A mobile loop in tyrosyl-tRNA synthetase, which corresponds to the KMSKS signature sequence of class I aminoacyl-tRNA synthetases, destabilizes the E.Tyr.ATP complex but stabilizes the following E.[Tyr-ATP]not equal to transition state for the formation of E.Tyr-AMP. Three amino acid residues in the mobile loop, K230, K233, and T234, are known to be primarily responsible for these effects. We now analyze the network of interactions between these three amino acids using multiple mutant free energy cycles. The complete characterization of the coupling energies within the mobile loop allows each of the steps leading to the formation of the transition state complex to be dissected into its energetic components. In particular, it is found that, in the absence of a functional mobile loop, there is synergistic coupling between the tyrosine and ATP substrates (i.e., each enhances the binding affinity of the other) which stabilizes the E.Tyr.ATP intermediate preceding the transition state complex. Thus, the mobile loop disrupts the synergism between the ATP and tyrosine substrates, using the ATP binding energy to stabilize the transition state for the reaction. Whereas the net effect of the mobile loop in the E.Tyr.ATP complex results from several conflicting side chain interactions that tend to offset each other, conflicting interactions in the E.[Tyr-ATP]not equal to transition state complex have been minimized and stabilizing pairwise interactions between the K230, K233, and T234 side chains are optimized. The tight coupling between the side chains of K230, K233, and T234 suggests that the mobile loop adopts a highly constrained conformation during formation of the transition state complex. These results quantitatively demonstrate the importance of side chain interactions in enzyme catalysis and illustrate the use of binding energy to stabilize the transition state of a reaction and the presence of unfavorable interactions to destabilize the ground state.