# Overview of ATP hydrolysis

The human body is amazingly complex and yet, it functions because of a molecule that supports many of our essential life processes - called ATP (adenosine triphosphate). ATP is the energy currency of the cell and provides energy for processes that build molecules and tissues, to transport molecules across the cell membrane, to facilitate enzyme activities, to facilitate motion, and is the product of a few steps in metabolism (which you’ll see below).
Think about what happens when you decide to train for that marathon, triathlon, or other physically demanding pastime. First, you need to improve your cardio and build up your muscles. You’ll need energy to build those muscles and you’ll need to be able to contract your muscles to train. ATP will help you with both of those, as well as all of the other essential processes going on in your body at the same time!
Where does ATP hydrolysis fit into cellular respiration?
1. Glycolysis - where the simple sugar glucose is broken down. (Occurs in the cytosol) $\text{**}$ ATP hydrolysis happens here!
2. The product from glycolysis is transformed for the next step. (Occurs in the mitochondria )
3. The citric acid cycle - where energy precursors are made (Occurs in the mitochondria)
4. Oxidative phosphorylation - this process uses the electron transport chain to produce the bulk of usable energy for the cell (Occurs in the mitochondria)
5. ATP hydrolysis happens after the bulk of the energy is made in oxidative phosphorylation too!

## What is ATP hydrolysis?

ATP hydrolysis is how the cell breaks down ATP to use its energy. How does it do it? There’s a big clue in the word hydrolysis:
• Hydro - is greek for “water”
• Lysis - is greek for “to split”
In ATP hydrolysis, water is used to split apart ATP to create ADP (adenosine diphosphate) to get ENERGY!
Below is a figure of how biologists and chemists depict our reactants: ATP and water. ATP is made up of three phosphate groups, a ribose sugar, and a nitrogenous base called adenine, which is found in DNA and RNA. This is where the term adenosine triphosphate, or ATP, comes from.
ATP and water reactants diagram
Diagram of ATP broken down into Adenine (Nitrogen Base), Phosphate Groups, and Ribose (sugar)

### Step 1

When water molecules are present, ATP can start to be hydrolyzed. The first water molecule will donate one of its extra pairs (or “lone pairs”) of electrons to form a bond with the hydrogen from the second water molecule. When the first water molecule forms that bond with the hydrogen, it steals the hydrogen from the other water molecule. This makes a hydronium ion (H$\text{}_{3} \text O$+), and turns the second water molecule into an unstable hydroxyl group (-OH).

### Step 2

As the hydronium ion was made in step 1, the electrons that make the bond between the stolen hydrogen and its original water molecule are donated as a third lone pair to the oxygen of the robbed molecule. The robbed molecule is a hydroxyl group that is unstable, meaning it is eager to form new bonds.

### Step 3

Because of the instability of the hydroxyl group, one of its lone pairs forms a bond with the phosphorus of the first phosphate group of the ATP molecule.

### Step 4

The phosphate group that made a bond with the hydroxyl group (now called a phosphoryl group) breaks off from the ATP molecule. The electrons that formed the bond that attached that phosphate group to ATP move onto the neighboring oxygen. These actions cause the oxygen molecule to become negatively charged, produce ADP, (a phosphoryl group) and produce energy. The bonds of the phosphate group contained high energy electrons (also seen in the different steps of cellular respiration). When the phosphate is broken off, the electrons come down to a lower energy state. The difference in energy states is the energy that is released by the reaction.

### Products:

Diagram of hydronium, Phosphoryl Group, and ATP molecules

### To summarize:

ReactantsProducts
2 Water1 hydronium ion
1 ATP1 phosphoryl group
ENERGY!; ΔG = -30.5 kJ/mol of ATP
ATP hydrolysis doesn’t usually occur on its own, but as the fuel for another reaction. You can think of it like this:
Reactants and products diagram
When ATP hydrolysis occurs at the same time as another reaction, the reactions are called coupled reactions. Often, during anabolic reactions (like when you’re trying to build extra muscle for that triathlon), ATP will donate its phosphate group to the reactant to build a polymer (a molecule that is a long chain of repeating units). This coupling of reactions with ATP hydrolysis is what physically helps to build longer molecules, while also providing energy for the reaction to take place. That’s pretty handy!

## Consider the following:

What happens when there is too much ATP hydrolysis? Lactic acidosis happens! Lactic acidosis is a health condition that occurs when our blood and tissues become acidic because of a build of hydronium ions from ATP hydrolysis. The buildup of hydronium ions increases the acidity of the tissues and blood (hydronium ions are interchangeable with hydrogen ions when talking about acidity). This happens when our cells aren’t receiving enough oxygen, like when you’re doing anaerobic exercise (sprinting). Recall from cellular respiration that when oxygen is absent, fermentation replaces the citric acid cycle and oxidative phosphorylation, but glycolysis continues to occur so that some ATP are still being made. On top of increasing hydronium ions being made, there is also a buildup of lactate from fermentation, which is where lactic acidosis gets its name.

## Related articles:

• Glycolysis and gluconeogenesis
• The citric acid cycle
• Oxidative phosphorylation