Differences in Carbon Isotope Discrimination of Three Variants of D-Ribulose-1,5-bisphosphate Carboxylase/Oxygenase Reflect Differences in Their Catalytic Mechanisms

Dennis McNevin, Murray Badger, Spencer Whitney, Susanne Von Caemmerer, Guillaume Tcherkez, Graham Farquhar

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    The carboxylation kinetic (stable carbon) isotope effect was measured for purified D-ribulose-1,5-bisphosphate carboxylases/oxygenases (Rubiscos) with aqueous CO2 as substrate by monitoring Rayleigh fractionation using membrane inlet mass spectrometry. This resulted in discriminations (Δ) of 27.4 ± 0.9‰ for wild-type tobacco Rubisco, 22.2 ± 2.1‰ for Rhodospirillum rubrum Rubisco, and 11.2 ± 1.6‰ for a large subunit mutant of tobacco Rubisco in which Leu335 is mutated to valine (L335V). These Δ values are consistent with the photosynthetic discrimination determined for wild-type tobacco and transplastomic tobacco lines that exclusively produce R. rubrum or L335V Rubisco. The Δ values are indicative of the potential evolutionary variability of Δ values for a range of Rubiscos from different species: Form I Rubisco from higher plants; prokaryotic Rubiscos, including Form II; and the L335V mutant. We explore the implications of these Δ values for the Rubisco catalytic mechanism and suggest that Rubiscos that are associated with a lower Δ value have a less product-like carboxylation transition state and/or allow a decarboxylation step that evolution has excluded in higher plants
    Original languageEnglish
    Pages (from-to)36068-36076
    Number of pages9
    JournalThe Journal of Biological Chemistry
    Issue number49
    Publication statusPublished - 2007


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