ATP hydrolysis is the catabolic reaction process by which
chemical energy
Chemical energy is the energy of chemical substances that is released when the substances undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (20 ...
that has been stored in the
high-energy phosphoanhydride bonds in
adenosine triphosphate
Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...
(ATP) is released after splitting these bonds, for example in
muscle
Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
s, by producing work in the form of
mechanical energy
In physical sciences, mechanical energy is the sum of macroscopic potential and kinetic energies. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, then the mechanical ...
. The product is
adenosine diphosphate
Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbon ...
(ADP) and an
inorganic phosphate (P
i). ADP can be further hydrolyzed to give energy,
adenosine monophosphate
Adenosine monophosphate (AMP), also known as 5'-adenylic acid, is a nucleotide. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine. It is an ester of phosphoric acid and the nucleoside adenosine. As a substituent it t ...
(AMP), and another inorganic phosphate (P
i).
ATP hydrolysis is the final link between the energy derived from food or sunlight and useful work such as
muscle contraction
Muscle contraction is the activation of Tension (physics), tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in musc ...
, the establishment of
electrochemical gradient
An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts:
* The chemical gradient, or difference in Concentration, solute concentration across ...
s across membranes, and biosynthetic processes necessary to maintain life.
Anhydridic bonds are often labelled as "''high-energy bonds"''. P-O bonds are in fact fairly strong (~30 kJ/mol stronger than C-N bonds)
[ Darwent, B. deB. (1970). "Bond Dissociation Energies in Simple Molecules", Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.) 31, 52 pages.][ ] and themselves not particularly easy to break. As noted below, energy is released by the hydrolysis of ATP. However, when the P-O bonds are broken, ''input'' of energy is required. It is the formation of new bonds and lower-energy inorganic phosphate with a ''release of a larger amount of energy'' that lowers the total energy of the system and makes it more stable.
Hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of the
phosphate
Phosphates are the naturally occurring form of the element phosphorus.
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
groups in ATP is especially
exergonic, because the resulting inorganic phosphate molecular ion is greatly stabilized by multiple
resonance structures, making the products (ADP and P
i) lower in energy than the reactant (ATP). The high negative charge density associated with the three adjacent phosphate units of ATP also destabilizes the molecule, making it higher in energy. Hydrolysis relieves some of these electrostatic repulsions, liberating useful energy in the process by causing conformational changes in enzyme structure.
In humans, approximately 60 percent of the energy released from the hydrolysis of ATP produces metabolic heat rather than fuel the actual reactions taking place.
Due to the acid-base properties of ATP, ADP, and inorganic phosphate, the hydrolysis of ATP has the effect of lowering the pH of the reaction medium. Under certain conditions, high levels of ATP hydrolysis can contribute to
lactic acidosis.
Amount of energy produced
Hydrolysis of the terminal phosphoanhydridic bond is a highly exergonic process. The amount of released energy depends on the conditions in a particular cell. Specifically, the energy released is dependent on concentrations of ATP, ADP and P
i. As the concentrations of these molecules deviate from values at equilibrium, the value of
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol is a thermodynamic potential that can be used to calculate the maximum amount of Work (thermodynamics), work, other than Work (thermodynamics)#Pressure–v ...
change (Δ''G'') will be increasingly different. In standard conditions (ATP, ADP and P
i concentrations are equal to 1M, water concentration is equal to 55 M) the value of Δ''G'' is between -28 and -34 kJ/mol.
The range of the Δ''G'' value exists because this reaction is dependent on the concentration of Mg
2+ cations, which stabilize the ATP molecule. The cellular environment also contributes to differences in the Δ''G'' value since ATP hydrolysis is dependent not only on the studied cell, but also on the surrounding tissue and even the compartment within the cell. Variability in the Δ''G'' values is therefore to be expected.
The relationship between the standard Gibbs free energy change Δ
r''G''
o and chemical equilibrium is revealing. This relationship is defined by the equation Δ
r''G''
o = -''RT'' ln(''K''), where ''K'' is the
equilibrium constant
The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...
, which is equal to the
reaction quotient
In chemical thermodynamics, the reaction quotient (''Q''r or just ''Q'') is a dimensionless quantity that provides a measurement of the relative amounts of products and reactants present in a reaction mixture for a reaction with well-defined overal ...
''Q'' in equilibrium. The standard value of Δ''G'' for this reaction is, as mentioned, between -28 and -34 kJ/mol; however, experimentally determined concentrations of the involved molecules reveal that the reaction is not at equilibrium.
Given this fact, a comparison between the equilibrium constant, ''K'', and the reaction quotient, ''Q'', provides insight. ''K'' takes into consideration reactions taking place in standard conditions, but in the cellular environment the concentrations of the involved molecules (namely, ATP, ADP, and P
i) are far from the standard 1 M. In fact, the concentrations are more appropriately measured in mM, which is smaller than M by three orders of magnitude.
Using these nonstandard concentrations, the calculated value of ''Q'' is much less than one. By relating ''Q'' to Δ''G'' using the equation Δ''G'' = Δ
r''G''
o + ''RT'' ln(''Q''), where Δ
r''G''
o is the standard change in Gibbs free energy for the hydrolysis of ATP, it is found that the magnitude of Δ''G'' is much greater than the standard value. The nonstandard conditions of the cell actually result in a more favorable reaction.
In one particular study, to determine Δ''G'' in vivo in humans, the concentration of ATP, ADP, and P
i was measured using nuclear magnetic resonance.
In human muscle cells at rest, the concentration of ATP was found to be around 4 mM and the concentration of ADP was around 9 μM. Inputing these values into the above equations yields Δ''G'' = -64 kJ/mol. After
ischemia
Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...
, when the muscle is recovering from exercise, the concentration of ATP is as low as 1 mM and the concentration of ADP is around 7 μM. Therefore, the absolute Δ''G'' would be as high as -69 kJ/mol.
By comparing the standard value of Δ''G'' and the experimental value of Δ''G'', one can see that the energy released from the hydrolysis of ATP, as measured in humans, is almost twice as much as the energy produced under standard conditions.
See also
*
Dephosphorylation
References
Further reading
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{{DEFAULTSORT:Atp Hydrolysis
Cellular respiration
Exercise physiology