How does an enzyme recognize CO2?

Phosphoenolpyruvate carboxykinase (PCK) reversibly catalyzes the carboxylation of phosphoenolpyruvate to oxaloacetate. Carbon dioxide, and not bicarbonate ion, is the substrate utilized. Assays of the carboxylation reaction show that initial velocities are 7.6-fold higher when CO2 is used instead of HCO3-. Two Escherichia coli PCK-CO2 crystal structures are presented here. The location of CO2 is the same for both structures; however the orientation of CO2 is significantly different, likely from the presence of a manganese ion in one of the structures. PCK and the other three known protein-CO2 crystal structure complexes have been compared; all have CO2 hydrogen bonding with a basic amino acid side chain (Arg65 or Lys213 in PCK), likely to polarize CO2 to make the central carbon atom more electrophilic and thus more reactive. Kinetic studies found that the PCK mutant Arg65Gln increased the K-M for substrates PEP and oxaloacetate but not for CO2. The unchanged K-M for CO2 can be explained since the Arg65Gln mutant likely maintains a hydrogen bond to one of the oxygen atoms of carbon dioxide. (C) 2007 Elsevier Ltd. All rights reserved.