In the present work, we use a combination of conventional and enhanced sampling molecular dynamics simulations, as well as empirical valence bond calculations, to explore the role of loop dynamics in dictating the selectivity and evolvability of the evolutionarily important model enzymes, HisA and TrpF, which selectively catalyze the isomerization of substrates ProFAR and PRA, respectively (Figure ), as well as the bifunctional isomerase PriA, which catalyzes both reactions in bacteria such as M. tuberculosis. The roles of loop dynamics and ligand-gated conformational changes in TIM-barrel proteins and proteins from other folds have been a topic of substantial research interest (e.g. refs. , among many others). However, what makes the current enzymes stand out from these prior studies is the importance of not one but two (TrpF) or even three (HisA and PriA) long, mobile loops (Figure ), the specific conformations of which have been suggested to play an important role in facilitating the selectivity of PriA and evolved HisA variants. Thus, these enzymes undergo more complex loop dynamics than the aforementioned systems. In addition, prior enzymes that have been characterized as being activated by ligand-gated conformational changes, such as TPI and OMPDC, are extremely proficient enzymes. In contrast, all enzymes studied here are relatively inefficient (Table ), , with turnover numbers of ~10 s -1 or less, despite catalyzing a reaction that is intrinsically very fast. Related to this, while loop motions in highly proficient TIM-barrel enzymes such as TPI are relatively fast (on the μs timescale 96 ), motions of up to the ms timescale have been detected in the evolved HisA variants, and thus loop motions are likely to be (at least partially) rate-limiting in these enzymes.