Proteins: ATP as an Energy Source

A cell is a dynamic place. Enzymes are speeding up chemical reactions. DNA, RNA, and proteins are being synthesized. Molecular motors are carrying organelles around the cell. Enzymes are repairing DNA lesions. The cell membrane is selectively letting specific molecules and ions in and out of the cell. The list goes on. It's a bustling environment!

But what is the energy source fueling all this activity? One good answer is "food." The food we eat consists of various kinds of molecules comprised of chemical bonds that, when broken, release energy that fuels the cell's countless activities.

But there's another more proximal, more correct answer: ATP. "ATP" stands for "adenosine triphosphate." This is the same adenosine that serves as one of the four DNA nucleotides. But with three phosphate ions attached, ATP serves another massively important role in the cell. Basically, it's the cell's energy source--the cell's "food" so to speak.

Let's take a step backward. When we eat, our gastrointestinal tract turns the carbohydrates, proteins and lipids in our food into nutrients. These nutrients are more or less the monomers that made up each of these polymers: sugars, amino acids and fatty acids, respectively.

These nutrients enter the bloodstream to be delivered to cells throughout the body. The amount of nutrients delivered to a given cell depends on the energy demands of that cell. Muscle, liver, brain and heart cells require a lot of nutrients and, and as we will see, make a lot of ATP.

In most cells, most of the ATP production occurs in the mitochondria. Metabolically active cells like heart muscle cells might have 5,000-8,000 mitochondria whereas less metabolically active cells like skin cells might have a few to a few hundred mitochondria.

The bottom line is that, at the cellular level, the energy in the chemical bonds of nutrients is converted into energy-filled ATP chemical bonds. The ATP molecules then float around the cell, ready to be used to fuel most cellular processes.

Now let's take a quick look at the ATP molecule itself (top half of the figure). Most of the molecule is adenosine (adenine + ribose). But the adenosine has three yellow phosphates ions attached to each other in a row--thus the name: adenosine triphosphate.

As also shown in the figure, there's energy in the bonds between the second and third phosphate and between the first and second phosphate. When either of these bonds is broken (especially the bond between the second and third phosphate) energy is released and we are left with ADP (adenosine diphosphate) and a phosphate ion.

I realize the idea of "energy being released" is vague. Energy is a tricky concept, and not one I want to get into here (that's physics). So once again, let's use an analogy. Think of the bonds between phosphates as loaded springs. When one of the bonds breaks, it's like the spring being released. When a spring pops open, it expends energy which can be used to literally move physical objects.

Likewise, when the third phosphate is released from ATP, the energy released can literally move physical molecules or parts of molecules. If the spring analogy isn't working for you, you can think of the breaking of that phosphate bond as a small explosion that, like a released spring, has the ability to move molecules or molecule domains.

The cellular world's incredible dynamism is due to ATP. Think of it as the energy currency of the cell. As we progress, we'll see many instances where ATP actually imparts movement on molecules or their parts, or domains.

Finally, once ATP is cleaved into ADP and a phosphate ion, the cell uses energy from the food we eat to reload that third phosphate, returning the molecule to the much more energetically powerful ATP form. The figure to the right depicts the cyclic transitioning from ATP to ADP and then back to ATP, etc.

In the next post, we'll look more closely at how one specific protein (myosin) uses ATP to generate muscle movement.