Based on earlier data concerning the reaction enthalpy \(\:{\varDelta\:}_{\text{r}}H\) of Ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO) from spinach (Frank et al. Phys Chem Chem Phys 2:1301–1304, 2000), it is shown that the specificity constant \(\:{\Gamma\:}\) , indicating the ability of RUBISCO to discriminate between CO2 and O2 as substrate, can be determined from an isothermal titration calorimetric (ITC) measurement of \(\:{\varDelta\:}_{\text{r}}H\) as a function of the substrate concentration ratio \(\:\rho\:=\left[\text{C}{\text{O}}_{2}\right]/\left[{\text{O}}_{2}\right]\) . The approach does not need any radioactive materials and might be a cost-effective alternative for RUBISCO engineering to screen for variants with a high specificity constant.
