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If bond break absorbs energy, why ATP hydrolysis releases it?

If bond break absorbs energy, why ATP hydrolysis releases it?



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We know that bond formation releases energy and bond break absorbs energy. I still don't figure out why ATP hydrolysis (that breaks a bond between oxygen and phosphorous) releases energy, and it releases a LOT.


ATP synthesis is actually an endergonic process, hence the need for oxiditative phosphorylation to drive the formation of ATP. Although the P-O bond releases a large amount of energy, the reverse of this process is not occurring in ATP hydrolysis. During hydrolysis, the ATP is broken down into two products, with the addition of water, these products are more thermodynamically stable, so energy is released. Breaking of the P-O bond does require energy, but because more energy released by adding the water in, the overall reaction releases energy. ATP is a thermodynamically unstable molecule (bond breakage releases free energy), but is kinetically stable (bond breakage has a high activation energy). I am not too sure what the exact reasons for ATPs hydrolysis being exergonic are, textbooks state it is because the major sources of the free energy associated with the P−O bond dissociation are the relaxation of the electronic repulsion, increased resonance of the products and the stabilisation of the solvation free energies of the relevant species after the hydrolyses. However, this paper (DOI: 10.1021/acs.jpcb.7b00637) claims that the solvation effects upon hydrolysis actually destabilise the products, but I am not sure of the validity of this paper.


If bond break absorbs energy, why ATP hydrolysis releases it? - Biology

Cells couple the exergonic reaction of ATP hydrolysis with endergonic reactions to harness the energy within the bonds of ATP.

Learning Objectives

Explain the role of ATP as the currency of cellular energy

Key Takeaways

Key Points

  • Adenosine triphosphate is composed of the nitrogenous base adenine, the five-carbon sugar ribose, and three phosphate groups.
  • ATP is hydrolyzed to ADP in the reaction ATP+H2O→ADP+Pi+ free energy the calculated ∆G for the hydrolysis of 1 mole of ATP is -57 kJ/mol.
  • ADP is combined with a phosphate to form ATP in the reaction ADP+Pi+free energy→ATP+H2O.
  • The energy released from the hydrolysis of ATP into ADP is used to perform cellular work, usually by coupling the exergonic reaction of ATP hydrolysis with endergonic reactions.
  • Sodium-potassium pumps use the energy derived from exergonic ATP hydrolysis to pump sodium and potassium ions across the cell membrane while phosphorylation drives the endergonic reaction.

Key Terms

  • energy coupling: Energy coupling occurs when the energy produced by one reaction or system is used to drive another reaction or system.
  • endergonic: Describing a reaction that absorbs (heat) energy from its environment.
  • exergonic: Describing a reaction that releases energy (heat) into its environment.
  • free energy: Gibbs free energy is a thermodynamic potential that measures the useful or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure (isothermal, isobaric).
  • hydrolysis: A chemical process of decomposition involving the splitting of a bond by the addition of water.

ATP: Adenosine Triphosphate

Adenosine triphosphate (ATP) is the energy currency for cellular processes. ATP provides the energy for both energy-consuming endergonic reactions and energy-releasing exergonic reactions, which require a small input of activation energy. When the chemical bonds within ATP are broken, energy is released and can be harnessed for cellular work. The more bonds in a molecule, the more potential energy it contains. Because the bond in ATP is so easily broken and reformed, ATP is like a rechargeable battery that powers cellular process ranging from DNA replication to protein synthesis.

Molecular Structure

Adenosine triphosphate (ATP) is comprised of the molecule adenosine bound to three phosphate groups. Adenosine is a nucleoside consisting of the nitrogenous base adenine and the five-carbon sugar ribose. The three phosphate groups, in order of closest to furthest from the ribose sugar, are labeled alpha, beta, and gamma. Together, these chemical groups constitute an energy powerhouse. The two bonds between the phosphates are equal high-energy bonds (phosphoanhydride bonds) that, when broken, release sufficient energy to power a variety of cellular reactions and processes. The bond between the beta and gamma phosphate is considered “high-energy” because when the bond breaks, the products [adenosine diphosphate (ADP) and one inorganic phosphate group (Pi)] have a lower free energy than the reactants (ATP and a water molecule). ATP breakdown into ADP and Pi is called hydrolysis because it consumes a water molecule (hydro-, meaning “water”, and lysis, meaning “separation”).

Adenosine Triphosphate (ATP): ATP is the primary energy currency of the cell. It has an adenosine backbone with three phosphate groups attached.

ATP Hydrolysis and Synthesis

ATP is hydrolyzed into ADP in the following reaction:

Like most chemical reactions, the hydrolysis of ATP to ADP is reversible. The reverse reaction combines ADP + Pi to regenerate ATP from ADP. Since ATP hydrolysis releases energy, ATP synthesis must require an input of free energy.

ADP is combined with a phosphate to form ATP in the following reaction:

ATP and Energy Coupling

Exactly how much free energy (∆G) is released with the hydrolysis of ATP, and how is that free energy used to do cellular work? The calculated ∆G for the hydrolysis of one mole of ATP into ADP and Pi is −7.3 kcal/mole (−30.5 kJ/mol). However, this is only true under standard conditions, and the ∆G for the hydrolysis of one mole of ATP in a living cell is almost double the value at standard conditions: 14 kcal/mol (−57 kJ/mol).

ATP is a highly unstable molecule. Unless quickly used to perform work, ATP spontaneously dissociates into ADP + Pi, and the free energy released during this process is lost as heat. To harness the energy within the bonds of ATP, cells use a strategy called energy coupling.

Energy Coupling in Sodium-Potassium Pumps

Energy Coupling: Sodium-potassium pumps use the energy derived from exergonic ATP hydrolysis to pump sodium and potassium ions across the cell membrane.

Cells couple the exergonic reaction of ATP hydrolysis with the endergonic reactions of cellular processes. For example, transmembrane ion pumps in nerve cells use the energy from ATP to pump ions across the cell membrane and generate an action potential. The sodium-potassium pump (Na + /K + pump) drives sodium out of the cell and potassium into the cell. When ATP is hydrolyzed, it transfers its gamma phosphate to the pump protein in a process called phosphorylation. The Na + /K + pump gains the free energy and undergoes a conformational change, allowing it to release three Na + to the outside of the cell. Two extracellular K + ions bind to the protein, causing the protein to change shape again and discharge the phosphate. By donating free energy to the Na + /K + pump, phosphorylation drives the endergonic reaction.

Energy Coupling in Metabolism

During cellular metabolic reactions, or the synthesis and breakdown of nutrients, certain molecules must be altered slightly in their conformation to become substrates for the next step in the reaction series. In the very first steps of cellular respiration, glucose is broken down through the process of glycolysis. ATP is required for the phosphorylation of glucose, creating a high-energy but unstable intermediate. This phosphorylation reaction causes a conformational change that allows enzymes to convert the phosphorylated glucose molecule to the phosphorylated sugar fructose. Fructose is a necessary intermediate for glycolysis to move forward. In this example, the exergonic reaction of ATP hydrolysis is coupled with the endergonic reaction of converting glucose for use in the metabolic pathway.


Contents

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 Pi. As the concentrations of these molecules deviate from values at equilibrium, the value of Gibbs free energy change (ΔG) will be increasingly different. In standard conditions (ATP, ADP and Pi concentrations are equal to 1M, water concentration is equal to 55 M) the value of ΔG is between -28 to -34 kJ/mol. [5] [6]

The range of the ΔG 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 ΔG 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 ΔG values is therefore to be expected. [6]

The relationship between the standard Gibbs free energy change ΔrG o and chemical equilibrium is revealing. This relationship is defined by the equation ΔrG o = -RT ln(K), where K is the equilibrium constant, which is equal to the reaction quotient Q in equilibrium. The standard value of ΔG 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. [6] 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 Pi) 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. [6] Using these nonstandard concentrations, the calculated value of Q is much less than one. By relating Q to ΔG using the equation ΔG = ΔrG o + RT ln(Q), where ΔrG o is the standard change in Gibbs free energy for the hydrolysis of ATP, it is found that the magnitude of ΔG is much greater than the standard value. The nonstandard conditions of the cell actually result in a more favorable reaction. [7]

In one particular study, to determine ΔG in vivo in humans, the concentration of ATP, ADP, and Pi was measured using nuclear magnetic resonance. [6] 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 ΔG = -64 kJ/mol. After ischemia, 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 ΔG would be as high as -69 kJ/mol. [8]

By comparing the standard value of ΔG and the experimental value of ΔG, 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. [6] [7]


Energy Coupling in Sodium-Potassium Pumps

Figure (PageIndex<1>): Energy Coupling: Sodium-potassium pumps use the energy derived from exergonic ATP hydrolysis to pump sodium and potassium ions across the cell membrane.

Cells couple the exergonic reaction of ATP hydrolysis with the endergonic reactions of cellular processes. For example, transmembrane ion pumps in nerve cells use the energy from ATP to pump ions across the cell membrane and generate an action potential. The sodium-potassium pump (Na + /K + pump) drives sodium out of the cell and potassium into the cell. When ATP is hydrolyzed, it transfers its gamma phosphate to the pump protein in a process called phosphorylation. The Na + /K + pump gains the free energy and undergoes a conformational change, allowing it to release three Na + to the outside of the cell. Two extracellular K + ions bind to the protein, causing the protein to change shape again and discharge the phosphate. By donating free energy to the Na + /K + pump, phosphorylation drives the endergonic reaction.


Why does removing a phosphate group from ATP release energy?

ATP has three different phosphate groups, but the bond holding the third phosphate group is unstable and is very easily broken. Where does ADP come from? When phosphate is removed, energy is released and ATP becomes ADP.

Furthermore, what happens when a phosphate molecule is removed from an ATP compound? Usually only the outer phosphate is removed from ATP to yield energy when this occurs ATP is converted to adenosine diphosphate (ADP), the form of the nucleotide having only two phosphates. ATP is able to power cellular processes by transferring a phosphate group to another molecule (a process called phosphorylation).

Secondly, where does the phosphate group from ATP go?

In the first reaction, a phosphate group is transferred from ATP to glucose, forming a phosphorylated glucose intermediate (glucose-P). This is an energetically favorable (energy-releasing) reaction because ATP is so unstable, i.e., really "wants" to lose its phosphate group.

Which reaction shows ATP releasing its energy?

ADP is combined with a phosphate to form ATP in the reaction ADP+Pi+free energy&rarrATP+H2O. The energy released from the hydrolysis of ATP into ADP is used to perform cellular work, usually by coupling the exergonic reaction of ATP hydrolysis with endergonic reactions.


Why energy released when bonds formed

Classical mechanics doesn't really work to describe how/why two species bond. For instance the positive charges of 2 Hydrogen nuclei should be repelled from each other and therefore you would think that Hydrogen gas should exist as Hydrogen atoms. However, Hydrogen gas exists as 2 Hydrogen atoms sharing a pair of electrons because the configuration of the electrons around the two protons is such that it is of lower energy when the electrons are "shared" then when they are not.

In Chemistry we do not talk about electrons bonding with protons. In Chemistry, a bond is when an ion/atom/molecule "decides" to share electrons with another ion/atom/molecule. The two species will "decide" to bond if the energy of the overall system will decrease by them forming bonds.

There are two main factors involved, entropy and enthalpy, commonly thought of as disorder and heat, respectively. The relation of these factors to energy is known as the Gibbs' Free Energy equation ΔG=ΔH-TΔS (H is enthalpy and S is entropy). In order for a process to proceed spontaneously, you need the ΔG to be negative (all systems tend to the lowest energy state possible).

When two species become one, the entropy of the system (typically) is decreasing making the [-TΔS] term positive. In order to have that process be spontaneous, the ΔH term must be negative enough to overcome the positive TΔS term. Physically this is observed as heat or spark or an explosion or what have you. Now you have a system where covalent bonds were formed and the system is at its lowest energy state. In order to go the other way, you need to put energy back into the system to break those bonds.

The funny thing is, that blanket statements like that tend to be a little misleading. Even if a process is spontaneous (overall negative ΔG), there is a certain amount of energy that you need to put into the system to "get things started." This is called the Activation Energy and is the reason why the wood that is around you right now is not bursting into flames as you read this message.


How Is Energy Released From ATP?

Energy is released from ATP by the breaking of the phosphate bond, states the University of Illinois at Chicago. Adenosine triphosphate, or ATP, consists of a sugar called ribose, the molecule adenine and three phosphate groups. During the hydrolysis of ATP, the last phosphate group is transferred to another molecule, thus breaking the phosphate bond. This reaction causes energy to be released to power other activities within the cell.

ATP is made by breaking down glucose, as stated by Dr. Dawn Tamarkin at Springfield Technical Community College. By breaking down the bonds in glucose in the presence of oxygen, energy is produced in order to add a phosphate group to ADP to form ATP. In this way, 38 ATPs are formed. This process is called cellular respiration.

The energy of the ATP molecule lies in the bonds between the phosphate groups, or pyrophosphate bonds, states Dr. Mike Farabee of Estrella Mountain Community College. The bond between the second phosphate and last phosphate groups yields the most energy, about seven kilocalories per mole. When this bond is broken, adenosine diphosphate, or ADP, is formed.

Because ATP is constantly being used, it needs to be replenished. A single muscle cell, probably one of the greatest users of ATP, uses and replenishes 10,000,000 ATP molecules per second, according to the University of Illinois at Chicago.


Introduction

ATP is an unstable molecule which hydrolyzes to ADP and inorganic phosphate when it is in equilibrium with water. The high energy of this molecule comes from the two high-energy phosphate bonds. The bonds between phosphate molecules are called phosphoanhydride bonds. They are energy-rich and contain a &DeltaG of -30.5 kJ/mol.

Figure 1: Structure of ATP molecule and ADP molecule, respectively. The adenine ring is at the top, connected to a ribose sugar, which is connected to the phosphate groups. Used with permission from Wikipedia Commons.


30 ATP: Adenosine Triphosphate

By the end of this section, you will be able to do the following:

  • Explain ATP’s role as the cellular energy currency
  • Describe how energy releases through ATP hydrolysis

Even exergonic, energy-releasing reactions require a small amount of activation energy in order to proceed. However, consider endergonic reactions, which require much more energy input, because their products have more free energy than their reactants. Within the cell, from where does energy to power such reactions come? The answer lies with an energy-supplying molecule scientists call adenosine triphosphate , or ATP . This is a small, relatively simple molecule ((Figure)), but within some of its bonds, it contains the potential for a quick burst of energy that can be harnessed to perform cellular work. Think of this molecule as the cells’ primary energy currency in much the same way that money is the currency that people exchange for things they need. ATP powers the majority of energy-requiring cellular reactions.

As its name suggests, adenosine triphosphate is comprised of adenosine bound to three phosphate groups ((Figure)). Adenosine is a nucleoside consisting of the nitrogenous base adenine and a five-carbon sugar, ribose. The three phosphate groups, in order of closest to furthest from the ribose sugar, are alpha, beta, and gamma. Together, these chemical groups constitute an energy powerhouse. However, not all bonds within this molecule exist in a particularly high-energy state. Both bonds that link the phosphates are equally high-energy bonds ( phosphoanhydride bonds ) that, when broken, release sufficient energy to power a variety of cellular reactions and processes. These high-energy bonds are the bonds between the second and third (or beta and gamma) phosphate groups and between the first and second phosphate groups. These bonds are “high-energy” because the products of such bond breaking—adenosine diphosphate (ADP) and one inorganic phosphate group (Pi)—have considerably lower free energy than the reactants: ATP and a water molecule. Because this reaction takes place using a water molecule, it is a hydrolysis reaction. In other words, ATP hydrolyzes into ADP in the following reaction:

Like most chemical reactions, ATP to ADP hydrolysis is reversible. The reverse reaction regenerates ATP from ADP + Pi. Cells rely on ATP regeneration just as people rely on regenerating spent money through some sort of income. Since ATP hydrolysis releases energy, ATP regeneration must require an input of free energy. This equation expresses ATP formation:

Two prominent questions remain with regard to using ATP as an energy source. Exactly how much free energy releases with ATP hydrolysis, and how does that free energy do cellular work? The calculated ∆G for the hydrolysis of one ATP mole into ADP and Pi is −7.3 kcal/mole (−30.5 kJ/mol). Since this calculation is true under standard conditions, one would expect a different value exists under cellular conditions. In fact, the ∆G for one ATP mole’s hydrolysis in a living cell is almost double the value at standard conditions: –14 kcal/mol (−57 kJ/mol).

ATP is a highly unstable molecule. Unless quickly used to perform work, ATP spontaneously dissociates into ADP + Pi, and the free energy released during this process is lost as heat. The second question we posed above discusses how ATP hydrolysis energy release performs work inside the cell. This depends on a strategy scientists call energy coupling. Cells couple the ATP hydrolysis’ exergonic reaction allowing them to proceed. One example of energy coupling using ATP involves a transmembrane ion pump that is extremely important for cellular function. This sodium-potassium pump (Na + /K + pump) drives sodium out of the cell and potassium into the cell ((Figure)). A large percentage of a cell’s ATP powers this pump, because cellular processes bring considerable sodium into the cell and potassium out of it. The pump works constantly to stabilize cellular concentrations of sodium and potassium. In order for the pump to turn one cycle (exporting three Na+ ions and importing two K + ions), one ATP molecule must hydrolyze. When ATP hydrolyzes, its gamma phosphate does not simply float away, but it actually transfers onto the pump protein. Scientists call this process of a phosphate group binding to a molecule phosphorylation. As with most ATP hydrolysis cases, a phosphate from ATP transfers onto another molecule. In a phosphorylated state, the Na + /K + pump has more free energy and is triggered to undergo a conformational change. This change allows it to release Na + to the cell’s outside. It then binds extracellular K + , which, through another conformational change, causes the phosphate to detach from the pump. This phosphate release triggers the K + to release to the cell’s inside. Essentially, the energy released from the ATP hydrolysis couples with the energy required to power the pump and transport Na + and K + ions. ATP performs cellular work using this basic form of energy coupling through phosphorylation.

One ATP molecule’s hydrolysis releases 7.3 kcal/mol of energy (∆G = −7.3 kcal/mol of energy). If it takes 2.1 kcal/mol of energy to move one Na + across the membrane (∆G = +2.1 kcal/mol of energy), how many sodium ions could one ATP molecule’s hydrolysis move?

Often during cellular metabolic reactions, such as nutrient synthesis and breakdown, certain molecules must alter slightly in their conformation to become substrates for the next step in the reaction series. One example is during the very first steps of cellular respiration, when a sugar glucose molecule breaks down in the process of glycolysis. In the first step, ATP is required to phosphorylze glucose, creating a high-energy but unstable intermediate. This phosphorylation reaction powers a conformational change that allows the phosphorylated glucose molecule to convert to the phosphorylated sugar fructose. Fructose is a necessary intermediate for glycolysis to move forward. Here, ATP hydrolysis’ exergonic reaction couples with the endergonic reaction of converting glucose into a phosphorylated intermediate in the pathway. Once again, the energy released by breaking a phosphate bond within ATP was used for phosphorylyzing another molecule, creating an unstable intermediate and powering an important conformational change.

See an interactive animation of the ATP-producing glycolysis process at this site.

Section Summary

ATP is the primary energy-supplying molecule for living cells. ATP is comprised of a nucleotide, a five-carbon sugar, and three phosphate groups. The bonds that connect the phosphates (phosphoanhydride bonds) have high-energy content. The energy released from ATP hydrolysis into ADP + Pi performs cellular work. Cells use ATP to perform work by coupling ATP hydrolysis’ exergonic reaction with endergonic reactions. ATP donates its phosphate group to another molecule via phosphorylation. The phosphorylated molecule is at a higher-energy state and is less stable than its unphosphorylated form, and this added energy from phosphate allows the molecule to undergo its endergonic reaction.

Visual Connection Questions

(Figure) One ATP molecule’s hydrolysis releases 7.3 kcal/mol of energy (∆G = −7.3 kcal/mol of energy). If it takes 2.1 kcal/mol of energy to move one Na + across the membrane (∆G = +2.1 kcal/mol of energy), how many sodium ions could one ATP molecule’s hydrolysis move?

(Figure) Three sodium ions could be moved by the hydrolysis of one ATP molecule. The ∆G of the coupled reaction must be negative. Movement of three sodium ions across the membrane will take 6.3 kcal of energy (2.1 kcal × 3 Na + ions = 6.3 kcal). Hydrolysis of ATP provides 7.3 kcal of energy, more than enough to power this reaction. Movement of four sodium ions across the membrane, however, would require 8.4 kcal of energy, more than one ATP molecule can provide.

Review Questions

The energy released by the hydrolysis of ATP is____

  1. primarily stored between the alpha and beta phosphates
  2. equal to −57 kcal/mol
  3. harnessed as heat energy by the cell to perform work
  4. providing energy to coupled reactions

Which of the following molecules is likely to have the most potential energy?

Critical Thinking Questions

Do you think that the EA for ATP hydrolysis is relatively low or high? Explain your reasoning.

The activation energy for hydrolysis is very low. Not only is ATP hydrolysis an exergonic process with a large −∆G, but ATP is also a very unstable molecule that rapidly breaks down into ADP + Pi if not utilized quickly. This suggests a very low EA since it hydrolyzes so quickly.

Glossary


Watch the video: ATP hydrolysis (August 2022).