Form I and II Rubiscos Exhibit Temperature Dependent Carbon Kinetic Isotope Effects

Nearly all carbon on Earth today is fixed by the enzyme ribulose-1,5-bisphopshate carboxylase/oxygenase ('rubisco'), which converts carbon dioxide (CO2) to sugar phosphates. All rubiscos measured thus far display a kinetic isotope effect (KIE) where 12CO2 is fixed at a faster rate than 13CO2. The relationship between rubisco's KIE and the carbon isotope composition of plants, algae, and organic matter is central to many fields in the Earth sciences, plant biology, and biochemistry. Currently, all applications assume that the KIE does not vary with temperature. Here, we examine this assumption experimentally with in vitro KIE measurements of two rubiscos from phylogenetically distinct host or