Although it has been proposed that the IGPS structure 3zr4 (structurally consistent with our Inactive model) is in a catalytically competent conformation with the Gly52 N-H completing the oxyanion hole, our data instead indicate that the energy associated with the Inactive geometry hinders thioester formation. There are multiple structural differences between Active and Inactive models. These include the Val51 backbone flip and fGln123 proximity, but also less apparent variations, such as the relative position of the oxyanion strand (Figure ). The capacity of Gly52 to serve as an H-bond donor along the reaction coordinate may depend on the subtle positioning of the oxyanion strand. Additionally, it is unclear from the energy profiles alone how proximity of fGln123 influences the reaction. We therefore pursued additional computational studies to deconvolute the energetic effects due to specific structural variations in the IGPS acylation reaction.