Kinetic isotope effects of RuBisCO

The kinetic isotope effect (KIE) of ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) is the isotopic fractionation associated solely with the step in the Calvin-Benson cycle where a molecule of carbon dioxide (CO2) is attached to the 5-carbon sugar ribulose-1,5-bisphosphate (RuBP) to produce two 3-carbon sugars called 3-phosphoglycerate (3 PGA).

[2][3] Understanding the intrinsic KIE of RuBisCO is of interest to earth scientists, botanists, and ecologists because this isotopic biosignature can be used to reconstruct the evolution of photosynthesis and the rise of oxygen in the geologic record, reconstruct past evolutionary relationships and environmental conditions, and infer plant relationships and productivity in modern environments.

The observed intrinsic KIEs of RuBisCO have been correlated with two aspects of its enzyme kinetics: 1) Its "specificity" for CO2 over O2, and 2) Its rate of carboxylation.

The reactive enodiolate species is also sensitive to oxygen (O2), which results in the dual carboxylase / oxygenase activity of RuBisCO.

[13] The dual carboxylase / oxygenase activity of RuBisCO is exacerbated by the fact that O2 and CO2 are small, relatively indistinguishable molecules that can bind only weakly, if at all, in Michaelis-Menten complexes.

Form I is used extensively by higher plants, eukaryotic algae, cyanobacteria, and Pseudomonadota (formerly proteobacteria).

In cyanobacteria, concentrations of CO2 are increased using a carboxysome, an icosahedral protein compartment about 100 nm in diameter that selectively uptakes bicarbonate and converts it to CO2 in the presence of RuBisCO.

However, the opposite is true for organisms without a CCM, who must optimize Sc/o at the expense of KC because O2 is readily present in the atmosphere.

This trade-off between Sc/o and VC or KC observed in extant organisms suggest that RuBisCO has evolved through geologic time to be maximally optimized in its current, modern environment.

This is supported by the biochemical characterization of an ancestral RuBisCO enzyme, which has intermediate values of VC and SC/O between the extreme end-members.

[14] It has been theorized that this ecological trade-off is due to the form that 2-carboxy-3-keto-D-arabinitol 1,5-bisphophate in its transient transition state before cleaving into two 3PGA molecules.

This diversity in enzyme structure is reflected in the resulting Δ13C values measured from different photosynthetic organisms.