Differences in carbon isotope discrimination of three variants of d-ribulose-1,5-bisphosphate carboxylase/oxygenase reflect differences in their catalytic mechanisms

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 (Delta) of 27.4 +/- 0.9 parts per thousand for wild-type tobacco Rubisco, 22.2 +/- 2.1 parts per thousand for Rhodospirillum rubrum Rubisco, and 11.2 +/- 1.6 parts per thousand for a large subunit mutant of tobacco Rubisco in which Leu(335) is mutated to valine (L335V). These Delta 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 Delta values are indicative of the potential evolutionary variability of Delta 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 Delta values for the Rubisco catalytic mechanism and suggest that Rubiscos that are associated with a lower Delta value have a less product-like carboxylation transition state and/or allow a decarboxylation step that evolution has excluded in higher plants.