If there is one molecule that could be called the ‘currency’ of life, it is ATP: adenosine triphosphate. It belongs to a large class of molecules of life, which are all high in energy and have constituent parts that would – given enough time – much prefer to separate.
ATP has three parts:
1. adenosine, a ‘molecular handle’ comprising a complex double ring,
2. ribose, a five-carbon sugar, and
3. the ‘business end’, with three phosphates in a row. These phosphates would prefer to be whizzing about in the lovely, polar water rather than stuck rigidly in a row. Each has a fearsomely negative charge, so they jostle against one another.
When hydrolysis does cause a phosphate to fall off, ATP is reduced to ADP (adenosine diphosphate), and when the next one goes it becomes AMP (adenosine monophosphate).
However, ATP doesn’t just burn itself away. Left to their own devices, ATP’s phosphates hydrolyse rather slowly, so their stored chemical frustration can be released by proteins as required.
Highly traded currency
ATP is constructed mainly by harnessing proton gradients (see water, the first molecule of Christmas). It is a practical form of energy that fuels virtually every other process in the cell – like transporting molecules in and out of the cell, bringing proteins into contact with each other, moving vesicles about, and making new chemicals and proteins.
There are many similar, high-energy, kinetically stable molecules in life aside from ATP. Notable examples include triphosphates that contain other nucleotide bases: guanine (GTP), and cytosine (CTP). Some things use GTP as an energy source, but ATP is the most widely traded currency for energy.
How proteins plug in
The “A” in ATP is a distinctive shape that nestles comfortably into proteins. Nearly all of the proteins that do use ATP for energy have a special ‘pocket’ just for adenine.
When the adenine nestles into this protein pocket on one side of the protein, that pulls the pent-up phosphates into position on another spot – they are then lined up perfectly for the last one in line to be removed, and energy to be released.
The removal of that last phosphate depends on the protein. The energy can only be released if the protein is exactly the right shape to cut that bond. The cutting of the bond will often move aspects of protein.
Sometimes (as in the case of myosin, a muscle protein) that energy is used for physical movement overall. Sometimes, it’s used to join things together and make a new molecule – like when an amino acid is joined to a tRNA so it can get ready to make new protein.
Sometimes the phosphate is transferred to another molecule – a protein or another small molecule – but more often it is simply released. Once the job is done, the resulting two-phosphate ADP falls out of the pocket, the free phosphate floats off, surrounded by solvating waters, and the cycle starts again.
These cycles are happening billions of times every second in every living thing. Our living is a constant hum of ATP changing to ADP and P, and then being recreated elsewhere.
In theory, the A – adenosine – could be anything relatively stable with a distinctive, recognisable shape. But it is not by chance that this is exactly the same molecule as part of RNA, best known for its mediation between DNA instruction and protein manufacture.
This goes to the likely origin of life, where everything was RNA: the genetic code, the catalysis engines and the ability to capture energy. Remnants of that RNA world are everywhere in the main, massive enzymes of life, such as the Ribosome – and in every protein which binds ATP. Everywhere, the currency of energy is still this ancient molecule.
ATP in EMBL-EBI data resources
Find out more about ATP on the EMBL-EBI website