A quantum mechanics/molecular mechanics study of the acylation reaction ofTEM1 beta-lactamase and penicillanate

The acylation step in beta-lactamase catalyzed hydrolysis of beta-lactams h as been explored by means of a quantum mechanics/molecular mechanics approa ch (AM1/CHARMM). The TEM1 enzyme, a class A beta-lactamase, and the penicil lanate constitute the system employed in our study. The entire molecular sy stem is divided into a quantum and a classical region: the quantum part is composed by the substrate, the serine Ser70 and the essential moieties of k ey active site residues, Lys73, Ser130 and Glu166, as well as a water molec ule present in the active site region, while the classical part is formed b y the remaining residues and structural waters of the enzyme. In particular , the sequence of steps proposed by Strynadka et al. (Nature, 1992, 359, 70 0) for the acylation reaction is analyzed. Minimal and transition structure s for the mechanism are reported and an energy activation of 18.29 kcal mol (-1) has been calculated for the rate-limiting step, the formation of an in itial tetrahedral adduct. From this structure, two different mechanistic ro utes have been found to achieve the acyl-enzyme intermediate. In the first of them a simultaneous beta-lactam ring opening and proton transfer from Se r130 to the beta-lactam nitrogen atom occurs, presenting an energy barrier of 12.91 kcal mol(-1) with respect to the tetrahedral intermediate. In the second route, these processes take place in a sequential way. From an energ etic point of view, the sequential mechanism is favored, requiring the ring opening step (7.66 kcal mol(-1)) and the subsequent nitrogen protonation ( 2.76 kcal mol(-1)). Some reflections arising from the preference of sequent ial processes in this system are exposed.