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Why is ATP the Energy Currency, Not the Others?

Author: Louis Li

Editors: Joshua Payne, Miriam Heikal, Jonathan Chen

Artist: Helen Gong


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If a student has ever taken a biology class in middle or high school, they are likely familiar with the abbreviation ATP. With the full name adenosine triphosphate, ATP is the most common energy molecule in the bodies of organisms, carrying energy in its high- energy phosphate bonds, which release energy when broken to fuel cellular reactions.  However, ATP is not the only molecule that contains high-energy phosphate bonds. There are also molecules like GTP (guanosine triphosphate), CTP (cytidine triphosphate), UTP (uridine triphosphate), and TTP (thymidine triphosphate) that contain the same high-energy bonds. In fact, the only difference between these triphosphate molecules is the nitrogenous base, a ring-like structure that contains nitrogen. This raises an important question: why is ATP, and not one of these other molecules, the universal energy currency of life? Theories have been proposed from the angles of molecular structure, evolutionary preference, and job division. First, when comparing the structure of these triphosphates, it turns out that the only structural difference—the nitrogenous bases—actually matters. Unlike other triphosphates, the nitrogenous base of ATP does not contain an oxygen double-bonded to carbon. Chemical bonds are, in reality, two electrons that pair up and cannot be easily separated. 

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Meanwhile, electrons are negative. Therefore, a double bond with four electrons will make the whole molecule more negative than a single bond with two electrons. The more negative a molecule is, the more “abnormal” it is compared to other molecules, forcing enzymes to adapt by reshaping themselves before catalysis can occur. The nitrogenous base of ATP is the only one that does not contain double-bonded oxygen, meaning it has the least negative charge. This structural advantage avoids the drastic shape change for ATP-related reactions, saving energy and lowering the threshold of ATP-related reactions. As a result, ATP can be used easily and efficiently in energy- transfer processes. 

From an evolutionary standpoint, the theory of ATP suggests that the choice of ATP is closely associated with Earth's early environment. Billions of years ago, the oxygen content on Earth was less than 0.1%, much lower than the 20% oxygen we have now. Considering that the nitrogenous base of ATP does not contain oxygen, while the nitrogenous bases of the rest of the triphosphates all regard oxygen as an essential component, this suggests that adenine may have appeared earlier. This preexistence allowed the massive accumulation of adenine in the early Earth environment. When ATP and other phosphates were later synthesized, the ready availability of adenine led to ATP vastly outnumbering the rest of the triphosphates. This advantage made it natural and convenient for biological reactions to favor ATP as their main energy carrier.

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Even with ATP’s advantages, other triphosphates can still take on its role of transporting energy, sometimes because of their same high-energy phosphate bonds. ATP is not replaced, and its status is not threatened because of the cells’ tendency to divide jobs for different molecules. For instance, ATP mainly converts energy in biochemical reactions and is the energy currency. GTP, on the other hand, plays a significant role in signal transduction and protein synthesis. UTP is mainly used for synthesizing glycogen, the form in which sugar is stored in the animal body. TTP participates in cell division and repair. CTP is used to synthesize another important molecule and engage in protein reactions. The specialized functions of different phosphates contribute to the specificity and precise regulation of cellular processes, enabling various functions of the organism to operate tightly and efficiently.

In conclusion, by studying the structure, evolution, and many other aspects of triphosphates, scientists have proposed different hypotheses to explain the ‘victory’ of ATP as life’s energy currency. While the complete story remains a mystery, it will possibly spawn more debates in the future. However, there is no doubt that breakthroughs about its secret will be crucial in our understanding of structure, function, evolution, and their intertwined relationship.

Citations:

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