The hydrolysis of ATP is coupled with many biological reactions changing the value of their delta G into a negative value.
The ΔG of ATP hydrolysis in a test tube under standard conditions is -7.3 kcal/mol. The ΔG for the reaction A + B = C under the same conditions is +8.2 kcal/mol.
Why does the hydrolysis of ATP have a negative ∆ G value?
The negative value of delta G in the hydrolysis of ATP is that since the products of this hydrolysis process are stable and have lower energy than ATP.
How much energy is released by ATP hydrolysis?
Energy Output in ATP Hydrolysis The hydrolysis of 1M of ATP into ADP and inorganic phosphate, releases −7.3 kcal/mol of energy. The energy released in the living cell is almost double the value in standard conditions, it is equal to −14 kcal/mol.
The calculated ∆G for the hydrolysis of one mole of ATP into ADP and P
Overview
ATP structure, ATP hydrolysis to ADP, and reaction coupling.
Introduction
A cell can be thought of as a small, bustling town. Carrier proteins move substances into and out of the cell, motor proteins carry cargoes along microtubule tracks, and metabolic enzymes busily break down and build up macromolecules.
Even if they would not be energetically favorable (energy-releasing, or exergonic) in isolation, these processes will continue merrily along if there is energy available to power them (much as business will continue to be done in a town as long as there is money flowing in). However, if the energy runs out, the reactions will grind to a halt, and the cell will begin to die.
Energetically unfavorable reactions are “paid for” by linked, energetically favorable reactions that release energy. Often, the "payment" reaction involves one particular small molecule: adenosine triphosphate, or ATP.
ATP structure and hydrolysis
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Introduction
A cell can be thought of as a small, bustling town. Carrier proteins move substances into and out of the cell, motor proteins carry cargoes along microtubule tracks, and metabolic enzymes busily break down and build up macromolecules.
Even if they would not be energetically favorable (energy-releasing, or exergonic) in isolation, these processes will continue merrily along if there is energy available to power them (much as business will continue to be done in a town as long as there is money flowing in). However, if the energy runs out, the reactions will grind to a halt, and the cell will begin to die.
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ATP structure and hydrolysis
Adenosine triphosphate, or ATP, is a small, relatively simple molecule. It can be thought of as the main energy currency of cells, much as money is the main economic currency of human societies. The energy released by hydrolysis (breakdown) of ATP is used to power many energy-requiring cellular reactions.
Structurally, ATP is an RNA nucleotide that bears a chain of three phosphates. At the center of the molecule lies a five-carbon sugar, ribose, which is attached to the nitrogenous base adenine and to the chain of three phosphates.
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Hydrolysis of ATP
Why are the phosphoanhydride bonds considered high-energy? All this really means is that an appreciable amount of energy is released when one of these bonds is broken in a hydrolysis (water-mediated breakdown) reaction. ATP is hydrolyzed to ADP in the following reaction:
ATP+H2O⇋ADP+Pi+energy
Note: Pi just stands for an inorganic phosphate group (PO43−) .
Like most chemical reactions, the hydrolysis of ATP to ADP is reversible. The reverse reaction, which regenerates ATP from ADP and Pi , requires energy. Regeneration of ATP is important because cells tend to use up (hydrolyze) ATP molecules very quickly and rely on replacement ATP being constantly produced1 .
You can think of ATP and ADP as being sort of like the charged and uncharged forms of a rechargeable battery (as shown above). ATP, the charged battery, has energy that can be used to power cellular reactions. Once the energy has been used up, the uncharged battery (ADP) must be recharged before it can again be used as a power source. The ATP regeneration reaction is just the reverse of the hydrolysis reaction:
energy+ADP+Pi⇋ATP+H2O
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Reaction coupling
How is the energy released by ATP hydrolysis used to power other reactions in a cell? In most cases, cells use a strategy called reaction coupling, in which an energetically favorable reaction (like ATP hydrolysis) is directly linked with an energetically unfavorable (endergonic) reaction. The linking often happens through a shared intermediate, meaning that a product of one reaction is “picked up” and used as a reactant in the second reaction.
When two reactions are coupled, they can be added together to give an overall reaction, and the ΔG of this reaction will be the sum of the ΔG values of the individual reactions. As long as the overall ΔG is negative, both reactions can take place. Even a very endergonic reaction can occur if it is paired with a very exergonic one (such as hydrolysis of ATP). For instance, we can add up a pair of generic reactions coupled by a shared intermediate, B, as follows2 :
A⇋BΔG=X+B⇋C+DΔG=YA⇋C+DΔG=X+Y
You might notice that the intermediate, B, doesn't appear in the overall coupled reaction. This is because it appears as a both a product and a reactant, so two Bs cancel each other out when the reactions are added.
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ATP in reaction coupling
When reaction coupling involves ATP, the shared intermediate is often a phosphorylated molecule (a molecule to which one of the phosphate groups of ATP has been attached). As an example of how this works, let’s look at the formation of sucrose, or table sugar, from glucose and fructose3,4 .
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Case study: Let's make sucrose
The formation of sucrose requires an input of energy: its ΔG is about +27 kJ/mol (under standard conditions). ATP hydrolysis has a ΔG around −30 kJ/mol under standard conditions, so it can release enough energy to “pay” for the synthesis of a sucrose molecule:
glucose +fructose⇋sucroseΔG=+27 kJ/molATP+H2O⇋ADP+PiΔG=−30 kJ/molglucose +fructose+ATP⇋sucrose+ADP+PiΔG=−3kJ/mol
How is the energy released in ATP hydrolysis channeled into the production of a sucrose molecule? As it turns out, there are actually two reactions that take place, not just one big reaction, and the product of the first reaction acts as a reactant for the second.
•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.
•In the second reaction, the glucose-P intermediate reacts with fructose to form sucrose. Because glucose-P is relatively unstable (thanks to its attached phosphate group), this reaction also releases energy and is spontaneous.
This example shows how reaction coupling involving ATP can work through phosphorylation, breaking a reaction down into two energetically favored steps connected by a phosphorylated (phosphate-bearing) intermediate. This strategy is used in many metabolic pathways in the cell, providing a way for the energy released by converting ATP to ADP to drive other reactions forward.
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Different types of reaction coupling in the cell
The example above shows how ATP hydrolysis can be coupled to a biosynthetic reaction. However, ATP hydrolysis can also be coupled to other classes of cellular reactions, such as the shape changes of proteins that transport other molecules into or out of the cell.
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Case study: Sodium-potassium pump
It’s energetically unfavorable to move sodium (Na+ ) out of, or potassium (K+ ) into, a typical cell, because this movement is against the concentration gradients of the ions. ATP provides energy for the transport of sodium and potassium by way of a membrane-embedded protein called the sodium-potassium pump (Na+/K+ pump).
In this process, ATP transfers one of its phosphate groups to the pump protein, forming ADP and a phosphorylated “intermediate” form of the pump. The phosphorylated pump is unstable in its original conformation (facing the inside of the cell), so it becomes more stable by changing shape, opening towards the outside of the cell and releasing sodium ions outside. When extracellular potassium ions bind to the phosphorylated pump, they trigger the removal of the phosphate group, making the protein unstable in its outward-facing form. The protein will then become more stable by returning to its original shape, releasing the potassium ions inside the cell.
Although this example involves chemical gradients and protein transporters, the basic principle is similar to the sucrose example above. ATP hydrolysis is coupled to a work-requiring (energetically unfavorable) process through formation of an unstable, phosphorylated intermediate, allowing the process to take place in a series of steps that are each energetically favorable.
[Attribution and references]
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