Robust reaction rate enhancement and allosteric regulation are hallmarks of enzyme catalysis, and both aspects may be at least in part underpinned by protein conformational flexibility. The catalytic prowess of enzymes can be significantly ascribed to substrate binding to an electrostatically preorganised active site, which minimises the reorganisation energy required for optimum stabilisation of the charge redistribution as the reaction progresses from the reactant state to the transition state. Yet several lines of evidence also suggest a contribution from protein dynamics, from nonstatistical, femtosecond-timescale vibrations coupled directly to transition-state barrier crossing, to slower, thermally equilibrated motions reshaping the enzyme conformational ensemble towards populations where active-site preorganisation is optimised. Nonetheless, this topic is still controversial possibly due to the inherent flexibility of proteins which makes it difficult to isolate motions that may have evolved to facilitate reaction. Allosteric modulation of enzymes, i.e. the alteration of reaction rate and/or substrate affinity upon ligand binding to, mutation of, or post-translational modification at a site remote from the active site, is a fundamental regulatory mechanism of biochemical reactions. It finds applications in drug discovery to facilitate drug-target selectivity as allosteric sites tend to be less conserved than active sites across homologous proteins, and in enzyme engineering and synthetic biology, where allosteric control may need to be introduced or, more often, eliminated. While enzymes subject to allosteric regulation by ligand binding can be broadly classified as K-type, those where substrate affinity is altered, and V-type, those where the steady-state catalytic rate constant (kcat) is altered, the specific kinetic steps affected can vary depending on the enzyme. For instance, in Mycobacterium tuberculosis -isopropylmalate synthase, the rate-limiting step changes from product release to chemistry upon allosteric inhibition by leucine. The role of protein dynamics in allostery has been much less controversial when discussed in terms of conformational changes to promote physical events such as substrate binding and product release, or the interconversion rate among conformations. However, in systems where allosteric regulation affects the rate of the chemical step itself, the intersection at which local and remote protein motions, active-site electrostatic preorganisation, and ultimately catalysis meet remains challenging to pinpoint, despite recent advances toward this goal with Kemp eliminase. ATP phosphoribosyltransferase (ATPPRT) (EC 2.4.2.17) catalyses the Mg 2+dependent formation of N 1 -(5-phospho-β-D-ribosyl)-ATP (PRATP) and inorganic pyrophosphate (PPi) from ATP and 5-phospho-α-D-ribose 1-pyrophosphate (PRPP) (Fig. ), the first and flux-controlling step of histidine biosynthesis, and is allosterically inhibited by histidine in a negative feedback control loop. ATPPRT is the focus of synthetic biology efforts to enable the production of histidine in bacteria and a promising drug target against some pathogenic bacteria. Short-form ATPPRTs form an intricate allosteric system comprising catalytic (HisGS) and regulatory (HisZ) subunits assembled as a heterooctamer with a tetrameric core of HisZ sandwiched by two dimers of HisGS. HisGS on its own is catalytically active and insensitive to inhibition by histidine. Binding of HisZ, which has no catalytic power of its own, to form the ATPPRT holoenzyme allosterically activates catalysis by HisGS. However, HisZ also harbours the pocket where histidine binds and allosterically inhibits ATPPRT. Thus, the regulatory protein plays a dual role, as allosteric activator of catalysis in the absence of histidine and mediator of allosteric inhibition in the presence of histidine. showed Arg56 of one of the catalytic subunits reaching across the dimer interface to form a salt-bridge with the pyrophosphate moiety of PRPP in the active site of the other subunit (Fig. ) in the PaATPPRT structure but not in the PaHisGS structure. Therefore, allosteric activation was proposed to lead to more efficient leaving group departure at the transition state by stabilisation of the negative charge build-up on the pyrophosphate upon nucleophilic attack of ATP N1 on PRPP C1. Second, no burst in product formation was observed for the reaction catalysed by PaHisGS, and the multiple-turnover, pre-steady-state rate constant was in agreement with kcat, suggesting chemistry is rate limiting in the nonactivated enzyme reaction. In contrast, a burst was observed in the reaction catalysed by PaATPPRT, producing a rate-constant (kburst) much higher than kcat, supporting a mechanism where allosteric activation speeds up the chemical step, making product release rate limiting. Finally, replacement of Mg 2+ by Mn 2+ , which more efficiently offsets the negative charge at the transition state, led to an ~3-fold enhancement of PaHisGS kcat, as would be predicted, qualitatively, for a rate-limiting chemical step, but had no effect on PaATPPRT kcat, where chemistry was already much faster than subsequent steps. As expected, an R56A-PaHisGS mutant had a reduced reaction rate in the nonactivated enzyme as measured at a fixed concentration of substrates, since R56 was posited to be important for leaving group departure in PaHisGS as well, only less efficiently.