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in our bodies , the conversion of metabolite A
to metabolite B with release of free energy is
coupled to another reaction in which free
energy is required to convert metabolite C to D.
Many coupled reactions use ATP to generate a
common intermediate. These reactions may
involve ATP cleavage—that is, the transfer of a
phosphate group from ATP to another
molecule. Other reactions lead to ATP synthesis
by transfer of phosphate from an energy-rich
intermediate to ADP, forming ATP.
In order to maintain living processes, all
organisms must obtain supplies of free energy
from their environment.
Generally, human beings obtain energy by
coupling their metabolism to the breakdown
of complex organic molecules in their
environments.
ATP plays a central role in the transference of
free energy from the exergonic to endorgonic
reactions.
Reactions or processes that have a large
positive ∆G, such as moving ions against a
concentration gradient across a cell
membrane, are made possible by coupling
the endergonic movement of ions with a
second spontaneous process with a large
negative ∆G, such as the hydrolysis of
adenosine triphosphate (ATP).
ATP consists of a molecule of adenosine (adenine
+ ribose) to which three phosphate groups are
attached.
Three phosphoryl groups are sequentially
linked to the fifth position of a ribose moiety
via the phosphoester bond, followed by two
phosphoanhydride bonds. Two terminal
phosphorly groups, i.e the ɣ and β phosphoryl
groups, are involved in phosphoric acid
anhydride bonding and are designated as
energy-rich bonds or high-energy bonds,
which are symbolised (῀ )
If one phosphate is removed, adenosine diphosphate
(ADP) is produced; if two phosphates are removed,
adenosine monophosphate (AMP) results. The
standard free energy of hydrolysis of ATP, ∆G°, is
approximately -7.3 K cal/mol for each of the two
terminal phosphate groups. Because of this large,
negative ∆G°, ATP is called a high-energy phosphate
compound.
the second reaction (ADP→AMP+Pi) is of less
biological importance than the ATP to ADP hydrolysis.
ATP act as energy transporter (energy
currency), it can provide energy through
hydrolysis and release of phosphate
groups to force endergonic reactions to
take place, to provide mechanical energy
(muscle movement) and heat energy ( to
maintain body temperature).
It is also possible to go directly from ATP to AMP by
cleaving a pyrophosphate group (but the
produced energy for such reaction is only -10.9k
cal/ molecule).
ATP → AMP + PPi
Note 1: not only ATP act as energy transporter but
other nucleotides also can play the same function
like GTP and UTP and produce the same energy as
ATP.
Note2: in our bodies there are different high energy
molecules that when catabolized can yields
energy to be transported to the site of utilization
by ATPs
e.g1:phosphoenolpyruvate can yields(-14.8
Kcal/molec.), 1,3 bisphosphoglycerate (-11.8
Kcal/molec.) And acetyl phosphate (-10.3
Kcal/molec.) which are important for the
conservation of energy.
e.g2: phosphocreatine (-10.3 Kcal/molec.) is stored
in muscles and can rapidly converted to ATP to
supply muscle contractions.
Production of phosphocreatine occurs when ATP
concentration is high and the reverse occur
when ATP concentration is low.
ATP
ADP+phosphocreatine
+creatine
Table 11–1. Standard Free Energy of Hydrolysis of Some
Organophosphates of Biochemical Importance
High-Energy Phosphates Are Designated by
The symbol
indicates that the group attached
to the bond, on transfer to an appropriate
acceptor, results in transfer of the larger
quantity of free energy. Thus, ATP contains
two high-energy phosphate groups and ADP
contains one, whereas the phosphate in AMP
(adenosine monophosphate) is of the lowenergy type, since it is a normal ester link
(Figure 11–5).
Structure of ATP, ADP, and AMP showing the position and the
number of high-energy phosphates (
).
HIGH-ENERGY PHOSPHATES ACT AS THE "ENERGY
CURRENCY" OF THE CELL
ATP is able to act as a donor of high-energy phosphate
to form those compounds below it in Table 11–1.
Likewise, with the necessary enzymes, ADP can
accept high-energy phosphate to form ATP from
those compounds above ATP in the table. In effect, an
ATP/ADP cycle connects those processes that
generate P to those processes that utilize
(Figure
11–6), continuously consuming and regenerating ATP.
This occurs at a very rapid rate, since the total
ATP/ADP pool is extremely small and sufficient to
maintain an active tissue for only a few seconds.
Role of ATP/ADP cycle in transfer of high-energy phosphate.
There are three major sources of
taking part in
energy conservation or energy capture:
1.Oxidative phosphorylation: The greatest
quantitative source of
in aerobic organisms.
Free energy comes from respiratory chain
oxidation using molecular O2 within mitochondria.
2.Glycolysis: A net formation of two results from
the formation of lactate from one molecule of
glucose, generated in two reactions catalyzed by
phosphoglycerate kinase and pyruvate kinase,
respectively .
3.The citric acid cycle: One is generated directly in
the cycle at the succinate thiokinase step.
When ATP acts as a phosphate donor to form
those compounds of lower free energy of
hydrolysis (Table11–1), the phosphate group is
invariably converted to one of low energy, eg,
ATP Allows the Coupling of Thermodynamically
Unfavorable
Reactions to Favorable Ones
• The phosphorylation of glucose to glucose 6phosphate, the first reaction of glycolysis , is
highly ender-gonic and cannot proceed under
physiologic conditions.
To take place, the reaction must be coupled with
another—more exergonic—reaction such as
the hydrolysis of the terminal phosphate of
ATP.
When (1) and (2) are coupled in a reaction
catalyzed by hexokinase, phosphorylation of
glucose readily.
proceeds in a highly exergonic reaction that
under physiologic conditions is irreversible.
Many "activation“ reactions follow this
pattern.
Adenylyl Kinase (Myokinase)
Interconverts Adenine Nucleotides
This enzyme is present in most cells. It catalyzes
the following reaction:
This allows:
1. High-energy phosphate in ADP to be used in
the synthesis of ATP.
2. AMP, formed as a consequence of several
activating reactions involving ATP, to be
recovered by rephosphorylation to ADP.
3. AMP to increase in concentration when ATP
becomes depleted and act as a metabolic
(allosteric) signal to increase the rate of
catabolic reactions, which in turn lead to the
generation of more ATP .
When ATP Forms AMP, Inorganic Pyrophosphate
(PPi) Is Produced
ATP can also be hydrolyzed directly to AMP, with
the release of PPi (Table 11–1). This occurs, for
example, in the activation of long-chain fatty
acids:
This reaction is accompanied by loss of free
energy as heat, which ensures that the
activation reaction will go to the right; and is
further aided by the hydrolytic splitting of PPi,
catalyzed by inorganic pyrophosphatase, a
reaction that itself has a large ∆G° of –19.2
kJ/mol. Note that activations via the
pyrophosphate pathway result in the loss of
two
rather than one, as occurs when ADP
and Pi are formed.
A combination of the above reactions makes it
possible for phosphate to be recycled and the
adenine nucleotides to inter-change (Figure 11–
8).
Other Nucleoside Triphosphates Participate in
the Transfer of High-Energy Phosphate
By means of the enzyme nucleoside diphosphate
kinase, UTP, GTP, and CTP can be synthesized
from their diphosphates, eg,
All of these triphosphates take part in
phosphorylations in the cell. Similarly, specific
nucleoside monophosphate kinases catalyze
the formation of nucleoside diphosphates
from the corresponding monophosphates.
Thus, adenylyl kinase is a specialized
monophosphate kinase.
Chemically, oxidation is defined as the removal
of electron(s) and reduction is defined as the
gain of electron(s).
biological oxidation ( which mean the oxidation
that occurs in biological systems ) is very
important to supply the body with energy.
Oxidation in biological systems can occur by :
1. removal of electrons .
2. removal of hydrogen .
3. addition of oxygen (less common).
Electrons are not stable in free state, so their
removal from a substance (oxidation) must be
accompanied by their acceptance by another
substance (reduction) hence the reaction is
called oxidation reduction reaction or redox
reaction and the involved enzymes in the
catalyzing of the reaction are called oxidoreductases.
Redox potential: it is the affinity of a
substance to accept electrons i.e it is the
potential for a substance to become reduced.
Hydrogen has the lowest redox potential (-0.42
volt ) while oxygen has the highest redox
potential (+0.82 volt).
The redox potential of all other substances lie
between that of hydrogen and oxygen.
Electrons are transferred from substances with
low redox potential to substances with higher
redox potential . this transfer of electrons is an
energy yielding process and the amount of
energy librated depends on the redox
potential difference between the electron
donor and acceptor.
Oxido-reductases:
These are group of enzymes that catalyze
oxidation reduction reactions which include 4
classes.
1. oxidases.
2. dehydrogenases
3. hydroxy peroxidases
4. oxygenases.
1-Oxidases:
they catalyze the removal of hydrogen from a
substrate using oxygen as a hydrogen
acceptor, they form water (H20) or hydrogen
peroxidase (H2O2) as a reaction product.
e.g : cytochrome oxidase is a hemo protein
widely distributed in many tissues, having a
typical heme prosthatic group (which also
present in other cytochromes, myoglobin and
hemoglobin).
Cytochrome oxidase is an important member of
the respiratory chain which found in the
mitochondria , it is also called cytochrome a3
or aa3.
Note: some of the oxidases cannot catalyze the
direct redox reactions but only with the help of
transporters (FMN+ and FAD+ co enzymes )
which act to transfer the reducing equivalents
from the substrate to the acceptor , such
enzymes are called the flavoprotin oxidases.
In these enzymes FMN (flavin mono nucleotide)
or FAD (flavin adenine di nucleotide) presented
as prosthetic group that tightly bound to their
enzyme.
Different flavoproteins are presented in our
bodies like L-amino acid oxidase which is FMN
linked enzyme found in the kidney which is
important for the oxidative metabolism of Lamino acids.
Note : some of these flavoproteins contain one
of the metals as essential co-factor and such
enzymes is called metalloflavoproteins like :
xanthine oxidase ( contain molybdenum) and
plays an important role in the conversion of
purine base to uric acid.