The ATP Molecule

Adenosine triphosphate (ATP) is the nucleotide known in biochemistry as the “molecular currency” of intracellular energy transfer; that is, ATP is able to store and transport chemical energy within cells. ATP also plays an important role in the synthesis of nucleic acids.

Chemical properties

Chemically, ATP consists of adenosine and three phosphate groups. It has the empirical formula C₁₀H₁₆N₅O₁₃P₃, and the chemical formula C₁₀H₈N₄O₂NH₂(OH)₂(PO₃H)₃H, with a molecular mass of 507.184 u.

Functions

Energy is released by hydrolysis of the third phosphate group. After this third phosphate group is released, the resulting ADP (adenosine diphosphate) can absorb energy and regain the group, thus regenerating an ATP molecule; this allows ATP to store energy like a rechargeable battery.

Phosphoryl positions

The phosphoryl groups starting with that on AMP are referred to as the alpha, beta, and gamma phosphates.

Synthesis

ATP can be produced by various cellular processes, most typically in mitochondria by oxidative phosphorylation under the catalytic influence of ATP synthase or in the case of plants in chloroplasts by photosynthesis.

The main fuels for ATP synthesis are glucose and fatty acids. Initially glucose is broken down into pyruvate in the cytosol. Two molecules of ATP are generated for each molecule of glucose. The terminal stages of ATP synthesis are carried out in the mitochondrion and can generate up to 34 ATP.

ATP in the human body

The total quantity of ATP in the human body is about 0.1 mole. The energy used daily by an adult calls for the hydrolysis of 200 to 300 moles of ATP. This means that each ATP molecule has to be recycled 2000 to 3000 times during the day. ATP cannot be stored and so its synthesis has to closely follow its consumption.

Other triphosphates

Living cells also have other “high-energy” nucleoside triphosphates, such as guanine triphosphate. Between them and ATP, energy can be easily transferred with reactions such as those catalyzed by nucleoside diphosphokinase: Energy is released when hydrolysis of the phosphate-phosphate bonds is carried out. This energy can be used by a variety of enzymes, motor proteins, and transport proteins to carry out the work of the cell. Also, the hydrolysis yields free inorganic phosphate and adenosine diphosphate, which can be broken down further to another phosphate ion and adenosine monophosphate. ATP can also be broken down to adenosine monophosphate directly, with the formation of pyrophosphate. This last reaction has the advantage of being an effectively irreversible process in aqueous solution.

Reaction of ADP with GTP

ADP + GTP > ATP + GDP

There is talk of using ATP as a power source for nanotechnology and implants. Artificial pacemakers could become independent of batteries.