Structural basis for a disfavored elimination reaction in catalytic antibody 1D4

Murine antibody 1D4 selectively catalyzes a highly disfavored P-elimination reaction. Crystal structures of unliganded 1D4 and 1D4 in complex with a t ransition-state analog (TSA) have elucidated a possible general base mode o f catalysis. The structures of the unliganded and liganded Fabs were determ ined to 1.80 and 1.85 A resolution, respectively. The structure of the comp lex reveals a binding pocket with high shape complementarity to the TSA, wh ich is recruited to coerce the substrate into the sterically demanding, ecl ipsed conformation that is required for catalysis. A histidine residue and two water molecules are likely involved in the catalysis. The structure sup ports either a concerted E2 or stepwise E1cB-like mechanism for elimination . Finally, the liganded 1D4 structure shows minor conformational rearrangem ents in CDR H2, indicative of induced-fit binding of the hapten. 1D4 has pu shed the boundaries of antibody-mediated catalysis into the realm of disfav ored reactions and, hence, represents an important milestone in the develop ment of this technology. (C) 2001 Academic Press.