Lecture no 6
The Adenosine triphosphate
(ATP)
Lecture no 6
The Adenosine triphosphate (ATP)
Adenosine-5'-triphosphate (ATP) is a multifunctional
nucleotide used in cells as a coenzyme.
ATP transports chemical energy within cells.
ATP is produced by phosphorylation and cellular
respiration and used by enzymes and structural proteins
in many cellular processes, including:
- Metabolism, synthesis, and active transport.
- Roles in cell structure and locomotion.
- Cell signaling.
Lecture no 6
Lecture no 6
One molecule of ATP contains three
phosphate groups, and it is produced by
ATP synthase from inorganic phosphate
and adenosine diphosphate (ADP) or
adenosine monophosphate (AMP).
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Metabolic processes that use ATP as an
energy source convert it back into its
precursors. ATP is therefore continuously
recycled in organisms, with the human
body turning over its own weight in ATP
each day
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Structure and properties of ATP
ATP is a nucleotide consists of adenine (6
amino derivative of purine, the 5-carbon
sugar,ribose, attached to adenine through
glucosyl linkage and 3 phosphate group joined
to 5‘-position of ribose.
ATP, ADP and AMP occur in all cells.
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 The phosphoryl groups, starting with the group
closest to the ribose, are referred to as the (α), (β),
and (γ) phosphates.
ATP molecule as it exists in the intact cell is highly
charged at pH 7, the three phosphate groups are
completely ionized (4 negative charges) near the
linear phosphate structure.
Lecture no 6
ATP is high energy compound because it has a
large negative free energy of hydrolysis.
Two phosphoanhydride bonds (those that connect
adjacent phosphates) in an ATP molecule are
responsible for the high energy content of this
molecule.
The first phosphate of
ATP is an ester bonded
phosphate and this is not
a high energy bond.
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These anhydride bonds are frequently referred to as
high-energy bonds.
Energy stored in ATP maybe released upon hydrolysis
of the anhydride bonds.
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ATP Hydrolysis
When ATP is incubated under suitable
conditions with muscle fibers, it undergoes
enzymatic hydrolysis to give ADP +Pi
When this hydrolysis proceeds it gives large
liberation of heat.
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The bonds formed after hydrolysis—or the
phosphorylation of a residue by ATP—are
lower in energy than the phosphoanhydride
bonds of ATP.
During enzyme-catalyzed hydrolysis of ATP
or phosphorylation by ATP, the available free
energy can be harnessed by a living system to
do work.
Compounds with Δ G more negative than 7
Kcal/mole or 30 KJ/ mole are regarded as high
energy compounds.
Lecture no 6
The overall equation for ATP hydrolysis is
usually written:
ATP + H2O <=> ADP + inorganic phosphate (Pi)
ΔGo' = -7.3 kcal. mol-1
Note, the H2O is omitted from the logarithmic
term because it is the solvent with a standard
state of 1M.
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Mechanism of ATP hydrolysis
When the terminal phosphate bond is
hydrolyzed, some electrostatic stress is
relieved, the similar charges are separated as
ADP3- + HPO4 2- and they will have very little
tendency to combine again because their
similar charges repel each other.
ATP 4- + H2O
ADP3- + HPO4 2-
ADP and phosphate as soon as they are
formed undergo stabilization by resonance.
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Ionization in biological systems
ATP has several negatively-charged groups in
neutral solution, it can chelate metals with very
high affinity.
ATP forms stable complexes with certain
divalent cations as Mg2+. Most of ATP in the cell
present as Mg2+-complex Mg2+.
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Why G°' is so high for ATP hydrolysis?

Hydrolysis reduces electrostatic repulsion between
negative charged oxygen atoms on the phosphorus
atoms.

ADP and Pi are stabilised by resonance.

Entropy (disorder) is higher after the reaction.
These 3 factors make ATP hydrolysis favourable and
are responsible for the large amount of energy
released during hydrolysis.
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However, there are a number of factors which make
the true situation more complicated. Most important
are:
1- The conc of ATP, ADP and Pi in the cell
much lower than 1.0 M .
are
2-These substances are not present in equimolar
concentrations
+
2- The presence of Mg2 will also modify the value for
Δ Go, because the different reactants form complexes
with bivalent cations.
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If appropriate corrections are applied of all
these factors, the free energy of hydrolysis of
ATP to ADP under intracellular conditions
will be about -12 Kcal/ mole.
So free energy of ATP hydrolysis inside cells
is not necessarily constant.
Free energy of ATP vary from one cell to
another.
It may vary from time to time depending on
the concentration of Mg2+, ATP, ADP and Pi.
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ATP synthesis
For ATP synthesis, the reaction is written in
the reverse direction, and the sign of the free
energy change is reversed, so that:
ADP + inorganic phosphate (Pi) <=> ATP + H2O
ΔGo' = +7.3 kcal. mol-1
The modifying factors which affect the
hydrolysis also effect the reverse reaction