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Transcript
86
The Role of ATP in Metabolism
R W HANSON
Department of Biological Sciences
Plymouth Polytechnic
Drake Circus, Plymouth PL4 8AA, UK
Introduction
The thesis presented in most textbooks of biology, cell
biology and (to a lesser extent) biochemistry regarding the
metabolic role of ATP is that the nucleoside triphosphate
acts as the 'energy store' or as the 'energy currency' of the
cell. The following passages from the book by Darnell,
Lodish and Baltimore I are typical:
Cells extract energy from foods through a series of
reactions that have negative free energy changes.
Much of the free energy released is not allowed to
dissipate as heat, but is captured in chemical bonds
formed by other molecules for use throughout the
cell. In almost all organisms the most important
molecule for capturing and transferring free energy is
adenosine triphosphate, or A TP. The useful free
energy in an A TP molecule is contained in highenergy phosphoanhydride bonds (p 42).
The authors justify the designation of a phosphoanhydride
bond as a high energy bond as follows (p 43).
Although this bond is an ordinary bond it is referred
to as a high energy bond because it releases about 7.3
kcal/mol of free energy when it is broken, as in
hydrolysis.
When summarising the involvement of ATP in energetically unfavourable reactions (p 49) it is stated that, All of
these reactions are fueled by the hydrolysis of A TP.
Several aspects of this type of explanation of the role of
ATP are, however, questionable. Most fundamentally,
Banks 2 has claimed that classical thermodynamics, which
applies only to closed systems, cannot be used to analyse
metabolic reactions which occur in open systems and are
under kinetic control. In contrast, Atkinson, 3 and Crabtree and Taylor 4 have argued convincingly that thermodynamics can be used with advantage in discussions of
metabolism. Each component reaction in a metabolic
pathway must move towards equilibrium in the direction
of the pathway, that is, the change in Gibbs function for
every reaction in a pathway must be negative; endergonic
reactions do not occur in metabolism, or indeed elsewhere, despite claims to the contrary. 5 Furthermore, each
reaction must provide products at concentrations which
allow the enzyme catalysing the next step in the pathway
to maintain the pathway flux. Atkinson, 3 and Crabtree
and Taylor 4 have given excellent accounts of how these
two criteria can be assessed using the principles of
equilibrium thermodynamics. It should be recognised,
however, that data calculated assuming that a reaction is
at equilibrium relate to a limiting situation because the
reaction will, in fact, be in a steady state somewhat
displaced from equilibrium.
If it is accepted that thermodynamics is relevant to
BIOCHEMICAL EDUCATION 17(2) 1989
discussions of the role of ATP in metabolism the use of
free energy changes remains a source of much confusion
to graduates and undergraduates alike. Free energy is not
a conventional type of energy, like thermal or electrical
energy, which is conserved according to the first law of
thermodynamics. It is a function of state which was
introduced by Gibbs as an indirect measure of the net
production of entropy in a closed system and its surroundings. 6 In real, spontaneous processes the net
entropy always increases and the Gibbs function (free
energy) always decreases: neither is conserved. Furthermore the change in Gibbs function which accompanies a
reaction is determined by the difference between the
initial and final states of the entire system. It cannot be
ascribed to a single component in the system and still less
to a single bond in any component. These points have
been discussed at length by several authors 2-4'7-1°. It is
surprising, therefore, that many other authors continue to
treat free energy as a caloric substance and that the
concept of the 'high-energy bond' continues to be
propagated. This last particularly in view of the fact that
experimental evidence has been available for some time
which shows that in at least two enzyme-catalysed
reactions cleavage of the terminal phospho-anhydride
bond in ATP does not "liberate significant amounts of
free energy". Bagshaw and Trentham 11 have made a
detailed study of the hydrolysis of ATP catalysed by the
myosin ATPase of muscle fibre. Their results show that
the step in which enzyme-bound ATP and water are
converted to enzyme-bound ADP plus Pi has a Gibbs
function change of only - 5 . 5 kJ mol -~ whereas nonenzymic hydrolysis in free solution (Eqn 1) has AG O' =
-30.5 kJ mo1-1, Keq' = 1.3 x 105.
ATP + H20 ~:~ ADP + Pi
(1)
Furthermore, Boyer and co-workers 12 have shown that
ATP, ADP and Pi are essentially in equilibrium (AG' =
0 kJ mo1-1) when bound to the active site of the
mitochondrial ATP synthetase complex. In both cases,
the steps with large changes in Gibbs function are those
involving substrate binding and product release. Perhaps
when these facts become more widely known the highenergy bond will eventually be laid to rest!
Finally, considerable confusion must arise from the
failure of authors to distinguish between the thermodynamic and kinetic stability of ATP towards hydrolysis,
and the frequent mis-use of the word hydrolysis. The
moderately large change in Gibbs function accompanying
the hydrolysis of ATP (Eqn 1) gives no information
whatsoever about the kinetic stability of ATP in the
presence of water. The nucleoside triphosphate is, in fact,
stable in water for up to 6 months if stored at 4°C; 13 it is
certainly not easily hydrolysed as claimed by Alberts et
al. 14 Furthermore although ATP participates in many
coupled reactions in which it is converted to ADP and Pi
(eg Eqns 11 and 12), they do not involve hydrolysis, that
is, bond cleavage as a result of reaction with water. 15
87
Indeed, the reaction catalysed by the myosin ATPase may
be the only significant metabolic reaction which involves
hydrolysis of the nucleotide. Here, the role of hydrolysis is
not to "generate free energy" but is to displace strongly
bound ATP from the protein so that another cycle of
filament contraction can begin.16
The areas of confusion noted above make the metabolic
role of ATP a very difficult topic to present to undergraduate students, particularly to those, such as biologists,
who may have only a weak background in the physical
sciences. Fortunately a coherent presentation, which
avoids the pitfalls associated with interpretations of
thermodynamic functions of state, can be given in terms of
equilibrium constants and mass action quotients as
described in this article. The concept of an equilibrium
constant is simpler than that of 'free energy' and, unlike
the latter, should be familiar to any student who has
followed an introductory course in chemistry.
Energy, the Capacity for Change
As noted in the introduction, the role of ATP in
metabolism is usually discussed in terms of energy
transduction. Energy, however, is a difficult concept to
grasp. It is defined as the capacity to do work, but since
work itself is often-defined in terms of changes in energy
the definition is somewhat circular. Progress can be made
when it is realised that the performance of work always
involves a change in the condition of the system and/or
surroundings in which it occurs; energy can then be
defined as the capacity to produce, or undergo, change.
This definition is particularly apposite to biological
systems, which are essentially networks of integrated
chemical reactions, because energy transduction in such
systems can then be discussed in terms of the capacity for
chemical change.
The capacity or potential for change in a chemical
system is usually assessed in terms of the associated
change in Gibbs function (AG). The latter is related to the
equilibrium constant for the reaction (K'q) by Eqn 2 in
which R is the gas constant, T the absolute temperature
and Q' is the mass action quotient ([C] ¢ [D]d/[A]a [B] b)
AG' = - R T In
(geq/Q')
(2)
for a generalised reaction aA + bB x=~ cC + dD:
note that the terms refer to initial concentrations, not to
equilibrium values. Primes indicate that each concentration term has been divided by the appropriate standard
state concentration: [H20] = 55.5 mol dm -3, [H ÷] = 1 x
10-7 mol dm -3, all others = 1 mol dm -3. It follows that
any deduction regarding the potential for, and direction
of, reaction made on the basis of AG' can be made equally
well on the basis of the value of the ratio K~q/Q'. If the
ratio is greater than unity, then the reaction will move
towards equilibrium in the forward direction; the concentrations of C and D increase at the expense of those of A
and B until, at equilibrium, Q' = K~q and the larger the
value of the ratio, the greater the potential for change
BIOCHEMICAL EDUCATION 17(2) 1989
towards the equilibrium state. Conversely, if g'q/Q' is less
than unity, the reaction will proceed in the reverse
direction. Again, the value of K'q/Q' is a measure of the
potential for change; in this case the smaller the ratio, the
greater the potential.
It is important to be familiar with a number of other
properties of equilibrium constants 9'17'1s before using
them to explain the metabolic role of ATP. These are:
(1) The equilibrium constant of a reaction is equal to
geq/Q' when Q' is unity; it is thus a measure of the
potential for the reaction to move towards equilibrium
when all of the reactants are present in their standard
states.
(2) Because, by convention, product concentrations
always appear in the numerator of the expression for an
equilibrium constant and starting material concentrations
in the denominator, K'q for the reverse component of a
reaction is the reciprocal of the equilibrium constant for
the forward component.
(3) The ratio K~q/O', like AG', gives no information
whatsoever about the rate at which a chemical change
occurs.
(4) The potential for change, as measured by K'q/Q', is
entirely independent of the route by which the change
occurs. This follows from the fact that the Gibbs function,
G, is a function of state.
(5) The equilibrium composition of a reaction mixture is
unaffected by a second reaction, occurring in the same
phase, provided that there is no interaction between the two
sets of reactants. The situation changes radically, however,
if chemical coupling exists between the two reactions.
Consider, for example, two reactions (Eqns 3 and 4)
coupled by a common intermediate, D, the net effect of
the coupled reactions being described by Eqn 5.
A+Bx~C+D
D + E~F
(3)
+ G
A+B+Ex~C+F+G
(4)
(5)
At equilibrium the concentration of D must be such that it
satisfies the equilibrium expressions for both reactions 3
and 4 (Eqn 6). It then follows, via Eqn 7, that K~q for the
overall reaction is equal to the product of the constants of
the component reactions (Eqn 8).
[DI =
K~q3 [A][B]
[F][G]
[C] = K~q4[E]
(6)
K~q3 x K~q4 = [C][FI[G]
[AI[B][E]
(7)
Keq3 × Keq4 = K~q5
(8)
Since AG' values are additive, this deduction also follows
from the logarithmic relationship between K'q and AG'.
88
Coupling of reactions by a common intermediate is
encountered throughout biochemistry and is extremely
important in enabling cells to produce acceptable concentrations of certain metabolites; this will be illustrated in a
subsequent section.
The Involvement of ATP in Cellular Processes
The importance of ATP in metabolism is indicated by the
estimate that the average human turns over his/her own
body weight (60-90 kg) of it per day. 7 This very large
turnover reflects the fact that ATP is involved, directly or
indirectly* in virtually every area of biosynthesis in cells,
as well as in 'active' solute transport, and in muscle
contraction and similar phenomena. The role of ATP in
each of these areas will now be discussed in more detail.
The Role of ATP in Biosynthesis
An examination of the biosynthetic reactions in which
ATP (or another NTP) is involved shows that the great
majority give rise to condensation products such as
amides, glycosides or esters, (eg, see Eqn 13). Direct
condensation reactions (which do not involve ATP) have
equilibrium constants which are usually very much less
than unity, reflecting the fact that water must be liberated
into a medium in which its chemical potential is already
very high. It is easy to show that reactions of this type
could give rise to only very low concentrations of products
in cells. These generalisations should, ideally, be illustrated using data pertaining to protein, polysaccharide,
nucleic acid or lipid biosynthesis. This, unfortunately, is
not possible. The data for such processes are bound to be
incomplete due to difficulties associated with assigning
realistic values to the concentration of a nascent peptide
chain attached to a ribosome or to the concentration of a
hydrated solid such as glycogen, etc. Data are available,
however, relating to the reactions by which acetyl coenzyme A is produced in certain bacterial (but not in
eukaryotic) cells and, because the role of ATP in the
production of the thiol ester is identical with its role in all
other cellular condensation reactions, these data will be
used to illustrate the role of the nucleoside triphosphate.
The direct condensation of acetic acid with coenzyme A
(Eqn 9)
CH3CO2H + HSCoA ~=~ CH 3 COSCoA + H 2 0 (9)
has Keq = 3 × 10 -6 (see ref 19). This means that if the
steadystate concentrations of acetic acid and of coenzyme
A in a typical bacterial cell are 1 x 10 -3 mol dm -3 and 1 ×
10 -5 tool dm -3, respectively, 3 then the highest concentration of acetyl coenzyme A which could co-exist with
starting materials would be only 3 × 10 -14 mol dm -3.
Most enzyme-substrate combinations have Km values in
the micromolar range or above; a substrate concentration
*All other nucleoside triphosphates can be considered to be
equivalent to ATP because they are derived from the corresponding
monophosphates by two successive phosphorylations, each involving
ATP.
B I O C H E M I C A L E D U C A T I O N 17(2) 1989
of the order of 10 -14 mol dm -3 would mean, therefore,
that the rate of any reaction involving that substrate would
be negligible. Furthermore product concentrations of this
order would clearly be unsatisfactory in situations where a
material, such as glycogen, has to be accumulated. It is
not surprising therefore, that condensation products are
produced indirectly in vivo by processes which, overall,
have much larger equilibrium constants than the corresponding direct condensation.
The corollary of the statement that condensation
reactions have small equilibrium constants is that hydrolytic reactions have large ones. For example, as previously
noted, the equilibrium constant for reaction 1 is 1.3 × 105.
The hydrolysis of ATP shares a reactant, water, with all
condensation reactions. It is, therefore, theoretically
possible to couple the two processes and use the relatively
high potential for ATP to undergo hydrolysis to facilitate
condensation. In the case of acetyl coenzyme A, the sum
of reactions 1 and 9 is given by Eqn 10, for which Keql0 =
K e q l X Keq9 = 0.39.
CH3CO2H + ATP + HSCoA-~CH3COSCoA + ADP + Pi
(10)
Thus the potential for synthesis of the condensation
product from reactants in their standard state would be
increased by a factor equal to K'ql. In more explicit
terms, water produced by the condensation would be
removed by the hydrolysis of ATP. This would maintain
K'qg/Q'9 above unity, and hence allow reaction 9 to
proceed until the concentration of all reactants satisfied
the equilibrium equation for K'ql0. The maximum concentration of acetyl coenzyme A which could be produced
by the coupled reactions would be 1.95 × 10 -5 mol dm -3
assuming the steadystate concentrations of acetic acid and
coenzyme A to be as before and those of ATP, ADP and
Pi to be 5 x 10 -3 mol dm -3, 1 x 10 -4 mol dm -3 and 1 x
10 -2 mol dm -3, respectively. 3 This concentration,
although still small, is well within the normal physiological
range of metabolite concentrations. Thus coupling the
condensation to the hydrolysis of ATP would increase the
maximum concentration of acetyl coenzyme A which can
be formed by a factor of 6.5 x 108. This very large
increase in concentration reflects both the fact that the
equilibrium constant for the condensation would be
increased by a factor of 1.3 x 105 (Keql) and the fact that
cells maintain the mass action ratio (Q'I) for ATP
hydrolysis more than eight orders of magnitude lower
than the equilibrium constant (K~ql/Q'I = 6.5 x 108).
In practice, however, reactions 1 and 9 cannot be
coupled by water. Cells contain approximately 70% water
and both reactions would proceed independently. That is,
a hypothetical enzyme catalysing reaction 1 would not be
able to recognise the water molecules produced by
reaction 9 and only those molecules. Evolution has solved
this problem in an elegant manner, utilising the fact,
stated previously, that the equilibrium constant of a
reaction (and, therefore, the potential for change towards
89
the equilibrium state) is independent of the route by
which the reaction occurs. Enzymes have evolved which
catalyse coupled reactions of the following type. In the
first reaction of a coupled pair, ATP phosphorylates one
starting material of the condensation, producing a product
which, unlike water, can be recognised unambiguously by
the enzyme catalysing the second reaction of the coupled
pair; the material phosphorylated is usually a carboxylic
acid or a reducing carbohydrate. In the second reaction of
the coupled pair the condensation product is generated by
the remaining starting material of the condensation
displacing phosphate from the common, phosphorylated,
intermediate. The coupled reactions of this type which are
used to produce acetyl coenzyme A in certain bacterial
cells are described by equations 11, (AK = acetate
kinase), and 12 (PTA = phosphotransacetylase) and their
net effect by Eqn 13.
CH3CO2H + ATP A__~g
2+K/M CH3CO2P + ADP (11)
acetyl phosphate
PTA
CH3CO2P + H S C o A x=~ C H 3 C O S C o A + Pi (12)
anhydride. The inclusion of the nucleoside triphosphate as
a reactant in a condensation allows the elements of water
to be removed from the reactants, RIOH and HXR 2, and
liberated, not as water, but as protons or as part of
phosphate. The chemical potential of both of the latter
species will be lower in the system than that of water and
on this basis the potential for formation of RIXR 2, will
clearly be increased in comparison with the direct condensation.
Some variation in the general theme described above is
seen in as much as the common intermediate produced by
ATP in coupled reactions is sometimes an adenylic acid
derivative, as in the activation of amino-acids during
protein biosynthesis, or a pyrophosphate derivative as in
the biosynthesis of polysaccharides. In all cases, however,
the immediate role of ATP in biosynthesis can be
summarised as follows: the participation of ATP in
biosynthesis allows cells to produce physiologically
acceptable concentrations of condensation products. The
latter are produced by coupled reactions, involving a
common phosphorylated, pyrophosphorylated, or adenylated intermediate, such that the high potential for ATP to
be converted to ADP + Pi or to AMP + PPi, etc, is
realised without the nucleotide being hydrolysed.
C H 3 C O 2 H + ATP + HSCoA ~ C H 3 C O S C o A
+ADP+Pi
(13)
The latter is identical with Eqn 10 and in keeping with this
the equilibrium constants for Eqns 11 and 12 are 3.6 x
10 -3, and 74, respectively19'2° giving Keq'13 = 0.27*; as
noted previously the maximum concentration of acetyl
coenzyme A which can be produced by reaction (10/13) is
of the order of 10-5 mol dm -3. Thus the high potential for
an aqueous solution of ATP to be converted to an
aqueous solution of ADP and Pi is realised, not by direct
hydrolysis, but by a route which concomitantly produces a
condensation product.
A plausible mechanism for coupled reactions of the
type just described might be as shown in Eqns 14 to 17:
RIOH xo--R10-: + H +
R10-: + M g A D P - P O ] - ~
R
~
~
R
(14)
R1OPO 2- + MgADP(15)
z ~_ R I ~ R z + PO34-
I
H
~:~ R1XR 2 + HPOa 2-
(16)
HPO 2- + H + ~ H2PO 4-
(17)
From this it can be seen that ATP is acting as a
dehydrating agent as would be expected of an acid
*The small difference between this figure and that given previously for
reaction 10 reflects the difficulties associated with the determination of
very large, or very small, equilibrium constants.
BIOCHEMICAL EDUCATION 17(2) 1989
The Role of ATP in Solute Transport
One of the most thoroughly investigated solute transport
systems involving ATP is the calcium pump of the
sarcoplasmic reticulum. 16 In a resting muscle cell the
pump maintains the concentration of calcium ions in the
sarcoplasmic reticulum at a level of 1 x 10-3 mol dm -3
and in the cytoplasm at a level of 1 x 10 -6 mol dm -3.
Unfortunately, quantitative analysis of the system in
terms of Keqand Q' for the ion transfer (Eqn 18)
2+
2Cacyto
~ 2Ca~2+
(18)
is complicated by the existence of an electrical potential
gradient across the membrane. 4 It is clear, however, that
in the absence of the pump, the potential for change
would be such that calcium would re-enter the cytoplasm
(as it does when gated calcium channels are opened on
receipt of a nerve impulse). The presence of the pump
allows ion transfer to occur against the opposing chemical
and electrical potential gradients by coupling it to the
conversion of ATP to ADP and Pi. The pump, which
spans the membrane, is thought to bind two calcium ions
at a high affinity site (E in Eqns 19 to 24) on its
cytoplasmic face (Eqn 19). ATP is then bound at a
separate site and the 13-carboxyl group of an aspartic acid
residue is phosphorylated (Eqn 20). It is suggested that
the introduction of the phosphoryl group allows additional
non-covalent bonds to be formed between it and the
protein such that the conformation of the latter is
changed; the calcium binding site is converted to a low
affinity site (E* in Eqns 21 to 24) which is now accessible
to the sarcoplasmic reticulum (Eqn 21). The calcium ion is
released from this site (Eqn 22) and, finally, the pump is
90
1
returned to its original conformation by hydrolysis of the
13-aspartylphosphate group (Eqns 23 and 24).
2+ ~=~ E.Ca22+
E + 2Cacyto
(19)
E.Ca 2+ + ATP x=~ E - P . C a 2+ + ADP
(20)
M
2
(21)
E*-P.Ca22+ x=~ E * - P + 2Ca~+
(22)
E * - P + H 2 0 Xr~ E* + Pi
(23)
E* x=~ E
(24)
2
L
~p
r
A
E - P - C a 2+ x=~ E*-P.Ca22+
1
M
1
A
t
M
1
~'2
)
A
4
'
2
~)~ADP'Pi
M
2+
2Cacyto + ATP + H 2 0 x=~ 2Ca2~++ ADP + Pi (25)
The overall function of the pump is described by Eqn
25, which is the sum of Eqns 18 and 1. As in the case of the
synthesis of acetyl coenzyme A, the inclusion of ATP as a
reactant in the process will increase the potential for
change in the required direction by a factor of 6.5 × 108.
Thus, the role of the nucleotide is exactly the same as in
biosynthesis - - the high potential for it to be converted to
ADP and Pi is realised, not by direct hydrolysis, but in
such a way that a process which would otherwise have a
low potential for change is facilitated.
ATP and Muscle Contraction
Although the details remain elusive, Huxley's 'sliding
filament' model is widely accepted as providing the best
explanation of the mechanism of muscle contraction. 16'21
Muscle fibres contain interdigitating arrays of thin actin
and thick myosin filaments. 'Cross-bridges' are present
between the filaments where protrusions (headpieces) on
the myosin filament make contact with binding sites on
the actin filament. The essence of Huxley's model is that
the cross-bridges go through an oar-like motion, sliding
the thick and thin filaments past each other, thus
shortening the muscle fibre. After an individual crossbridge has undergone a power stroke, the myosin head
detaches from the actin filament, returns to its original
conformation (the back-stroke of the oar) and then
attaches to an adjacent binding site on the actin filament
(Fig. 1). The sequence of events is then repeated.
In the absence of ATP ADP and Pi, the potential for
the overall change depicted in Fig 1 and Eqn 26 to occur
would be very small.
A1.M + A2 xr:~A2.M + A 1
(26)
Steps 1 and 3 involve mechanical work, while step 2
involves breaking strong protein-protein interactions.
Only step 4, which involves bond formation, would be
expected to have a favourable equilibrium constant. As in
the cases of the biosynthesis of condensation products and
of solute transport, evolution has overcome this problem
by coupling the low potential process to the conversion of
BIOCHEMICAL EDUCATION 17(2) 1989
Figure 1 The cross-bridge model of muscle contraction
ATP to ADP and Pi. The steps in the process may be as
shown in Fig 1 and Eqns 27 to 30.
A1.M.ADP.Pi ~ AI.M * + ADP + Pi
(27)
AI.M * + ATP ~ M*.ATP + A 1
(28)
M*.ATP + H 2 0 ~=~ M.ADP.Pi
(29)
M.ADP.Pi + A 2 ~ A2.M.ADP.Pi
(30)
A1.M.ADP.Pi + A 2 + ATP + H 2 0 ~:~
A2.M.ADP-Pi + A' + ADP + Pi
(31)
In the equations, M represents a myosin head in a
conformation such that it makes an angle of approximately 90 ° to the actin filament to which it binds, while
M* represents a myosin head in a 45° conformation with
respect to the actin filament. As noted previously,
Bagshaw and Trentham's 11 analysis of the hydrolysis of
ATP catalysed by myosin showed that the steps with large
equilibrium constants are those involving substrate binding and product release.
In keeping with this, it is suggested that the force
generating step in muscle contraction is associated with
product (specifically ADP) release from an actomyosin
ADP.Pi complex (Step 1, Fig. 1; Eqn 27). As a result of
product release, the conformation of the myosin head
changes from the 90° state to the 45 ° state and the actin
filament moves with respect to the myosin filament. In
order for further movement to occur, the strongly-bound
myosin must be displaced from the actomyosin complex.
This is achieved by ATP binding when one set of strong
bonds is replaced by another (Step 2, Fig 1; Eqn 28).
Strongly bound ATP must then, in turn, be displaced from
myosin; this is accomplished by hydrolysis (Step 3, Fig 1;
Eqn 29). The free energy change associated with hydrolysis ( - 5 . 5 kJ mol-1), corresponding to a value of K~q/Q'
of approximately 10, indicates that there is a small, but
significant, potential for the change to occur. It is not
91
certain whether the myosin head reverts to the 90°
conformation immediately after detachment from the
actin filament, or whether the conformational change
accompanies hydrolysis of ATP. Finally the myosin
ADP.Pi complex re-binds to actin at a site adjacent to the
previous binding site (Step 4, Fig 1; Eqn 30). The
complete cross bridge cycle is described by Eqn 31.
During muscle contraction, in contrast to the synthesis
of condensation products and ion transport, ATP is
actually hydrolysed. It is important to remember, however, that it is the steps involving ATP binding and
ADP.Pi release which have large equilibrium constants
and which are used to increase the potential for the
change described by Eqn 28 to occur. Hydrolysis of the
nucleotide appears to be used only to convert it from a
tightly-bound form (ATP) to a form (ADP.Pi) which is
potentially easily lost to the medium.
Summary
The preceding discussions of the role of ATP in biosynthesis, solute transport and muscle contraction appear to
be compatible with the description of A T P a s the 'energy
currency, or store' of the cell. The high potential for ATP
to be converted to ADP and Pi is used to increase the
potential for change in other systems. Thus ATP could be
said to supply energy to these systems. 'This view,
however, is too restrictive; in order to obtain a more
realistic assessment of the role of ATP, the overall
metabolism of the cell must be considered.
The concentration of ATP in cells is no higher than that
of many other metabolites and certainly not high enough
for it to act as a store of energy. For it to continue to
function as described, it must be continually re-synthesised from ADP and Pi. Re-synthesis occurs concomitantly with the oxidation of carbon compounds
(oxidative phosphorylation), especially carbohydrates and
fats. The potential for these oxidative reactions to occur is
enormous. For example, the complete oxidation of
glucose (Eqn 32) has Keq = 1 × 1048°:
C6H1206 + 602 = 6CO2 -t- 1 2 H 2 0
(32)
(AG o' = -2850 kJ mo1-1) while the oxidation of palmitic
acid (Eqn 33) has Keq = 1 x 101648 (AG o' = -9781 kJ
m o l - 1 ) . 22
C16H3202 + 2302 = 16CO2 + 16H20
(33)
Comparison of these values with those for reaction 1
shows clearly that ingested carbon compounds, not ATP,
are the energy currency of cells. The immense potential
for oxidative reactions to occur cannot, however, affect
the potential for change in other systems unless some form
of chemical coupling exists between them; ATP provides,
in part, the requisite coupling (Fig 2). The qualification,
'in part', is necessary because the A T P - A D P - P i system is
not the only one which couples oxidation to low potential
processes. The proton circuit which exists across the inner
BIOCHEMICAL E D U C A T I O N 17(2) 1989
~
tm,
irtimo l~id ~
n ~ vt,~am.
~ute l
C02 + 1"t20
Figure 2 Schematic summary o f cellular metabolism
mitochondrial membrane of eukaryotic cells and across
the cytoplasmic membrane in prokaryotic cells also
couples oxidative reactions to a small number of low
potential processes. Thus besides coupling oxidative
reactions to ATP biosynthesis the proton circuit is also
claimed to drive heat production in brown fat mitochondria, the rotation of bacterial flagella and the transport of
a number of solutes. ~ The utility of the two coupling
systems, however, is very different. The proton circuit
must, of necessity, operate across a membrane; it is a
vectorial device which cannot be used in free solution.
Additionally, the chemistry and kinetic versatility of
protons is limited.
In contrast, ATP can be used in free solution, it
participates in a wide variety of reactions and processes
and is clearly very versatile in a kinetic sense. The
numerous functional groups in the nucleotide provide
great potential for bond formation with metal ions and
with functional groups within the active sites of enzymes.
Bond formation can alter the electronic properties of the
nucleotide and also the accessibility of functional groups
within it (by alterations in conformation) and can thus
determine its kinetic reactivity. Indeed, the kinetic properties of ATP are probably at least as important, if not
more important, than its thermodynamic properties in
determining the central role which the nucleotide plays in
metabolism. The equilibrium constant for the hydrolysis
of ATP is important in as much that it must be large
enough to offset the small equilibrium constants associated with condensation reactions and other low potential
processes; it should not, however, be extreme, otherwise
ATP could not be easily synthesised. 2 In contrast, the
kinetic versatility of ATP in the presence of appropriate
enzymes and, usually, magnesium ions allows it to couple
oxidation to a large number of biosynthetic reactions and
other processes and its kinetic stability in the absence of
enzymes allows these reactions and processes to be
controlled by variations in enzyme activity.
In conclusion, the preceding discussions suggest that if
the role of ATP in metabolism is to be dealt with in
'energetic' terms, the nucleotide can be described only as
an energy transducing agent, not as a source, or store, of
energy. ATP is probably best described as a kinetically
versatile coupling agent which allows the high potential
for ingested carbon compounds to be oxidised to be
92
realised in such a way that low potential processes such as
biosynthesis, active transport and muscle contraction are
facilitated.
References
l Darnell, J, Lodish, H and Baltimore, D (1986) 'Molecular Cell
Biology', Scientific American Books, W H Freeman, New York,
42-49
/Banks, B E C (1969) Chemistry in Britain 5,514-519
3Atkinson, D E (1977) 'Cellular Energy Metabolism and its Regulation', Academic Press, New York
4Crabtree, B and Taylor, D J (1979) in 'Biochemical Thermodynamics',
Edited by Jones, M N, Elsevier, Amsterdam, 333-379
5Watson, J D, Hopkins, N H, Roberts, J W, Steitz, J A and Weiner, A
M (1987) 'Molecular Biology of The Gene', Vol 1, Fourth Edition,
Benjamin-Cummings, Menlo Park, p 167
6Allen, A (1983) Trends in Biochem Sci 8, 81-82
7Banks, B E C and Vernon, C A (1978) Trends in Biochem Sci 3,
N156-158
8Novick, S (1976) J Biol Educ 10, 116-118
9Morris, J C (1974) 'A Biologist's Physical Chemistry', Second Edition,
Edward Arnold, London
l°Chappeli, J B (1977) 'ATP', Carolina Biology Reader No 50, Carolina
Biological Supply Co, Burlington, NC, USA
llBagshaw, C R and Trentham, D R (1983) Biochem J 133,323-328
~2Reported by Hatefi, Y (1985) Ann Rev Biochem 54, 1075-1089
~3Bioehemica Information II (1975), Boehringer Mannheim GmbH, 21
a4Alberts, B, Bray, D, Lewis, J, Raft, M, Roberts, K and Watson, J D
(1983) 'The Molecular Biology of the Cell', Garland, New York, p 67
15Hawsley, M G G (1981) in 'The Condensed Chemical Dictionary',
Tenth Edition, Edited by Sobers, H A, Van Nostrand Reinhold, New
York, p 546
a6Eisenberg, E and Hill, T L (1985) Science 227, 999-1006
17Harris, W F (1982) J Chem Educ 59, 1034-1036
18McPartland, A A and Segal, I H (1986) Biochem Educ 14, 137-141
~9jencks, W P (1968) in 'Handbook of Biochemistry', Second Edition,
CRC Press, Cleveland, OH, J181-J186
2°Stadman, E R (1955) Meth Enzymol Vol 1,596-599
21Hibberd, M G and Trentham, D R (1986) Ann Rev Biochem 15,
119-161
22Lehninger, A L (1970) 'Biochemistry', Worth, New York, p 298
23Harold, F M (1986) 'The Vital Force: A Study of Bio-energetics', W H
Freeman, New York
BIOCHEMICAL EDUCATION 17(2) 1989
Does Free-energy Change of Individual Reactions
Alone Determine the Flux through a Metabolic
Pathway?
ABRAHAM K ABRAHAM
Department of Biochemistry
Faculty of Medicine
University of Kuwait, Kuwait
Introduction
One of the most important facts that needs to be known
about a metabolic pathway is the flux through the
pathway. This can be determined from the free energy
change, AG, of its individual reactions. Most textbooks of
biochemistry emphasise this point. Thus for any given
metabolic pathway, the flux through the pathway in the
forward direction is possible only if the AG values of all
individual reactions are negative, except in the case of
coupled reactions. In other words, it is not sufficient that
the overall value of free-energy changes is negative. Yet,
when this point is described in more detail later in the
book by providing the AG values of glycolytic reactions, ~'2
the data presented seem to confuse the students.
Free Energy
Several text b o o k s 1-3 provide the standard free-energy
change, AG °, and the actual free-energy change, AG,
calculated from mass action ratios for individual reactions
of the glycolytic pathway. One of the reasons for
presenting these details is probably to illustrate that while
the standard free-energy change, AG ° , of some reactions
have a positive value, the actual free-energy change, AG,
of all reactions are negative, thus re-emphasising the basic
concept taught earlier under bioenergetics. However, the
data presented show that the actual free-energy change,
of several reactions of this pathway are positive. 1"2 This
increase in the free-energy depicted for several reactions
has been pointed out, and explained as deriving from
errors in experimental measurements, z
Another text book provides no explanation for the
positive AG values.1 Two out of the three reaction steps
with positive AG values do not confuse the majority of
students. 1 These reactions are: (1) conversion of dihydroxyacetone phosphate to glyceraldehyde 3-phosphate
by triosephosphate isomerase, and (2) the conversion of
1,3-glycerate bisphosphate to 3-phosphoglycerate by
phosphoglycerate kinase. In the first case, being a side
reaction, it is easy to understand that this reaction does
not prevent the flux of glucose along the glycolytic
pathway. In the second case, if a student is able to recall
that the reactions catalysed by glyceraldehyde 3-phosphate dehydrogenase and phosphoglycerate kinase are
coupled, they should be able to work out the overall freeenergy change which is in fact negative.
For the reaction catalysed by phosphoglyceromutase ie
conversion of 3-phosphoglycerate to 2-phosphoglycerate,
also shown with a positive AG value, 1 it is difficult for the
students to understand how glycolysis can proceed beyond