Infrared Spectroscopy Reveals Metal independent Carbonic Anhydrase Activity in Crotonyl CoA Carboxylase/Reductase

The conversion of CO2 by enzymes such as carbonic anhydrases or carboxylases plays a crucial role in many biological processes. However, methods to study the conversion of CO2 at the active site of enzymes in situ are still limited. Here, we used Fourier-transform infrared (FTIR) spectroscopy to study the interaction of CO2 with crotonyl-CoA carboxylase/reductase from Kitasaospora setae (KsCcr), one of the fastest CO2-fixing enzymes in nature. Our studies reveal that the enzyme possesses a so far unknown metal-independent carbonic anhydrase activity. Molecular dynamics (MD) simulations explain why substrate binding inhibits anhydrase activity, and mutations of active site residues of KsCcr suggest that an ‘activated’ water molecule, coordinated by a histidine and glutamate residue, forms the hydroxyl anion that attacks the CO2 molecule. Altogether, we demonstrate how in situ FTIR spectroscopy combined with MD simulations provides new means to investigate the interaction of different proteins with CO2, providing a simple, yet powerful approach to atomistic reaction mechanisms including CO2 hydration and enzymatic (de-)hydration reactions.