ATP is a molecule used by every cell known to temporarily store the energy derived from the food you eat. Just how temporary is my question. The beauty of ATP is that it is a pretty high energy molecule, and most processes in cells that require energy get it from ATP. So it connects the energy harvesting reactions of a cell to the energy spending ones. The analogy I use while teaching is that its like the cash in my wallet. I harvest money at work and spend money at stores, cash is the intermediary, and cash is not very stable in my wallet. Its not a great analogy (especially because I don’t rip two-inches off of the bills while holding them really close to the register and I generally pay for everything with credit, which doesn’t fit into the analogy at all) but it works.
There are a lot of energy spending reactions, and most of the test tube reactions I have done in the lab require the addition of ATP (else your reaction will not go). It comes as a white powder, and every researcher I have ever known considers it to be a very unstable molecule, meaning that it is a high enough energy molecule that it sort of just “falls apart”. We keep it on ice, we freeze it in ultra cold freezers, instead of freezing 1 ml of it in one tube we break that 1ml into 10 tubes so as to avoid freeze/thawing it. I had never thought about it too much, but had always considered ATP to be unstable and that made a certain amount of sense to me given its role as a temporary energy storage molecule (if it stored the energy stably, it would be hard to get the energy back out for the energy spending reactions). But this isn’t the kind of thing that we measure, none of the labs I have worked in cared specifically about the stability of ATP so long as there was enough of it in your tube to make your reaction go.
This thinking has even leaked out during my teaching… I don’t remember, but I am fairly sure that I have told classes that ATP IS a very temporary place to store energy and that once the ATP is made, that it doesn’t last very long. However I have never actually seen data that actually asks the question: “how long does a molecule of ATP last?”. All compounds fall apart at some rate, some faster than others, but I had always assumed that ATP’s stability would be measured in hours, if not minutes. Well, snake research to the rescue.
According to a 1997 study (1) looking at NTP levels (ATP is an NTP (there are three others, CTP, GTP and TTP) in snake red blood cells, you can leave ATP in a beaker for 24 hours with no drop in levels. I am sure this is underwhelming to many many people, but to me its an important fact when thinking and teaching about cellular metabolism: if ATP is very unstable, then it can not be an energy storage molecule, if it IS truly that stable, then it can. Probably in our mammalian cells this is never an issue – as endotherms we are constantly burning though energy to keep ourselves warm, but it COULD possibly be more of a real issue for ectotherms (which is to say, most animals (don’t be fooled, mammals are actually a very small subset of animals)). As an ectotherm cools down, could its metabolism slow down to the point where it is actually relying on the “stability of ATP”??. Given the known ability of mitchondrial electron transport chains to generate free radicals, I would think that it would be advantageous to bank on ATPs stability in order to slow ETC as much as possible. Which naturally leads me to question: is there any indication that the mitochondrial genomes of ectotherms is LESS damaged than those of endotherms??
And yes, I know that a cell isn’t a beaker of buffer and that there are many ATPases in every cell just waiting to lay waste. But come on, isn’t anyone else even a little bit surprised that ATP is THAT stable on its own? How many times have YOU saved that tube of NTPs floating in your ice bucket the next morning?
1. Ingerman et al 1997 J. Exp Biol “Stability of nucleoside triphosphate levels in the red cells of the snake”