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pH and kinetic isotope effects in D-amino acid oxidase catalysis : evidence for a concerted mechanism in substrate dehydrogenation via hydride transfer

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2001

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Harris, Christopher M.
Pollegioni, Loredano

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European Journal of Biochemistry. 2001, 268(21), pp. 5504-5520. eISSN 0014-2956. Available under: doi: 10.1046/j.1432-1033.2001.02462.x

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The effects of pH, solvent isotope, and primary isotope replacement on substrate dehydrogenation by Rhodotorula gracilis d-amino acid oxidase were investigated. The rate constant for enzyme-FAD reduction by d-alanine increases ≈ fourfold with pH, reflecting apparent pKa values of ≈ 6 and ≈ 8, and reaches plateaus at high and low pH. Such profiles are observed in all presteady-state and steady-state kinetic experiments, using both d-alanine and d-asparagine as substrates, and are inconsistent with the operation of a base essential to catalysis. A solvent deuterium isotope effect of 3.1 ± 1.1 is observed on the reaction with d-alanine at pH 6; it decreases to 1.2 ± 0.2 at pH 10. The primary substrate isotope effect on the reduction rate with [2-D]d-alanine is 9.1 ± 1.5 at low and 2.3 ± 0.3 at high pH. At pH 6.0, the solvent isotope effect is 2.9 ± 0.8 with [2-D]d-alanine, and the primary isotope effect is 8.4 ± 2.4 in D2O. Thus, primary and solvent kinetic isotope effects (KIEs) are independent of the presence of the other isotope, i.e. the double kinetic isotope effect is the product of the individual KIEs, consistent with a transition state in which rupture of the two bonds of the substrate to hydrogen is concerted. These results support a hydride transfer mechanism for the dehydrogenation reaction in d-amino acid oxidase and argue against the occurrence of any intermediates in the process. A pKa,app of ≈ 8 is interpreted to arise from the microscopic ionization of the substrate amino acid α-amino group, but also includes contributions from kinetic parameters.

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570 Biowissenschaften, Biologie

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flavoproteins, reaction mechanism, isotope effects, hydride transfer, pH effects

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ISO 690HARRIS, Christopher M., Loredano POLLEGIONI, Sandro GHISLA, 2001. pH and kinetic isotope effects in D-amino acid oxidase catalysis : evidence for a concerted mechanism in substrate dehydrogenation via hydride transfer. In: European Journal of Biochemistry. 2001, 268(21), pp. 5504-5520. eISSN 0014-2956. Available under: doi: 10.1046/j.1432-1033.2001.02462.x
BibTex
@article{Harris2001kinet-8675,
  year={2001},
  doi={10.1046/j.1432-1033.2001.02462.x},
  title={pH and kinetic isotope effects in D-amino acid oxidase catalysis : evidence for a concerted mechanism in substrate dehydrogenation via hydride transfer},
  number={21},
  volume={268},
  journal={European Journal of Biochemistry},
  pages={5504--5520},
  author={Harris, Christopher M. and Pollegioni, Loredano and Ghisla, Sandro}
}
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    <dcterms:abstract xml:lang="eng">The effects of pH, solvent isotope, and primary isotope replacement on substrate dehydrogenation by Rhodotorula gracilis d-amino acid oxidase were investigated. The rate constant for enzyme-FAD reduction by d-alanine increases ≈ fourfold with pH, reflecting apparent pKa values of ≈ 6 and ≈ 8, and reaches plateaus at high and low pH. Such profiles are observed in all presteady-state and steady-state kinetic experiments, using both d-alanine and d-asparagine as substrates, and are inconsistent with the operation of a base essential to catalysis. A solvent deuterium isotope effect of 3.1 ± 1.1 is observed on the reaction with d-alanine at pH 6; it decreases to 1.2 ± 0.2 at pH 10. The primary substrate isotope effect on the reduction rate with [2-D]d-alanine is 9.1 ± 1.5 at low and 2.3 ± 0.3 at high pH. At pH 6.0, the solvent isotope effect is 2.9 ± 0.8 with [2-D]d-alanine, and the primary isotope effect is 8.4 ± 2.4 in D2O. Thus, primary and solvent kinetic isotope effects (KIEs) are independent of the presence of the other isotope, i.e. the  double  kinetic isotope effect is the product of the individual KIEs, consistent with a transition state in which rupture of the two bonds of the substrate to hydrogen is concerted. These results support a hydride transfer mechanism for the dehydrogenation reaction in d-amino acid oxidase and argue against the occurrence of any intermediates in the process. A pKa,app of ≈ 8 is interpreted to arise from the microscopic ionization of the substrate amino acid α-amino group, but also includes contributions from kinetic parameters.</dcterms:abstract>
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