ATP Hydrolysis

ATP molecule

Let's begin our journey by defining ATP.

The structure of ATP consists of one adenosine and three phosphates (figure 1).

• Adenosine is a nucleoside, which are molecules containing an organic ring with nitrogen, and sugar.

• Phosphate is a functional group composed of a phosphate atom surrounded by four oxygen atoms.

Fig. 1. Molecular structure of Adenosine Triphosphate (ATP), and its functional groups, licensed by CC BY 3.0.

The main source of ATP synthesis in cells and living organisms is respiration.

• In plants, ATP is also synthesized during photosynthesis.

• In environments with little to no oxygen, ATP can alternatively be created by anaerobic respiration, such as fermentation by bacteria.

ATP Hydrolysis Definition

Just like it takes effort to hold hands, chemical bonds require a certain amount of energy to be maintained. When a bond is broken, the energy needed to hold the bond is now “freed”. In other words, the reaction is exergonic.

• An exergonic reaction is a chemical reaction where energy is released.

• An endergonic reaction is a chemical reaction where energy is absorbed.

Chemical reactions are interactions between molecules, and the release of energy from ATP is no exception. It needs a reaction partner: water.

Now, let's look at the definition of ATP hydrolysis.

ATP Hydrolysis Mechanism

To continue our journey of ATP hydrolysis, let's look at its mechanism. ATP stores and, more importantly, supplies energy in its phosphate bonds.

During ATP hydrolysis, dephosphorylation occurs.

Specifically, it loses an orthophosphate, which is a single, unbound phosphate group. The resulting molecule is called adenosine diphosphate, or ADP.

ATP Hydrolysis Equation

The equation for ATP hydrolysis is as follows:

ATP Hydrolysis Reaction

The ATP hydrolysis reaction is exergonic, which means it releases energy. This exergonic reaction releases 30.5 kJ per mole of ATP under standard conditions.

• A standard reaction (under standard condition) presumes an equal amount of ATP and water. Of course, in a cell, there is plenty of water and much less ATP. Correcting for a non-standard reaction, the ATP hydrolysis reaction has the potential to release 45 to 75 kJ/mol.

The reversal of ATP hydrolysis is called condensation. Since ATP hydrolysis is an exergonic reaction, then the reverse is clearly an endergonic reaction. This means energy must be added to the reaction to bind the orthophosphate on ADP. During condensation, the hydroxyl group on orthophosphate unbinds and bonds with a free hydrogen proton to form water.

Free Energy from ATP Hydrolysis

Now, let's talk about free energy.

At 30.5 kJ per mole, the phosphate bond is considered a high-energy bond because it releases a lot of free energy! The bond itself isn’t special, though. ATP contains phosphoanyhdride bonds, which are chemical bonds between two phosphate groups.

So, why is it labeled “high-energy”? Let's find out!

1. The unique structure of ATP contributes to its efficacy as an energy delivery molecule. The chain of phosphate groups on ATP, all with -3 charge, act like magnets with the same polarity. They exert repulsive forces against each other, so that when a reaction occurs that releases a phosphate group, it releases it strongly and willingly!

2. Also, ATP hydrolysis increases entropy. Recall the second law of thermodynamics, which say the natural state of a closed system favors entropy. Thus, ATP hydrolysis is spontaneous.

3. Orthophosphate is highly stable, more so than ATP. This implies the forward movement of the chemical reaction (i.e. ATP hydrolysis, not condensation) is favored.

Orthophosphate has four oxygen bonded to its central phosphorus atom. One of those bonds is a double bond that is mobile and can jump between the oxygen atoms (Fig. 2). The moving double bond rearranges the charge distribution and makes orthophosphate less prone to form or reform phosphoanhydride bonds.

Besides energy distribution, ATP hydrolysis also yields a phosphate group. This detached phosphate group doesn't go to waste, it is recycled during ATP synthesis!

During the glycolysis step, a free phosphate group attaches to glucose to become phosphorylated glucose. The phosphate group act as a way to label the glucose molecule so that it moves forward during ATP synthesis.

ATP hydrolase (ATPase)

If ATP hydrolysis is a spontaneous reaction, you may be imagining a torrent of ATP being produced by hydrolysis. Cells are full of water, after all! However, this is not the case. ATP hydrolysis in cells often requires a catalyst, such as an enzyme.

The use of ATP hydrolase allows for some control on when and where ATP hydrolysis. Energy coupling is the combination of two reactions, in which the energy producing reaction powers a second reaction. ATP hydrolysis, the exergonic reaction, is frequently coupled with an endergonic reaction which performs a vital cellular function.

Without energy coupling, ATP hydrolysis would occur aimlessly! Nearly all the energy produced would be converted to thermal energy.

Thermal energy is important because it allows cells and organisms to regulate their own temperature. Yet, energy regularly needs to be directed and converted to perform a specific function. Instead of heat, the energy can be used to perform movement, to create molecules, or for storage.

Here are some examples of energy coupling that use ATP hydrolysis:

• Muscle Contraction: In muscles, ATP binds to the contracting protein myosin. This triggers myosin to shift, which contracts the muscle.

• Anabolism: Sometimes, a cell needs to assemble molecules. To do so, it must form bonds between molecules, which requires the energy provided by ATP hydrolysis.

• Ion transport: The typical example is the sodium-potassium pump, a protein in the cell membrane. ATP provides energy to this protein to move sodium or potassium actively, against its concentration gradient.