Catalytic Role of Gln202 in the Carboligation Reaction Mechanism of Yeast AHAS: A QM/MM Study

Acetohydroxyacid synthase (AHAS) is a thiamin diphosphate-dependent enzyme involved in the biosynthesis of valine, leucine, isoleucine, and lysine. Experimental evidence has shown that mutation of the Gln202 residue results in a decrease in the enzymatic activity, thus suggesting the main role of the carboligation catalyzed by AHAS. It has been postulated that this residue acts as an acid/base group, protonating the carbonyl oxygen from the 2-ketoacid substrate, during the carboligation reaction. However, previous studies have revealed that 2-ketoacid is not engaged in catalytically relevant interactions with ionizable groups that can act as an acid/base group during the catalysis. Therefore, it has been proposed that the carboligation reaction could occur through an intramolecular proton transfer without the assistance of an amino acid residue with acid-base properties. To decipher the role of Gln202, in this work, we studied the last two catalytic steps of the AHAS through quantum mechanics/molecular mechanics calculations using a full enzyme model of the wild-type AHAS and the Gln202Ala mutant. Our results indicate that the carboligation mechanism occurs through an intramolecular proton transfer that does not require the action of an additional acid-base group. The mechanism is composed of two steps in which the last one is rate-limiting. Our findings reveal that Gln202 stabilizes a catalytic water molecule in the reactive site through electrostatic contributions that are mostly relevant during the carboligation step, in agreement with experimental evidence. The catalytic water engages in intermolecular hydrogen bonds with the reacting species and makes a strong electronic contribution to the stabilization of the reaction intermediate (AL-ThDP).