The Experts below are selected from a list of 201 Experts worldwide ranked by ideXlab platform
Vern L Schramm - One of the best experts on this subject based on the ideXlab platform.
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Binding Causes the Remote [5‘-3H]Thymidine Kinetic Isotope Effect in Human Thymidine Phosphorylase
Journal of the American Chemical Society, 2004Co-Authors: Matthew R. Birck, Vern L SchrammAbstract:The remote 5‘-3H V/K Kinetic Isotope Effect (KIE) observed in human thymidine phosphorylase (6.1%) is significantly larger than can be explained by the reaction chemistry. One hypothesis connects the 5‘-3H KIE in purine nucleoside phosphorylase to that enzyme's SN1 transition state. The transition state of thymidine phosphorylase, however, is an SN2 nucleophilic displacement. Here we report equilibrium binding Isotope Effects sufficiently large to explain the presence of this substantial KIE in thymidine phosphorylase.
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binding causes the remote 5 3h thymidine Kinetic Isotope Effect in human thymidine phosphorylase
Journal of the American Chemical Society, 2004Co-Authors: Matthew R. Birck, Vern L SchrammAbstract:The remote 5‘-3H V/K Kinetic Isotope Effect (KIE) observed in human thymidine phosphorylase (6.1%) is significantly larger than can be explained by the reaction chemistry. One hypothesis connects the 5‘-3H KIE in purine nucleoside phosphorylase to that enzyme's SN1 transition state. The transition state of thymidine phosphorylase, however, is an SN2 nucleophilic displacement. Here we report equilibrium binding Isotope Effects sufficiently large to explain the presence of this substantial KIE in thymidine phosphorylase.
Matthew R. Birck - One of the best experts on this subject based on the ideXlab platform.
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Binding Causes the Remote [5‘-3H]Thymidine Kinetic Isotope Effect in Human Thymidine Phosphorylase
Journal of the American Chemical Society, 2004Co-Authors: Matthew R. Birck, Vern L SchrammAbstract:The remote 5‘-3H V/K Kinetic Isotope Effect (KIE) observed in human thymidine phosphorylase (6.1%) is significantly larger than can be explained by the reaction chemistry. One hypothesis connects the 5‘-3H KIE in purine nucleoside phosphorylase to that enzyme's SN1 transition state. The transition state of thymidine phosphorylase, however, is an SN2 nucleophilic displacement. Here we report equilibrium binding Isotope Effects sufficiently large to explain the presence of this substantial KIE in thymidine phosphorylase.
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binding causes the remote 5 3h thymidine Kinetic Isotope Effect in human thymidine phosphorylase
Journal of the American Chemical Society, 2004Co-Authors: Matthew R. Birck, Vern L SchrammAbstract:The remote 5‘-3H V/K Kinetic Isotope Effect (KIE) observed in human thymidine phosphorylase (6.1%) is significantly larger than can be explained by the reaction chemistry. One hypothesis connects the 5‘-3H KIE in purine nucleoside phosphorylase to that enzyme's SN1 transition state. The transition state of thymidine phosphorylase, however, is an SN2 nucleophilic displacement. Here we report equilibrium binding Isotope Effects sufficiently large to explain the presence of this substantial KIE in thymidine phosphorylase.
Russ Hille - One of the best experts on this subject based on the ideXlab platform.
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coupled electron proton transfer in complex flavoproteins solvent Kinetic Isotope Effect studies of electron transfer in xanthine oxidase and trimethylamine dehydrogenase
Journal of Biological Chemistry, 2001Co-Authors: Russ Hille, Robert F AndersonAbstract:Abstract A solvent Kinetic Isotope Effect study of electron transfer in two complex flavoproteins, xanthine oxidase and trimethylamine dehydrogenase, has been undertaken. With xanthine oxidase, electron transfer from the molybdenum center to the proximal iron-sulfur center of the enzyme occurs with a modest solvent Kinetic Isotope Effect of 2.2, indicating that electron transfer out of the molybdenum center is at least partially coupled to deprotonation of the Mo(V) donor. A Marcus-type analysis yields a decay factor, β, of 1.4 A−1, indicating that, although the pyranopterin cofactor of the molybdenum center forms a nearly contiguous covalent bridge from the molybdenum atom to the proximal iron-sulfur center of the enzyme, it affords no exceptionally Effective mode of electron transfer between the two centers. For trimethylamine dehydrogenase, rates of electron equilibration between the flavin and iron-sulfur center of the one-electron reduced enzyme have been determined, complementing previous studies of electron transfer in the two-electron reduced form. The results indicate a substantial solvent Kinetic Isotope Effect of 10 ± 4, consistent with a model for electron transfer that involves discrete protonation/deprotonation and electron transfer steps. This contrasts to the behavior seen with xanthine oxidase, and the basis for this difference is discussed in the context of the structures for the two proteins and the ionization properties of their flavin sites. With xanthine oxidase, a rationale is presented as to why it is desirable in certain cases that the physical layout of redox-active sites not be uniformly increasing in reduction potential in the direction of physiological electron transfer.
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Electron transfer within xanthine oxidase: a solvent Kinetic Isotope Effect study.
Biochemistry, 1991Co-Authors: Russ HilleAbstract:Solvent Kinetic Isotope Effect studies of electron transfer within xanthine oxidase have been performed, using a stopped-flow pH-jump technique to perturb the distribution of reducing equivalents within partially reduced enzyme and follow the Kinetics of reequilibration spectrophotometrically. It is found that the rate constant for electron transfer between the flavin and one of the iron-sulfur centers of the enzyme observed when the pH is jumped from 10 to 6 decreases from 173 to 25 s{sup {minus}1} on going from HJ{sub 2}O to D{sub 2}O, giving an observed solvent Kinetic Isotope Effect of 6.9. An Effect of comparable magnitude is observed for the pH jump in the opposite direction, the rate constant decreasing form 395 to 56 s{sup {minus}1}. The solvent Kinetic Isotope Effect on k{sub obs} is found to be directly proportional to the mole fraction of D{sub 2}O in the reaction mix for the pH jump in each direction, consistent with the Effect arising from a single exchangeable proton. Calculations of the microscopic rate constants for electron transfer between the flavin and the iron-sulfur center indicate that the intrinsic solvent Kinetic Isotope Effect for electron transfer from the neutral flavin semiquinone to the iron-sulfur center designated Fe/S I ismore » substantially greater than for electron transfer in the opposite direction and that the observed solvent Kinetic Isotope Effect is a weighted average of the intrinsic Isotope Effects for the forward and reverse microscopic electron-transfer steps. The results emphasize the importance of prototropic equilibria in the Kinetic as well as thermodynamic behavior of xanthine oxidase and indicate that protonation/deprotonation of the isoalloxazine ring is concomitant with electron transfer in the xanthine oxidase system.« less
Robert F Anderson - One of the best experts on this subject based on the ideXlab platform.
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coupled electron proton transfer in complex flavoproteins solvent Kinetic Isotope Effect studies of electron transfer in xanthine oxidase and trimethylamine dehydrogenase
Journal of Biological Chemistry, 2001Co-Authors: Russ Hille, Robert F AndersonAbstract:Abstract A solvent Kinetic Isotope Effect study of electron transfer in two complex flavoproteins, xanthine oxidase and trimethylamine dehydrogenase, has been undertaken. With xanthine oxidase, electron transfer from the molybdenum center to the proximal iron-sulfur center of the enzyme occurs with a modest solvent Kinetic Isotope Effect of 2.2, indicating that electron transfer out of the molybdenum center is at least partially coupled to deprotonation of the Mo(V) donor. A Marcus-type analysis yields a decay factor, β, of 1.4 A−1, indicating that, although the pyranopterin cofactor of the molybdenum center forms a nearly contiguous covalent bridge from the molybdenum atom to the proximal iron-sulfur center of the enzyme, it affords no exceptionally Effective mode of electron transfer between the two centers. For trimethylamine dehydrogenase, rates of electron equilibration between the flavin and iron-sulfur center of the one-electron reduced enzyme have been determined, complementing previous studies of electron transfer in the two-electron reduced form. The results indicate a substantial solvent Kinetic Isotope Effect of 10 ± 4, consistent with a model for electron transfer that involves discrete protonation/deprotonation and electron transfer steps. This contrasts to the behavior seen with xanthine oxidase, and the basis for this difference is discussed in the context of the structures for the two proteins and the ionization properties of their flavin sites. With xanthine oxidase, a rationale is presented as to why it is desirable in certain cases that the physical layout of redox-active sites not be uniformly increasing in reduction potential in the direction of physiological electron transfer.
Arieh Warshel - One of the best experts on this subject based on the ideXlab platform.
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simulations of the large Kinetic Isotope Effect and the temperature dependence of the hydrogen atom transfer in lipoxygenase
Journal of the American Chemical Society, 2004Co-Authors: Mats H M Olsson, Per E M Siegbahn, Arieh WarshelAbstract:Elucidating the role of nuclear quantum mechanical (NQM) Effects in enzyme catalysis is a topic of significant current interest. Despite the great experimental progress in this field it is important to have theoretical approaches capable of evaluating and analyzing nuclear quantum mechanical contributions to catalysis. In this study, we use the catalytic reaction of lipoxygenase, which is characterized by an extremely large Kinetic Isotope Effect, as a challenging test case for our simulation approach. This is done by applying the quantum classical path (QCP) method with an empirical valence bond potential energy surface. Our computational strategy evaluates the relevant NQM corrections and reproduces the large observed Kinetic Isotope Effect and the temperature dependence of the H atom transfer reaction while being less successful with the D atom transfer reaction. However, the main point of our study is not so much to explore the temperature dependence of the Isotope Effect but rather to develop and val...
