Ain (AMPbd: residues 30?9), and the CORE domain (residues 1?9, 60?21, and 160?14) [13,14]. The relative positions and orientations of the AMPbd and LID domains characterize the major difference between various AdK conformations. Apart from experimental investigations [10,15,16], conformational transitions in AdK have also been extensively studied in get Apocynin molecular simulations [13,14,17?0] employing a variety of sampling techniques. Many simulation studies focus on the ligand-free state of AdK, which is believed to be part of the catalytic cycle. To name a few, Kubitzki and de Groot identified a transition pathway between the open and the closed conforma-tions of ligand-free AdK, using temperature-enhanced essential dynamics replica exchange [13]. Arora and Brooks [17] computed the free energy as a function of the difference in the root mean square deviations (RMSDs) with respect to the open and closed crystal structures. Beckstein et al. 11967625 applied dynamic importance sampling to reveal the conformational changes [14]. More recently, using the string method [21], Matsunaga et al. calculated the free energy profiles along the transition pathways for the ligand-free and ligand-bound AdK [18]. Currently, simulations employing different sampling methods do not seem to have reached a consensus conclusion concerning the AdK conformations. For the ligand-free AdK, e.g., some calculated free energies indicate that the closed conformation would not be stable, whereas other studies suggest that it is a metastable state MedChemExpress Microcystin-LR instead. In this study, we aim to examine the stability and dynamics of the open 23148522 and closed AdK conformations using molecular dynamics simulations. Specifically, our simulations are designed to offer insight into the following questions: Which conformation would a ligand-free AdK predominantly adopt at equilibrium? Are both the open and closed conformations metastable? What is the difference in the equilibrium probability (or equivalently, the free energy) between the two conformations? To help answer the questions above, we carry out two types of simulations here. The first type involves simulations starting from the open or the closed conformation of a ligand-free AdK, without any applied restraints. These unrestrained simulations could offer a robust and unbiased test on the stability of a given protein conformation, as they are not subject to the assumptions andAdenylate Kinase Conformationapproximations involved in the various enhanced sampling methods. Indeed, unrestrained simulations of AdK were reported in several earlier studies [13,22,23]. Brokaw and Chu simulated the open and closed conformations of AdK with and without the bound ligand [22], and observed some complete or partial spontaneous transitions between the two conformations. Ramanathan et al. also performed unrestrained simulations starting from the two AdK conformations, and analyzed the trajectories using a novel quasi-anharmonic technique [23]. Currently, the outcomes from the unrestrained simulations appear to vary somewhat from study to study. For the closed-state ligand-free AdK, e.g., in some simulations a complete closed-to-open transition was observed within ,100 ns, whereas in others only a partial opening event occurred. Such variation could arise either from the differences in the simulation protocols (protein force field, water model, etc.), or from the intrinsic protein flexibility. To clarify this issue, here we initiate multiple unrestrained simulations f.Ain (AMPbd: residues 30?9), and the CORE domain (residues 1?9, 60?21, and 160?14) [13,14]. The relative positions and orientations of the AMPbd and LID domains characterize the major difference between various AdK conformations. Apart from experimental investigations [10,15,16], conformational transitions in AdK have also been extensively studied in molecular simulations [13,14,17?0] employing a variety of sampling techniques. Many simulation studies focus on the ligand-free state of AdK, which is believed to be part of the catalytic cycle. To name a few, Kubitzki and de Groot identified a transition pathway between the open and the closed conforma-tions of ligand-free AdK, using temperature-enhanced essential dynamics replica exchange [13]. Arora and Brooks [17] computed the free energy as a function of the difference in the root mean square deviations (RMSDs) with respect to the open and closed crystal structures. Beckstein et al. 11967625 applied dynamic importance sampling to reveal the conformational changes [14]. More recently, using the string method [21], Matsunaga et al. calculated the free energy profiles along the transition pathways for the ligand-free and ligand-bound AdK [18]. Currently, simulations employing different sampling methods do not seem to have reached a consensus conclusion concerning the AdK conformations. For the ligand-free AdK, e.g., some calculated free energies indicate that the closed conformation would not be stable, whereas other studies suggest that it is a metastable state instead. In this study, we aim to examine the stability and dynamics of the open 23148522 and closed AdK conformations using molecular dynamics simulations. Specifically, our simulations are designed to offer insight into the following questions: Which conformation would a ligand-free AdK predominantly adopt at equilibrium? Are both the open and closed conformations metastable? What is the difference in the equilibrium probability (or equivalently, the free energy) between the two conformations? To help answer the questions above, we carry out two types of simulations here. The first type involves simulations starting from the open or the closed conformation of a ligand-free AdK, without any applied restraints. These unrestrained simulations could offer a robust and unbiased test on the stability of a given protein conformation, as they are not subject to the assumptions andAdenylate Kinase Conformationapproximations involved in the various enhanced sampling methods. Indeed, unrestrained simulations of AdK were reported in several earlier studies [13,22,23]. Brokaw and Chu simulated the open and closed conformations of AdK with and without the bound ligand [22], and observed some complete or partial spontaneous transitions between the two conformations. Ramanathan et al. also performed unrestrained simulations starting from the two AdK conformations, and analyzed the trajectories using a novel quasi-anharmonic technique [23]. Currently, the outcomes from the unrestrained simulations appear to vary somewhat from study to study. For the closed-state ligand-free AdK, e.g., in some simulations a complete closed-to-open transition was observed within ,100 ns, whereas in others only a partial opening event occurred. Such variation could arise either from the differences in the simulation protocols (protein force field, water model, etc.), or from the intrinsic protein flexibility. To clarify this issue, here we initiate multiple unrestrained simulations f.