ATP – twohundredandfifty grams of pure power

September 27, 2011

Approximately 250 gram of this chemical in our body give us the power to do what we want. Ok, the “power” (or better lets call it chemical potential energy from now on) we are talking about here is not free or pure. Actually, this potential energy originating from ATP (also called adenosine triphosphate) is stored in the covalent phosphoanhydride bonds which can be seen in Fig. 1 (red circles). I really like this molecules because every day I encounter its enormous significance for biology. In almost every biochemical reaction, every folding, or cell dynamical process there is almost a 100% certainty that ATP will be encountered. Per mol ATP 7.3 calories are released when one of the named phosphoanhydride bonds is broken. Because of the presence of two of these high-energy bonds ATP is present in large amounts in every cell and serves as the energy currency of the cell.

Fig. 1: A molecule of ATP. It is composed of three phosphate groups which are interlinked by phosphoanhydride bonds (red circles), a ribose sugar (circular molecule in the middle of the structure), and the so-called purine base adenosine. The four nucleotides that make up the DNA have actually a similar structure: Only two of the three phosphates are removed, one hydroxyl of the ribose is exchanged for a sole proton, and the bases are varying between purines and pyrimidines.

ATP is so special because some of its subparts. The two most important parts which lend ATP these behaviours are ADP, which is just missing one phosphate, and the other part is, what a surprise, the missing third phosphate called Pi. At neutral pH 7, which is usually is present in living cells, these to parts are strongly negatively charged. Two charges of the same kind do not combine very well. That is why the cell uses a lot of potential electrical energy called the proton-motive force (pmf) across the mitochondrial membrane to combine these charges via a protein called ATPase. Electrons required for the build-up of the pmf are transported across the mitochondrial membrane through the breakage of other energy storing chemical bonds in for example sugars during the process of glycolysis (oxidation reactions). The electron gradient of the pmf thus delivers the energy to let the endergonic reaction occur in which the  ADP and Pi molecules fuse into a newly formed covalent phosphoanhydride bond via ATPase. Since bonds release energy when they are cleaved, the already mentioned 7.3 calories/mol are available for doing work now. Using the chemical energy which was saved within these bonds, ATP can be used to transform this energy via one or more of so-called “energy coupling” reactions into mechanical kinetic energy. Resulting movement is often mediated by protein molecules which can now catalyze new chemical reactions or assist in folding other proteins due to their changed conformational state which “pushes” them into the right native state. But also “normal” movement can be induced, like the famous “power stroke” in which the muscular proteins actin and myosin are involved. Millions of power strokes at the same time finally result in the enormous strength a human leg or arm muscle can have.

All this and much more is based on this beautiful and relatively simple molecule. Fig. 2 depicts a self-made balls-and-sticks 3D model of ATP which is orientated in the same way as in Fig. 1. When observing carefully the same chemical groups which are also described in Fig. 1 can also be traced back here as well.

Fig. 2: A 3D ball-and-stick model of ATP. Atoms, bonds, and spacial forces between atoms can be clearly seen.

All depictions of chemical structures in this article were created by myself by making use of the program ACD/ChemSketch product version 12.01.

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