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Browsing by Author "Richards, Nigel G."
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Item Optimized CGenFF force-field parameters for acylphosphate and N-phosphonosulfonimidoyl functional groups(Springer, 2013-11) Hegazy, Lamees; Richards, Nigel G.; Department of Chemistry & Chemical Biology, IU School of ScienceWe report an optimized set of CGenFF parameters that can be used to model small molecules containing acylphosphate and N-phosphonosulfonimidoyl functional groups in combination with the CHARMM force field. Standard CGenFF procedures were followed to obtain bonded interaction parameters, which were validated by geometry optimizations, comparison to the results of calculations at the MP2/6-31+G(d) level of theory, and molecular dynamics simulations. In addition, partial atomic charges were assigned so that the energy of hydrogen bonding of the model compounds with water was correctly reproduced. The availability of these parameters will facilitate computational studies of enzymes that generate acyladenylate intermediates during catalytic turnover. In addition, given that the N-phosphonosulfonimidoyl moiety is a stable transition state analog for the reaction of ammonia with an acyladenylate, the parameters developed in this study should find use in efforts to develop novel and potent inhibitors of various glutamine-dependent amidotransferases that have been validated as drug targets. Topology and parameter files for the model compounds used in this study, which can be combined with other CGenFF parameters in computational studies of more complicated acylphosphates and N-phosphonosulfonimidates are made available.Item Probing the mechanism of Bacillus subtilis oxalate decarboxylase(2015-12-01) Zhu, Wen; Richards, Nigel G.; Li, Lei; Shah, Kavita; Mesecar, Andrew; Long, Eric C.Oxalate decarboxylase (EC 4. 1. 1. 2 OxDC) from Bacillus subtilis is a manganese-dependent enzyme that catalyzes the cleavage of the chemically inactive C-C bond in oxalate to yield formate and carbon dioxide. A mechanism involving Mn(III) has been proposed for OxDC, however no clear spectroscopic evidence to support this mechanism has yet been obtained. In addition, a recent study has shown that N-terminal metal binding site loop variants of OxDC were able to catalyze the oxidation of oxalate to yield hydrogen peroxide and carbon dioxide, which makes OxDc function as another oxalate degradation protein in the cupin superfamily, oxalate oxidase (EC 1.2.3.4 OxOx). In this work, wild-type (WT) Bacillus subtilis OxDC and a series of variants with mutations on conserved residues were characterized to investigate the catalytic mechanism of OxDC. The application of membrane inlet mass spectrometry (MIMS), electronic paramagnetic resonance (EPR) spectroscopy and kinetic isotope effects (KIEs) provided information about the mechanism. The Mn(III) was identified and characterized under acidic conditions in the presence of dioxygen and oxalate. Mutations on the second shell residues in the N-terminal metal binding site affected the enzyme activity properties of the metal. In the N-terminal domain, the functional importance of the residues in the active site loop region, especially Glu162, was confirmed, and evidence for the previously proposed mechanism in which OxDC and the OxDC/OxOx chimeric variant share the initial steps has been found. In addition, the mono-dentate coordination of oxalate in the N-terminal metal binding site was confirmed by X-ray crystallography. A proteinase cleavable OxDC was constructed and characterized, revealing the interaction between the N-terminal and C-terminal domains.