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Rubisco from Bacteria to Higher Plants

The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the first major step of carbon fixation in the Calvin cycle, a process in which atmospheric carbon dioxide is converted to energy-rich molecules such as sucrose. Rubisco is widely recognized as the most abundant enzyme on the planet, and it is a major determinant of global carbon dioxide levels. Despite decades of research, there are several aspects of the Rubisco catalytic mechanism that remain unresolved. The large isotope discrimination value () associated with the overall reaction with CO₂ varies evolutionarily across photosynthetic organisms from lower values in bacteria to higher ones in higher plants.

Arrhenius plots and free energy plots for three types of Rubisco.

This Paper of the Week by McNevin and colleagues provides a novel and clever approach to a comparative isotope discrimination analysis of partial reactions of several different forms of Rubisco and establishes a relationship between values and the partial reactions of the enzyme. This allows the authors to make some insightful deductions about the catalytic mechanism, particularly the nature of the transition state. Specifically, the results indicate that Rubiscos associated with a lower value (e.g. photosynthetic bacteria) have a less product-like carboxylation transition state and/or they allow a decarboxylation step that evolution has excluded in higher plants.

FOOTNOTES

See referenced article, J. Biol. Chem. 2007, 282, 36068-36076

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