Graphical Abstract.
Thermodynamic cycle analysis of the free energy contributions of the HxGH and KMSKS catalytic signatures in Leucyl-tRNA synthetase and its 129-residue urzyme identify dramatic changes in the relative contributions of the KMSKS sequence and the HVGH*KMSKS two-way interaction. Structural analysis shows that a conserved network of non-polar packing in the anticodon binding domain functions as an “Enforcer” to impose coupled behavior in the full-length enzyme.