Viewpoint: Isotopic fractionation by plant nitrate reductase, twenty years later

Functional Plant Biology - Tập 33 Số 6 - Trang 531 - 2006
Guillaume Tcherkez1,2, Graham D. Farquhar1
1Environmental Biology Group, Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, ACT, 2601, Australia
2Laboratoire d'Ecophysiologie Végétale, Bâtiment 362, Université Paris XI, 91405 Orsay, France.

Tóm tắt

Plant nitrate reductase, the enzyme that reduces nitrate (NO3–) to nitrite (NO2–), is known to fractionate N isotopes, depleting nitrite in 15N compared with substrate nitrate. Nearly 20 years ago, the nitrogen isotope effect associated with this reaction was found to be around 1.015. However, the relationships between the isotope effect and the mechanism of the reaction have not yet been examined in the light of recent advances regarding the catalytic cycle and enzyme structure. We thus give here the mathematical bases of the 14N / 15N and also the 16O / 18O isotope effects as a function of reaction rates. Enzymatic nitrate reduction involves steps other than NO3– reduction itself, in which the oxidation number of N changes from +V (nitrate) to +III (nitrite). Using some approximations, we give numerical estimates of the intrinsic N and O isotope effects and this leads us to challenge the assumptions of nitrate reduction itself as being a rate-limiting step within the nitrate reductase reaction, and of the formation of a bridging oxygen as a reaction intermediate.

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Functional Plant Biology

Tập 33 Số 6

531

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