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Kinetic characterization of an organic radical in the ascarylose biosynthetic pathway.
Biochemistry. 1996 Dec 10; 35(49):15846-56.B

Abstract

The lipopolysaccharide of Yersinia pseudotuberculosis V includes a 3,6-dideoxyhexose, ascarylose, as the nonreducing end of the O-antigen tetrasaccharide. The C-3 deoxygenation of CDP-6-deoxy-L-threo-D-glycero-4-hexulose is a critical reaction in the biosynthesis of ascarylose. The first half of the reaction is a dehydration catalyzed by CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase (E1), which is PMP-dependent and contains a redox-active [2Fe-2S] center. The second half is a reduction that requires an additional enzyme, CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase reductase (E3, formerly known as CDP-6-deoxy-delta 3,4-glucoseen reductase), which has a FAD and a [2Fe-2S] center in the active site. Using NADH as the reductant in the coupled E1-E3 reaction, we have monitored the kinetics of a radical intermediate using both stopped-flow spectrophotometry and rapid freeze-quench EPR under aerobic and hypoxic conditions. In the EPR studies, a sharp signal at g = 2.003 was found to appear at a rate which is kinetically competent, reaching its maximum intensity at approximately 150 ms. Stopped-flow UV-vis analysis of the reaction elucidated a minimum of six optically distinguishable states in the mechanism of electron transfer from NADH to substrate. Interestingly, one of the detected intermediates has a time course nearly identical to that of the radical detected by rapid freeze-quench EPR. The difference UV-vis spectrum of this intermediate displays a maximum at 456 nm with a shoulder at 425 nm. Overall, these results are consistent with an electron transfer pathway that includes a radical intermediate with the unpaired spin localized on the substrate-cofactor complex. Evidence in support of this mechanism is presented in this report. These studies add the PMP-glucoseen radical to the growing list of mechanistically important bioorganic radical intermediates that have recently been discovered.

Authors+Show Affiliations

Department of Chemistry, University of Minnesota, Minneapolis 55455, USA.No affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

8961949

Citation

Johnson, D A., et al. "Kinetic Characterization of an Organic Radical in the Ascarylose Biosynthetic Pathway." Biochemistry, vol. 35, no. 49, 1996, pp. 15846-56.
Johnson DA, Gassner GT, Bandarian V, et al. Kinetic characterization of an organic radical in the ascarylose biosynthetic pathway. Biochemistry. 1996;35(49):15846-56.
Johnson, D. A., Gassner, G. T., Bandarian, V., Ruzicka, F. J., Ballou, D. P., Reed, G. H., & Liu, H. W. (1996). Kinetic characterization of an organic radical in the ascarylose biosynthetic pathway. Biochemistry, 35(49), 15846-56.
Johnson DA, et al. Kinetic Characterization of an Organic Radical in the Ascarylose Biosynthetic Pathway. Biochemistry. 1996 Dec 10;35(49):15846-56. PubMed PMID: 8961949.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Kinetic characterization of an organic radical in the ascarylose biosynthetic pathway. AU - Johnson,D A, AU - Gassner,G T, AU - Bandarian,V, AU - Ruzicka,F J, AU - Ballou,D P, AU - Reed,G H, AU - Liu,H W, PY - 1996/12/10/pubmed PY - 1996/12/10/medline PY - 1996/12/10/entrez SP - 15846 EP - 56 JF - Biochemistry JO - Biochemistry VL - 35 IS - 49 N2 - The lipopolysaccharide of Yersinia pseudotuberculosis V includes a 3,6-dideoxyhexose, ascarylose, as the nonreducing end of the O-antigen tetrasaccharide. The C-3 deoxygenation of CDP-6-deoxy-L-threo-D-glycero-4-hexulose is a critical reaction in the biosynthesis of ascarylose. The first half of the reaction is a dehydration catalyzed by CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase (E1), which is PMP-dependent and contains a redox-active [2Fe-2S] center. The second half is a reduction that requires an additional enzyme, CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase reductase (E3, formerly known as CDP-6-deoxy-delta 3,4-glucoseen reductase), which has a FAD and a [2Fe-2S] center in the active site. Using NADH as the reductant in the coupled E1-E3 reaction, we have monitored the kinetics of a radical intermediate using both stopped-flow spectrophotometry and rapid freeze-quench EPR under aerobic and hypoxic conditions. In the EPR studies, a sharp signal at g = 2.003 was found to appear at a rate which is kinetically competent, reaching its maximum intensity at approximately 150 ms. Stopped-flow UV-vis analysis of the reaction elucidated a minimum of six optically distinguishable states in the mechanism of electron transfer from NADH to substrate. Interestingly, one of the detected intermediates has a time course nearly identical to that of the radical detected by rapid freeze-quench EPR. The difference UV-vis spectrum of this intermediate displays a maximum at 456 nm with a shoulder at 425 nm. Overall, these results are consistent with an electron transfer pathway that includes a radical intermediate with the unpaired spin localized on the substrate-cofactor complex. Evidence in support of this mechanism is presented in this report. These studies add the PMP-glucoseen radical to the growing list of mechanistically important bioorganic radical intermediates that have recently been discovered. SN - 0006-2960 UR - https://www.unboundmedicine.com/medline/citation/8961949/Kinetic_characterization_of_an_organic_radical_in_the_ascarylose_biosynthetic_pathway_ L2 - https://doi.org/10.1021/bi961370w DB - PRIME DP - Unbound Medicine ER -