Arginine kinase catalyzes reversible phosphoryl transfer between arginine and ATP. Crystal structures of arginine kinase in an open, substrate-free form and closed, transition state analog (TSA) complex indicate that the enzyme undergoes substantial domain and loop rearrangements required for substrate binding, catalysis, and product release. Nuclear magnetic resonance (NMR) has shown that substrate-free arginine kinase is rigid on the ps-ns timescale (average S-2 = 0.84 +/- 0.08) yet quite dynamic on the mu s-ms timescale (35 residues with R-ex, 12%), and that movements of the N-terminal domain and the loop comprising residues I182-G209 are rate-limiting on catalysis. Here, NMR of the TSA-bound enzyme shows similar rigidity on the ps-ns timescale (average S-2 = 0.91 +/- 0.05) and substantially increased mu s-ms timescale dynamics (77 residues; 22%). Many of the residues displaying mu s-ms dynamics in NMR Carr-Purcell-Meiboom-Gill (CPMG) N-15 backbone relaxation dispersion experiments of the TSA complex are also dynamic in substrate-free enzyme. However, the presence of additional dynamic residues in the TSA-bound form suggests that dynamics extend through much of the C-terminal domain, which indicates that in the closed form, a larger fraction of the protein takes part in conformational transitions to the excited state(s). Conformational exchange rate constants (k(ex)) of the TSA complex are all approximately 2500 s(-1), higher than any observed in the substrate-free enzyme (800-1900 s(-1)). Elevated mu s-ms timescale protein dynamics in the TSA-bound enzyme is more consistent with recently postulated catalytic networks involving multiple interconnected states at each step of the reaction, rather than a classical single stabilized transition state. (C) 2017 Elsevier Inc. All rights reserved.