  Life is an exergonic chemical reaction. Many individual reactions in
metabolism entail slightly endergonic processes that are coupled to free energy
release, typically as ATP hydrolysis, in order to go forward. ATP is almost
always supplied by the rotor-stator ATP synthetase (the ATPase), which
harnesses chemiosmotic ion gradients. Because the ATPase is a protein, it arose
after the ribosome did. Here we address two questions using comparative
physiology: What was the energy currency of metabolism before the origin of the
ATPase? How (and why) did ATP come to be the universal energy currency? About
27 percent of a cell's energy budget is consumed as GTP during translation. The
universality of GTP-dependence in ribosome function indicates that GTP was the
ancestral energy currency of protein synthesis. The use of GTP in translation
and ATP in small molecule synthesis are conserved across all lineages,
representing energetic compartments that arose in the last universal common
ancestor, LUCA.
