Why Do Our Cells Contain So Much ATP?

Credit: Luca Galuzzi

Credit: Luca Galuzzi

ATP is the universal energy currency in living organisms. So, the question why our cells contain so much of it may seem strange. Yet, ATP concentrations in the cell are about a thousand fold higher than the concentration typically required for enzymes to do their job. One explanation is that the high concentrations are needed to fuel the multitude of reactions taking place simultaneously within the cell. But now a new paper published in Science suggests that there’s another reason why our cells contain so high ATP concentrations, ATP helps to keep the proteins in our cell soluble. 
 
Everyone has experienced that salt or sugar has a limited solubility in water. No matter how much you stir or shake at a certain point no more salt or sugar will dissolve. Just like salt or sugar other molecules also have a limited solubility in water. Our cells contain an incredibly concentrated solution of proteins, metabolites, nucleic acids, and other molecules. Failure of molecules to stay soluble results in diseases. For example, in amyloid-beta proteins start to clump together in Alzheimer’s disease, losing their solubility and eventually forming insoluble protein clumps known as plaques. 
 
In this new paper, researchers demonstrate that ATP helps to keep proteins soluble by acting as a hydrotrope. First described in 1916 by the German biochemist Carl Neuberg, hydrotropes are molecules that greatly increase the solubility of otherwise badly soluble organic molecules in water. For example, many liquid detergents, liquid soaps, and shampoos contain hydrotropes to help keep the detergent molecules dissolved.
 
The authors speculate that the age-related decrease in cellular ATP could contribute to a loss of protein solubility and hence explain the increased incidence of neurodegenerative diseases, like Alzheimer’s disease, with age.     
 
Patel A et al. (2017). ATP as a biological hydrotrope. Science 356: 753-756.