
3, hexokinase; 4, phosphoglucomutase; 5, phosphoglucose isomerase; 6, PFK; 7, PFP; 8, PK;
9, PPDK.
PPi, instead of ADP used in the reaction catalysed by PK. In this
case, the summary equation will be:
Sucrose + 7PPi + 4AMP + 4ADP + 4NAD + → 4Pyr
+ 8ATP + 2Pi + 4NADH
This includes:
Sucrose + 3PPi → 4GAP + 2Pi
and then:
GAP + PPi + AMP + ADP + NAD + → Pyr + 2ATP
+ Pi + NADH
In this sequence of reactions, not only ADP but also AMP
and PPi will be used for ATP formation. In conjunction
with AK (reaction: 2ADP→AMP + ATP), PPDK (reaction:
AMP + PPi + PEP→ATP + Pi + Pyr) can be also involved in
extra ATP formation [12] in the following combined reaction
equation: 2ADP + PPi + PEP→2ATP + Pi + Pyr, and the gain of
ATP in glycolysis will be even higher (three ATP from GAP
or 12 ATP from sucrose).
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Received 1 February 2011/14 March 2011; accepted 21 March 2011
Published on the Internet 13 July 2011, doi:10.1042/BJ20110213
c The Authors Journal compilation c 2011 Biochemical Society