Elucidation of the Reaction Mechanism of l-Asparaginase: A QM/MM Study.

The l-asparaginase (l-ASNase) enzyme catalyzes the conversion of the non-essential amino acid l-asparagine into l-aspartic acid and ammonia. Importantly, the l-ASNases are used as a key part of the treatment of acute lymphoblastic leukemia (ALL); however, despite their benefits, they trigger severe side effects because they have their origin in bacterial species ( and ). Therefore, one way to solve these side effects is the use of l-ASNases with characteristics similar to those of bacterial types, but from different sources. In this sense, l-ASNase (CpA) of mammalian origin is a promising enzyme because it possesses similarities with bacterial species. In this work, the hydrolysis reaction for l-asparaginase was studied from an atomistic point of view. The QM/MM methodology was employed to describe the reaction, from which it was found that the conversion mechanism of l-asparagine into l-aspartic acid occurs in four steps. It was identified that the nucleophilic attack and release of the ammonia group is the rate-limiting step of the reaction. In this step, the nucleophile (Thr) attacks the substrate (ASN) leading to the formation of a covalent intermediate and release of the leaving group (ammonia). The calculated energy barrier is 18.9 kcal mol, at the M06-2X+D3(0)/6-311+G(2d,2p)//CHARMM36 level of theory, which is in agreement with the kinetic data available in the literature, 15.9 kcal mol (derived from the value of 38.6 s). These catalytic aspects will hopefully pave the way toward enhanced forms of CpA. Finally, our work emphasizes that computational calculations may enhance the rational design of mutations to improve the catalytic properties of the CpA enzyme.