Adenosine triphosphate or ATP is often called the energy currency of the cell because this molecule plays a key role in metabolism, particularly in energy transfer within cells. The molecule acts to couple the energy of exergonic and endergonic processes, making energetically unfavorable chemical reactions able to proceed.
Metabolic Reactions Involving ATP
Adenosine triphosphate is used to transport chemical energy in many important processes, including:
- aerobic respiration (glycolysis and the citric acid cycle)
- cellular division
- motility (e.g., shortening of myosin and actin filament cross-bridges as well as cytoskeleton construction)
- exocytosis and endocytosis
- protein synthesis
In addition to metabolic functions, ATP is involved in signal transduction. It is believed to be the neurotransmitter responsible for the sensation of taste. The human central and peripheral nervous system, in particular, relies on ATP signaling. ATP is also added to nucleic acids during transcription.
ATP is continuously recycled, rather than expended. It's converted back into precursor molecules, so it can be used again and again. In human beings, for example, the amount of ATP recycled daily is about the same as body weight, even though the average human being only has about 250 grams of ATP. Another way to look at it is that a single molecule of ATP gets recycled 500-700 times every day. At any moment in time, the amount of ATP plus ADP is fairly constant. This is important since ATP is not a molecule that can be stored for later use.
ATP may be produced from simple and complex sugars as well as from lipids via redox reactions. For this to occur, the carbohydrates must first be broken down into simple sugars, while the lipids must be broken into fatty acids and glycerol. However, ATP production is highly regulated. Its production is controlled via substrate concentration, feedback mechanisms, and allosteric hindrance.
As indicated by the molecular name, adenosine triphosphate consists of three phosphate groups (tri- prefix before phosphate) connected to adenosine. Adenosine is made by attaching the 9' nitrogen atom of the purine base adenine to the 1' carbon of the pentose sugar ribose. The phosphate groups are attached connecting and oxygen from a phosphate to the 5' carbon of the ribose. Starting with the group closest to the ribose sugar, the phosphate groups are named alpha (α), beta (β), and gamma (γ). Removing a phosphate group results in adenosine diphosphate (ADP) and removing two groups produces adenosine monophosphate (AMP).
How ATP Produces Energy
The key to energy production lies with the phosphate groups. Breaking the phosphate bond is an exothermic reaction. So, when ATP loses one or two phosphate groups, energy is released. More energy is released breaking the first phosphate bond than the second.
ATP + H2O → ADP + Pi + Energy (Δ G = -30.5 kJ.mol-1)
ATP + H2O → AMP + PPi + Energy (Δ G = -45.6 kJ.mol-1)
The energy that is released is coupled to an endothermic (thermodynamically unfavorable) reaction in order to give it the activation energy needed to proceed.
ATP was discovered in 1929 by two independent sets of researchers: Karl Lohmann and also Cyrus Fiske/Yellapragada Subbarow. Alexander Todd first synthesized the molecule in 1948.
|Molecular Mass||507.18 g.mol-1|
What Is ATP an Important Molecule in Metabolism?
There are essentially two reasons ATP is so important:
- It's the only chemical in the body that can be directly used as energy.
- Other forms of chemical energy need to be converted into ATP before they can be used.
Another important point is that ATP is recyclable. If the molecule was used up after each reaction, it wouldn't be practical for metabolism.
- Want to impress your friends? Learn the IUPAC name for adenosine triphosphate. It's (2"R",3"S",4"R",5"R")-5-(6-aminopurin-9-yl)-3,4-dihydroxyoxolan-2-ylmethyl(hydroxyphosphonooxyphosphoryl)hydrogen phosphate.
- While most students study ATP as it relates to animal metabolism, the molecule is also the key form of chemical energy in plants.
- The density of pure ATP is comparable to that of water. It's 1.04 grams per cubic centimeter.
- The melting point of pure ATP is 368.6°F (187°C).