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Kinetic comparison of reduction and intramolecular electron transfer in milk xanthine oxidase and chicken liver xanthine dehydrogenase by laser flash photolysis.
Biochemistry. 1991 Jun 18; 30(24):5912-7.B

Abstract

A comparative study using laser flash photolysis of the kinetics of reduction and intramolecular electron transfer among the redox centers of chicken liver xanthine dehydrogenase and of bovine milk xanthine oxidase is described. The photogenerated reductant, 5-deazariboflavin semiquinone, reacts with the dehydrogenase (presumably at the Mo center) in a second-order manner, with a rate constant (k = 6 x 10(7) M-1 s-1) similar to that observed with the oxidase [k = 3 x 10(7) M-1 s-1; Bhattacharyya et al. (1983) Biochemistry 22, 5270-5279]. In the case of the dehydrogenase, neutral FAD radical formation is found to occur by intramolecular electron transfer (kobs = 1600 s-1), presumably from the Mo center, whereas with the oxidase the flavin radical forms via a bimolecular process involving direct reduction by the deazaflavin semiquinone (k = 2 x 10(8) M-1 s-1). Biphasic rates of Fe/S center reduction are observed with both enzymes, which are due to intramolecular electron transfer (kobs approximately 100 s-1 and kobs = 8-11 s-1). Intramolecular oxidation of the FAD radical in each enzyme occurs with a rate constant comparable to that of the rapid phase of Fe/S center reduction. The methylviologen radical, generated by the reaction of the oxidized viologen with 5-deazariboflavin semiquinone, reacts with both the dehydrogenase and the oxidase in a second-order manner (k = 7 x 10(5) M-1 s-1 and 4 x 10(6) M-1 s-1, respectively). Alkylation of the FAD centers results in substantial alterations in the kinetics of the reaction of the viologen radical with the oxidase but not with the dehydrogenase. These results suggest that the viologen radical reacts directly with the FAD center in the oxidase but not in the dehydrogenase, as is the case with the deazaflavin radical. The data support the conclusion that the environments of the FAD centers differ in the two enzymes, which is in accord with other studies addressing this problem from a different perspective [Massey et al. (1989) J. Biol. Chem. 264, 10567-10573]. In contrast, the rate constants for intramolecular electron transfer among the Mo, FAD, and Fe/S centers in the two enzymes (where they can be determined) are quite similar.

Authors+Show Affiliations

Department of Biochemistry, University of Arizona, Tucson 85721.No affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Comparative Study
Journal Article
Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

2043632

Citation

Walker, M C., et al. "Kinetic Comparison of Reduction and Intramolecular Electron Transfer in Milk Xanthine Oxidase and Chicken Liver Xanthine Dehydrogenase By Laser Flash Photolysis." Biochemistry, vol. 30, no. 24, 1991, pp. 5912-7.
Walker MC, Hazzard JT, Tollin G, et al. Kinetic comparison of reduction and intramolecular electron transfer in milk xanthine oxidase and chicken liver xanthine dehydrogenase by laser flash photolysis. Biochemistry. 1991;30(24):5912-7.
Walker, M. C., Hazzard, J. T., Tollin, G., & Edmondson, D. E. (1991). Kinetic comparison of reduction and intramolecular electron transfer in milk xanthine oxidase and chicken liver xanthine dehydrogenase by laser flash photolysis. Biochemistry, 30(24), 5912-7.
Walker MC, et al. Kinetic Comparison of Reduction and Intramolecular Electron Transfer in Milk Xanthine Oxidase and Chicken Liver Xanthine Dehydrogenase By Laser Flash Photolysis. Biochemistry. 1991 Jun 18;30(24):5912-7. PubMed PMID: 2043632.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Kinetic comparison of reduction and intramolecular electron transfer in milk xanthine oxidase and chicken liver xanthine dehydrogenase by laser flash photolysis. AU - Walker,M C, AU - Hazzard,J T, AU - Tollin,G, AU - Edmondson,D E, PY - 1991/6/18/pubmed PY - 1991/6/18/medline PY - 1991/6/18/entrez SP - 5912 EP - 7 JF - Biochemistry JO - Biochemistry VL - 30 IS - 24 N2 - A comparative study using laser flash photolysis of the kinetics of reduction and intramolecular electron transfer among the redox centers of chicken liver xanthine dehydrogenase and of bovine milk xanthine oxidase is described. The photogenerated reductant, 5-deazariboflavin semiquinone, reacts with the dehydrogenase (presumably at the Mo center) in a second-order manner, with a rate constant (k = 6 x 10(7) M-1 s-1) similar to that observed with the oxidase [k = 3 x 10(7) M-1 s-1; Bhattacharyya et al. (1983) Biochemistry 22, 5270-5279]. In the case of the dehydrogenase, neutral FAD radical formation is found to occur by intramolecular electron transfer (kobs = 1600 s-1), presumably from the Mo center, whereas with the oxidase the flavin radical forms via a bimolecular process involving direct reduction by the deazaflavin semiquinone (k = 2 x 10(8) M-1 s-1). Biphasic rates of Fe/S center reduction are observed with both enzymes, which are due to intramolecular electron transfer (kobs approximately 100 s-1 and kobs = 8-11 s-1). Intramolecular oxidation of the FAD radical in each enzyme occurs with a rate constant comparable to that of the rapid phase of Fe/S center reduction. The methylviologen radical, generated by the reaction of the oxidized viologen with 5-deazariboflavin semiquinone, reacts with both the dehydrogenase and the oxidase in a second-order manner (k = 7 x 10(5) M-1 s-1 and 4 x 10(6) M-1 s-1, respectively). Alkylation of the FAD centers results in substantial alterations in the kinetics of the reaction of the viologen radical with the oxidase but not with the dehydrogenase. These results suggest that the viologen radical reacts directly with the FAD center in the oxidase but not in the dehydrogenase, as is the case with the deazaflavin radical. The data support the conclusion that the environments of the FAD centers differ in the two enzymes, which is in accord with other studies addressing this problem from a different perspective [Massey et al. (1989) J. Biol. Chem. 264, 10567-10573]. In contrast, the rate constants for intramolecular electron transfer among the Mo, FAD, and Fe/S centers in the two enzymes (where they can be determined) are quite similar. SN - 0006-2960 UR - https://www.unboundmedicine.com/medline/citation/2043632/Kinetic_comparison_of_reduction_and_intramolecular_electron_transfer_in_milk_xanthine_oxidase_and_chicken_liver_xanthine_dehydrogenase_by_laser_flash_photolysis_ DB - PRIME DP - Unbound Medicine ER -