The electrostatic driving force for nucleophilic catalysis in L-arginine deiminase: a combined experimental and theoretical study

Abstract

L-arginine deiminase (ADI) catalyzes the hydrolysis of L-arginine to form L-citrulline and ammonia via two partial reactions. A working model of the ADI catalytic mechanism assumes nucleophilic catalysis by a stringently conserved active site Cys and general acid-general base catalysis by a stringently conserved active site His. Accordingly, in the first partial reaction, the Cys attacks the substrate guanidino C zeta atom to form a tetrahedral covalent adduct, which is protonated by the His at the departing ammonia group to facilitate the formation of the Cys- S-alkylthiouronium intermediate. In the second partial reaction, the His activates a water molecule for nucleophilic addition at the thiouronium C zeta atom to form the second tetrahedral intermediate, which eliminates the Cys in formation of the L-citrulline product. The absence of a basic residue near the Cys thiol suggested that the electrostatic environment of the Cys thiol, in the enzyme-substrate complex, stabilizes the Cys thiolate anion. The studies described in this paper explore the mechanism of stabilization of the Cys thiolate. First, the log(k(cat)/K(m)) and log k(cat) pH rate profiles were measured for several structurally divergent ADIs to establish the pH range for ADI catalysis. All ADIs were optimally active at pH 5, which suggested that the Cys pKa is strongly perturbed by the prevailing electrostatics of the ADI active site. The p K a of the Bacillus cereus ADI (BcADI) was determined by UV-pH titration to be 9.6. In contrast, the pKa determined by iodoacetamide Cys alkylation is 6.9. These results suggest that the negative electrostatic field from the two opposing Asp carboxylates perturbs the Cys pKa upward in the apoenzyme and that the binding of the iodoacetamide (a truncated analogue of the citrulline product) between the Cys thiol and the two Asp carboxylates shields the Cys thiol, thereby reducing its pKa. It is hypothesized that the bound positively charged guanidinium group of the L-arginine substrate further stabilizes the Cys thiolate. The so-called "substrate-assisted" Cys ionization, first reported by Fast and co-workers to operate in the related enzyme dimethylarginine dimethylaminohydrolase [Stone, E. M., Costello, A. L., Tierney, D. L., and Fast, W. (2006) Biochemistry 45, 5618-5630], was further explored computationally in ADI by using an ab initio quantum mechanics/molecular mechanics method. The energy profiles for formation of the tetrahedral intermediate in the first partial reaction were calculated for three different reaction scenarios. From these results, we conclude that catalytic turnover commences from the active configuration of the ADI(L-arginine) complex which consists of the Cys thiolate (nucleophile) and His imidazolium ion (general acid) and that the energy barriers for the nucleophilic addition of Cys thiolate to the thiouronium C zeta atom and His imidazolium ion-assisted elimination from the tetrahedral intermediate are small.