Figure 9.

Concerted deprotonation and carboxylation catalyzes the reaction by dispersing negative charge. As CO₂ interaction is simultaneous with deprotonation, some of the negative charge developing on the γ carbon initiates bond formation with CO₂, which in turn results in the appearance of negative charge on the CO₂ oxygens. Additionally, the original glutamate carboxyl group can accept some negative charge from the γ carbon by formation of an aci-carboxylate structure. His160 could interact with either the aci-carboxylate moiety via a low-barrier hydrogen bond or with the negative charge on the incoming CO₂ via an electrostatic interaction to effect catalysis.