Energy metabolism overview
In the figure below, a general outline of energy metabolism is given
There are four distinct segments: glycolysis, conversion of pyruvate to acetyl-CoA
the citric acid cycle, and oxidative phosphorylation driven by the electron transport chain. In
some simple organisms, pyruvate is converted to ethanol
and glycolysis is the only source for generation of ATP. In muscle, this can also be the case but
with the conversion of pyruvate to lactate instead.
Remarkably, in muscle, glycolysis is a rapid way to generate ATP compared to the citric acid
cycle and electron transport generation of ATP. Thus in situations of heavy need, glycolysis does
the job, with the help of ATP regeneration from a phosphagen, the molecule phosphocreatine
that is stored for this purpose. During rest, the entire ATP generation pathway with the citric acid
cycle and the electron transport chain is used to generate ATP and to recharge the phosphagen
stores. Glycolysis produces a net of 2 ATPs per glucose molecule whereas the electron transport
chain accounts for 32 ATPs per glucose. The details of these processes are given below.
The overall result of glycolysis, a ten step reaction pathway, is the conversion of glucose
to pyruvate
glu cos e 2 NAD 2 ADP 2 P
2 pyruvate 2 NADH 2 ATP 2 H 2 O 2 H
The standard state Gibbs free energy change at pH 7 is G0 8.5kcal / mol . However,
inclusion of the actual physiological concentrations of reactants yields G 18.5kcal / mol . In
going from glucose to 2 pyruvates, 4 hydrogens and 2 H+s have been lost. Two hydrogens end up
on 2 NADH’s, as do 4 electrons leaving 4 H+s. The remaining 2 H+s are in the 2 water molecules
because the conversion of ADP and P to ATP is a bit more complicated at pH 7. At pH 7 the
correct charge states for these phosphate species are ADP3-, P2-, and ATP4-. Thus the generation
of of ATP actually is given by
ADP3 P2 ATP4 HO
+
and it takes an H to convert HO to H2O.
The overall result of the conversion of pyruvate to acetyl-CoA, a 6 step pathway
occurring on a multienzyme complex, is given by
2 pyruvate 2CoA SH 2 NAD
2acetyl CoA 2CO2 2 NADH
The Gibbs free energy change for this sequence is G0 16.0kcal / mol . Acetyl-CoA carries
the two carbon compound acetate to the citric acid cycle where it is completely oxidized.
The overall result of the citric acid cycle may be expressed by
2acetyl CoA 6 NAD 2 FAD 2GDP 2 P 4 H 2O
2CoA SH 6 NADH 4 H 2 FADH 2 2GTP 4CO2
The Gibbs free energy change is G0 25.6kcal / mol .
The overall result of the electron transport chain appears to be
10 NADH 6H 2FADH 2 6O2 12H 2O 10 NAD 2FAD
with a Gibbs free energy change of G0 672kcal / mol . However, both the citric acid cycle
and the pyruvate dehydrogenase enzyme complexes are contained within the mitochondrial
matrix and the 8 NADH’s they produce are readily available to the electron transport chain
complexes located in the inner mitochondrial membrane that borders the matrix. The 2 NADH’s
produced by glycolysis, however, are initially exterior to the mitochondrial outer membrane.
They easily cross the outer membrane but the inner membrane is impermeable to them. The
glycerol phosphate shuttle is the mechanism by which the electrons on NADH are communicated
to the interior of the inner membrane. NADH itself doesn’t actually get inside and the glycerol
phosphate shuttle places the electrons on FAD instead, forming FADH2. Thus, from the
perspective of the inner surface of the inner mitochondrial membrane where electron transport is
initiated, the overall reaction equation should read
8NADH 4H 4FADH 2 6O2 12H 2O 8NAD 4FAD
The NADH’s enter the electron transport chain at the top of the chain and they yield enough
energy in mitochondria for the synthesis of 3 ATP’s. The FADH2’s, however, enter the electron
transport chain further down the chain at the CoQ site. This yields enough energy for only 2
ATP’s. The net result is 24 ATP’s from 8 NADH’s, 8 ATP’s from 4 FADH2’s, 2 ATP’s from
glycolysis and 2 GTP’s from the citric acid cycle. This is a grand total of 36 high energy
phosphate bonds, 34 ATP’s and 2 GTP’s. Only 2 high energy phosphate bonds arise from
glycolysis, the only source in anaerobic organisms and in highly active muscle tissue.
When all of these steps are combined the overall result is
glu cos e 2 ADP 2 P 2GTP 2 P 6O2
6CO2 10 H 2 O 2 ATP 2GTP energy
The energy is maintained by the membrane potential and can be converted into the synthesis of
32 ATP molecules from 32 ADP’s and 32 P’s. This balance sheet for the complete oxidation of
glucose to CO2 and H2O differs from that for the simple burning of glucose in air which would
read
glu cos e 6O2 6CO2 6H 2O heat
in the presence of the phosphates and 10 H2O’s. The extra 4 H2O’s represent the dehydration
condensation of 2 ADP’s, 2 GDP’s and 4 P’s to 2 ATP’s and 2 GTP’s, i.e. the reversal of these
syntheses is hydrolysis.
In standard state the Gibbs free energy change for the simple burning of glucose is
0
G 6(94.3 56.7) (219.1)kcal / mol 686.9kcal / mol . In the slow burning of the energy
metabolism pathways 36 high energy phosphate bonds are harvested at a standard state cost of
about 7.3 kcal/mol. Thus, 262.8/686.9, or 38% of the available energy is harvested. Compared
with, say, man made photovoltaic cells that have efficiencies less than 10%, and the nuclear
power plant, the Connecticut Yankee on the Connecticut River that functions like a Carnot cycle
engine at 33%, this is very impressive.
Energy metabolism
Several energy metabolism pathways mentioned earlier are considered in greater detail
here. These pathways are central to metabolism generally and they contain mechanisms and
processes that are basic to an understanding of nanobiology. The presentation covers glycolysis,
the conversion of pyruvate into acetyl-CoA, the citric acid cycle, the glyoxylate cycle and the
Calvin cycle.
Glycolysis
Glycolysis ( from the Greek glycos for sweet and lysis for dissolution) is a ten step
sequence of reactions in which glucose is partially oxidized to two molecules of pyruvate along
with the generation of two molecules of NADH and a net gain of two molecules of ATP. The
pathway is given in the figure.
The first and third steps each require a molecule of ATP for the phosphorylation of glucose and
then of fructose-6-phosphate. This activates the pathway and results in the breaking up of the six
carbon fructose-1,6-bisphosphate into two three carbon molecules, both of which ultimately end
up as glyceraldehyde-3-phosphate. The rest of glycolysis involves the conversion of each
molecule of glyceraldehyde-3-phosphate into pyruvate, along with generation of NADH, two
ATP’s and one molecule of water. Steps six and seven make up the prototype for oxidative
phosphorylation in which an oxidation step, the oxidation of glyceraldehyde-3-phosphate by
NAD+, is coupled by a phosphorylated intermediate, 1,3-bisphosphoglycerate, to the
phosphorylation of ADP, making ATP. A subsequent phosphorylation generates another
molecule of ATP from ADP as phosphoenolpyruvate is converted into pyruvate in step ten. Both
phosphorylations involve chemical coupling and the the first was taken as the paradigm for
oxidative phosphorylation generally for many years. This conception of the mechanism made the
discovery and acceptance of the chemiosmotic mechanism of ATP generation much more
difficult to achieve.
The oxidative phosphorylation steps in glycolysis, steps six and seven, occur sequentially
on an enzyme that catalyzes both the oxidation and the phosphorylation. The NAD+ participates
as a coenzyme in this process. This means that a key catalytic step is mediated directly by NAD+
and not by the enzyme protein itself. The enzyme protein, called the apoenzyme (from the Greek
apo meaning away from), together with the coenzyme, in this case NAD+, make up the
holoenzyme (from the Greek holo meaning whole). The phosphorylation step, however, is
catalyzed by the enzyme through the mediation of a sulfhydryl group, -SH, on an amino acid
cysteine residue and the help of an appropriately positioned basic amino acid residue. This is
shown in the figure.
This mechanism introduces what will be a recurring theme, the catalytic involvement of sulfur
atoms in the formation of thioesters. There is reason to believe that this represents an
evolutionarily primitive step.
The reaction sequence begins with the holoenzyme containing bound coenzyme NAD+.
The basic amino acid residue takes up the sulfhydryl proton of cysteine as the sulfur reacts with
the aldehyde end of glyceraldehyde-3-phosphate by a nucleophilic attack of the aldehyde carbon
by sulfur, producing what is called a thiohemiacetyl intermediate. The next step is the oxidation
of the bound intermediate by NAD+ producing a thioester. Exchange of NADH for NAD+
recharges the coenzyme site with oxidized NAD+. The released NADH is either reoxidized later
on when pyruvate is reduced to ethanol or to lactate in anaerobic metabolism, or it is reoxidized
by the electron transport chain in aerobic metabolism. An oxygen atom of inorganic phosphate
nucleophilically attacks the thioester carbon atom and a transthiolation occurs in which 1,3-
bisphosphoglycerate ester is formed. Simultaneously, the reduced basic amino acid residue gives
its proton back to the sulfur atom of the apoenzyme’s cysteine residue. The holoenzyme is
completely regenerated for another cycle, although its NAD+ is not the same one with which it
started. The 1,3-bisphosphoglycerate is energy rich and can phosphorylate ADP making ATP
with an overall decrease in Gibbs free energy.
Notice that to initiate the pathway, two ATP’s are required and in the second half of the
pathway two ATP’s are produced for each of two glyceraldehyde-3-phosphates. Thus there is a
net gain of two ATP’s. However, the four ATP’s produced as output can initiated two copies of
the pathway, yielding eight ATP’s as output. These can prime four copies of the pathway,
yielding 16 ATP’s and so on. This creates an exponential increase in ATP production provided
there are enough copies of the enzymes for the pathway in the cell. The glycolysis enzymes exist
inside the cytosol of the cell in a great many multiple copies. They are loosely associated with
each other and substrate diffuses from reaction site to reaction site in order to complete the
pathway. This is quite primitive compared to the orderly arrangement of enzymes for the
electron transport chain in the mitochondrial membrane.
The requirement for priming and the subsequent exponential increase in ATP production
can be seen in in vitro experiments as an initial induction phase for the process. The enzymes,
glucose and ADP and phosphate are incubated together at appropriate temperature, pH and ionic
strength. Enough ATP will spontaneously form from ADP and phosphate to prime the reaction
although this takes some amount of time. This is seen as a delay in the onset of exponential ATP
production.
Conversion of pyruvate to acetyl-CoA
The coupling of glycolysis to the citric acid cycle requires the conversion of pyruvate into
acetyl-CoA, a thioester.
pyruvate CoA SH NAD acetyl CoA CO2 NADH H
This is a five step process that takes place on a multienzyme complex that self-assembles from
its constituent subunits. It involves five coenzymes, TPP (thiamine pyrophosphate), LSS (lipoic
acid), FAD (flavin-adenine dinucleotide), NAD+ and CoA-SH. Coenzymes TPP and LSS are
depicted in the figure.
Notice the frequent occurrence of phosphate and ribose in these five coenzymes. These
coenzymes had to evolve only after both ribose and phosphate were plentiful constituents of
primitive metabolism.
The multienzyme complex on which this process takes place contains multiple copies of
three enzyme subunits labeled E1, E2 and E3. These subunits self-assemble into a cubic structure.
The core is made up of 24 E2 subunits, two for each of the twelve edges of the cube. Assembled
on this core are 24 E1 subunits, again two per edge. In addition, 12 E3 subunits adorn each of the
cubic faces, two per face. A schematic of the reaction sequence is given in the figure.
The next figure shows the involvement of FAD in greater detail indicating that additional
sulfhydryl groups are part of the FAD/FADH2 redox cycle. This part of the pathway is
remarkably similar to the mechanism of glutathione reductase. In fact, the structure of E3 from
bacteria is incredibly similar to that of human glutathione reductase.
The carboxyl group of lipoic acid is connected to the enzyme complex through an amino
acid lysine residue of the E2 subunit. The -amino group of the lysine and the carboxyl group of
the lipoic acid form a peptide bond. This combination creates a freely rotating arm 1.4 nm long.
The transfer of acetate from thiamine pyrophosphate to coenzyme CoA is mediated by
attachment of acetate to the sulfhydryl group of the rotating lipoic acid. This results in a
reduction of the lipoate sulfur atoms. Their re-oxidation is catalyzed by the FAD’s on the E3
subunits
A very similar structure exists for the multienzyme complex -ketoglutarate
dehydrogenase that occurs in the citric acid cycle and produces succinyl-CoA.
ketoglutarate CoA SH NAD succinyl CoA CO2 NADH H
The same five coenzymes are involved and the lipoyllysine rotating arm serves the same
purpose. While the E1 and E2 subunits are different in -ketoglutarate dehydrogenase, the E3
subunit is the same as in pyruvate dehydrogenase. The degradation of the amino acids isoleucine,
leucine and valine involves a similar complex. This exhibits the conservative nature of molecular
and cellular evolution in that a mechanism is used for several different purposes once it has
evolved for a single purpose.
Citric acid cycle
The citric acid cycle is a major hub for metabolism
Fatty acid and amino acid metabolism are directly connected to citric acid intermediates. The
cycle generate reduced coenzymes NADH and FADH2 that ultimately drive the electron
transport chain and the production of chemiosmotic membrane energy. Thus, the citric acid cycle
is a major power generator as well. The cycle involves eight enzymes, all located in an organized
way inside the mitochondrial matrix
The enzymes are: (1) citrate synthase catalyzing the hydrolytic release of CoA-SH and
incorporation of acetate into oxaloacetate to form citrate; (2) aconitase, an FeS containing
protein [4Fe-4S] catalyzing the elimination of a molecule of water from citrate to form cisaconitate, an enzyme bound intermediate, and then catalyzing the rehydration of cis-aconitate
with the same eliminated water molecule to form isocitrate; (3) isocitrate dehydrogenase
catalyzing the oxidation of isocitrate by NAD+ to form enzyme bound intermediate,
oxalosuccinate, which is decarboxylated to -ketoglutarate; (4) -ketoglutarate dehydrogenase
catalyzing the oxidative (by NAD+) decarboxylation (elimination of CO2) and incorporation of
CoA-SH to form succinyl-CoA; (5) succinyl-CoA synthase catalyzing the formation of succinate
along with the elimination of CoA-SH and the phosphorylation of GDP to GTP; (6) succinate
dehydrogenase catalyzing the oxidation of succinate by FAD to form fumarate; (7) fumarase
catalyzing the hydration of fumarate to form malate; and (8) malate dehydrogenase catalyzing
the oxidation of malate by NAD+ to form oxaloacetate which completes the cycle. The similarity
of -ketoglutarate dehydrogenase to pyruvate dehydrogenase was already mentioned in the last
chapter. Unlike their role in the elctron transport chain where FeS proteins are associated with
redox processes, in aconitase the role is different. In this case, one special iron atom in the cluster
is centrally involved in the coordination of the water molecule that is first eliminated from citrate
and then used to rehydrate cis-aconitate. The formation of GTP is another example to be added
to the two examples from glycolysis of phosphorylation of a nucleotide diphosphate to make a
triphosphate by chemical coupling.
The mechanism of GTP formation in the citric acid cycle is of interest because of the
involvement of the amino acid histidine. This mechanism is shown in the figure.
Inorganic phosphate nucleophilically attacks the ester carbon of succinyl-CoA, releasing CoASH and forming succinyl-phosphate, a high energy phosphate intermediate. Histidine, an amino
acid constituent of the enzyme in turn nucleophilically attacks the phosporus atom of succinylphosphate to form the enzyme bound 3-phospho-histidine and free succinate. Finally, a terminal
oxygen of GDP nucleophilically attacks the phosphorus atom of 3-phospho-histidine to form
GTP and a normal histidine residue. Histidine plays a key role in the mechanisms of enzymes
(1), (2) and (7) as well and in (7) the FAD is covalently bound to a histidine residue of the
enzyme.
For the citric acid cycle, acetate is oxidized to 2 CO2’s, 3 (NADH+H+) and 1 FADH2,
along with 1 GTP. The electron transport chain can generate 3 ATP’s from 1 NADH and 2
ATP’s from 1 FADH2. Thus, one passage through the citric acid cycle can generate 3 x 3 + 2 + 1
= 12 high energy phosphate bonds. For glucose, 2 acetates are produced, as well as a net of 2
ATP’s during glycolysis. Thus 2 x 12 + 2 = 26 high energy phosphate bonds can be made. As
pointed out in the previous chapter, glycolysis also generates 2 NADH’s per glucose and the
pyruvate dehydrogenase reaction does too. At a possible 3 ATP’s per NADH, it appears that 12
more ATP’s are possible. However, those NADH’s from glycolysis have the get their electrons
to the electron transport chain and this is done through the intermediacy of FADH2 which can
only generate 2 ATP’s per FADH2. As described in the last chapter, the NADH2’s from pyruvate
dehydrogenase don’t have this problem. Thus, there are only 10 extra ATP’s possible for a grans
total of 26 + 10 = 36 high energy phosphate bonds.
Glyoxylate cycle
In bacteria and plants there exists the glyoxylate cycle which is a shortened version of the
citric acid cycle that generates the four carbon compound succinate.
2acetyl CoA NAD 2H 2O succinate 2CoA SH NADH H
This cycle is given in the figure.
In plants, this cycle involves the organelle called the glyoxysome. The reaction cycle is divided
between the glyoxysome and the plant mitochondria. The net result is
2acetyl CoA NAD 2H 2O oxaloaceta te NADH H
What happens is that a portion of the citric acid cycle occurring in the mitochondrial matrix is
coupled to a portion of the total glyoxylate cycle occurring in the glyoxysome that resides in the
cellular cytosol. Inside the mitochondrial matrix, oxaloacetate from the citric acid cycle is
aminated by the enzyme aspartate aminotransferase using the amino acid glutamate as amino
group donor producing aspartate, another amino acid, and -ketogluarate, the deamination
product of glutamate. Asparate and -ketoglutarate can be transported out of the mitochondria
into the cytosol and then into the glyoxysome. Inside the glyoxysome, -ketoglutarate and
aspartate react to produce oxaloacetate and glutamate, the reverse of the initial mitochondrial
reaction. This is also catalyzed by aspartate aminotransferase, but inside the glyoxysome. The
glutamate can be transported back into the mitochondrial matrix for another round. Inside the
glyoxysome the steps of the cycle leading from oxaloacetate to malate, with the production of
succinate, take place. The succinate is transported back into the mitochondrial matrix where it is
converted into oxaloacetate by a portion of the citric acid cycle. The malate is transported out of
the glyoxysome into the cellular cytosol where it is converted into oxaloacetate by malate
dehydrogenase. Here, the oxaloacetate is available for the synthesis of glucose, a process called
gluconeogenesis. This means that the acetate of acetyl-CoA is ultimately converted into glucose
that may fuel the germination of plant seeds. The acetyl-CoA is produced by the oxidation of
fatty acids, in particular triacylglycerols, that are the latent fuel for the seeds.
Calvin cycle
The pathways of glycolysis, the citric acid cycle and electron transport are designed to
catabolize glucose. They oxidize glucose completely to CO2 and water and capture much of the
energy released by this process as high energy phosphate bonds, principally in ATP, and as
energized membrane potential. The glyoxylate cycle, however, is aimed at anabolism, i.e. the
generation of glucose from the smaller acetate precursors. In plants, an even grander anabolic
process occurs in which water and CO2 are combined to make glucose and other sugars. This
process begins by harnessing energy from a light driven electron transport chain that partially
transduces the light energy into the energy of ATP and the energy of reducing potential in a
variant of NADH, NADPH (nicotinamide adenine dinucleotide phosphate)
The Calvin cycle utilizes this energy to condense water and CO2 into sugar molecules.
The Calvin cycle interconverts twelve different carbohydrate species ranging from
compounds with three carbons to those with seven. The twelve species names, abbreviations and
chemical structures are:
glyceraldehyde-3-phosphate (GAP)
dihydroxyacetone-phosphate (DHAP)
erythrose-4-phosphate (E4P)
fructose-1,6-bisphosphate (FBP)
fructose-6-phosphate (F6P)
sedoheptulose-1,7-bisphosphate (SBP)
sedoheptulose-7-phosphate S7P)
xylulose-5-phosphate (Xu5P)
ribulose-5-phosphate (Ru5P)
ribose-5-phosphate (R5P)
3-phosphoglycerate (3PG)
1,3-bisphosphoglycerate (BPG)
Using these abbreviations the reaction pathway is represented as
The key step is the incorporation of a molecule of CO2 and a molecule of H2O into Ru5P to
make rwo molecules of 3PG. Thus, a five carbon sugar-phosphate adds one more carbon from
CO2 to make two three carbon molecules of 3PG.
Imagine initiating the pathway with 5 molecules of GAP (15 carbon atoms). If two go by
path a and one each take paths b, c and d, then the result is three Ru5P’s (15 carbon atoms), one
by way of R5P and two by way of Xu5P. The ATP and NADPH activation steps convert these
three Ru5P’s into six GAP’s. This is a multiplicative factor of 6/5. Recall that in glycolysis the
ATP generation factor was 4/2 and caused exponential growth of the ATP population. Here, 6/5
will do the same thing, albeit at a slower rate of exponentiation. If instead, this pathway is
initiated with just three GAP’s (9 carbon atoms) and one each take paths a, b and c, then one
molecule of Xu5P (5 carbon atoms) and one molecule of E4P (4 carbon atoms) are produced.
The Xu5P converts to Ru5P which can be activated to make two GAP’s (6 carbon atoms). One
of these takes path a to combine with E4P to make S7P. The remaining GAP takes path d and
combines with S7P to ultimately make two Ru5P’s. These are then activated to produce 4 GAP’s
(12 carbon atoms). Save one of these GAP’s and convert the remaining three into four as was
just done and the result is five GAP’s, enough to prime exponential production.
The primary product of the Calvin cycle is GAP. By means of other pathways, GAP can
be converted into glucose-1-phosphate as well as many other products. The primary light
reactions of photosynthesis occur inside chloroplasts in the thylakoid (from the Greek thylakos
meaning sac or pouch) membranes
much like the electron transport chains in mitochondria. The Calvin cycle occurs in the stroma of
the chloroplast. The stroma is the analogue of the mitochondrial matrix, where the citric acid
cycle takes place. GAP must be exported to the chloroplast exterior in order to be utilized for
other syntheses.
View publication stats