Amino acid residues at the N- and C-termini are essential for the folding of active human butyrylcholinesterase polypeptide

Human serum butyrylcholinesterase (HuBChE) is currently the most suitable bioscavenger for the prophylaxis of highly toxic organophosphate (OP) nerve agents. A dose of 200 mg of HuBChE is envisioned as a prophylactic treatment that can protect humans from an exposure of up to 2 × LD50 of soman. The limited availability and administration of multiple doses of this stoichiometric bioscavenger make this pretreatment difficult. Thus, the goal of this study was to produce a smaller enzymatically active HuBChE polypeptide (HBP) that could bind to nerve agents with high affinity thereby reducing the dose of enzyme. Studies have indicated that the three-dimensional structure and the domains of HuBChE (acyl pocket, lip of the active center gorge, and the anionic substrate-binding domain) that are critical for the binding of substrate are also essential for the selectivity and binding of inhibitors including OPs. Therefore, we designed three HBPs by deleting some N- and C-terminal residues of HuBChE by maintaining the folds of the active site core that includes the three active site residues (S198, E325, and H438). HBP-4 that lacks 45 residues from C-terminus but known to have BChE activity was used as a control. The cDNAs for the HBPs containing signal sequences were synthesized, cloned into different mammalian expression vectors, and recombinant polypeptides were transiently expressed in different cell lines. No BChE activity was detected in the culture media of cells transfected with any of the newly designed HBPs, and the inactive polypeptides remained inside the cells. Only enzymatically active HBP-4 was secreted into the culture medium. These results suggest that residues at the N- and C-termini are required for the folding and/or maintenance of HBP into an active stable, conformation.

► Production of a smaller enzymatically active polypeptide of HuBChE was attempted. ► Smaller HBPs were cloned in various vectors and expressed in many cell lines. ► All the expressed HBPs were inactive and retained inside the cells. ► Both N- and C-terminal residues are required for an active, stable conformation.