Clyde A Smith,
Stanford University, USA
Title: Structural studies on Carbapenem-Hydrolyzing Class D serine β-lactamases from Acinetobacter baumannii
Biography
Biography: Clyde A Smith,
Abstract
The class D serine β-lactamases comprise a superfamily of almost 800 enzymes capable of conferring high-level resistance to β-lactam antibiotics, predominantly the penicillin’s including oxacillin and cloxacillin. In recent years it has been discovered that some members of the class D superfamily have evolved the ability to deactivate carbapenems, “last resort” β-lactam antibiotics generally held in reserve for highly drug resistant bacterial infections. These enzymes are collectively known as Carbapenem-Hydrolyzing Class D serine β-Lactamases or CHDLs (1). The mechanism of β-lactam deactivation by the class D serine β-lactamases involves the covalent binding of the antibiotic to an active site serine to form an acyl-enzyme intermediate (acylation). This is followed by hydrolysis of the covalent bond (deacylation), catalyzed by a water molecule activated by a carboxylated lysine residue (2). It was initially thought that the carbapenems acted as potent inhibitors of the class D enzymes since the formation of the covalent acyl-enzyme intermediate effectively expelled all water molecules from the active site, thus preventing the deacylation step. Our structural studies on two CHDLs (3,4) have indicated that their carbapenem hydrolyzing ability may be due to two surface hydrophobic residues which allow for the transient opening and closing of a channel through which water molecules from the milieu can enter the binding site to facilitate the deacylation reaction (Figure). Although the hydrophobic residues responsible for the channel formation are present in all class D β-lactamases, sequence and structural differences nearby may be responsible for the evolution of carbapenemase activity in the CHDLs. These mechanisms will be presented, including some insights into the carbapenemase activity of non-Acinetobacter CHDLs which show a variation in how deacylation is activated. Future work aimed at improved inhibitor design will also be explored.