Amino acid sequences and hapten binding of catalytic and noncatalytic antibodies against N(alpha)-(5'-phosphopyridoxyl)-L-lysine

Structural Basis for D-Amino Acid Transamination by the Pyridoxal 5′-Phosphate-dependent Catalytic Antibody 15A9

Antibody 15A9, raised with 5'-phosphopyridoxyl (PPL)-N(epsilon)-acetyl-L-lysine as hapten, catalyzes the reversible transamination of hydrophobic D-amino acids with pyridoxal 5'-phosphate (PLP). The crystal structures of the complexes of Fab 15A9 with PPL-L-alanine, PPL-D-alanine, and the hapten were determined at 2.3, 2.3, and 2.5A resolution, respectively, and served for modeling the complexes with the corresponding planar imine adducts. The conformation of the PLP-amino acid adduct and its interactions with 15A9 are similar to those occurring in PLP-dependent enzymes, except that the amino acid substrate is only weakly bound, and, due to the immunization and selection strategy, the lysine residue that covalently binds PLP in these enzymes is missing. However, the N-acetyl-L-lysine moiety of the hapten appears to have selected for aromatic residues in hypervariable loop H3 (Trp-H100e and Tyr-H100b), which, together with Lys-H96, create an anion-binding environment in the active site. The structural situation and mutagenesis experiments indicate that two catalytic residues facilitate the transamination reaction of the PLP-D-alanine aldimine. The space vacated by the absent L-lysine side chain of the hapten can be filled, in both PLP-alanine aldimine complexes, by mobile Tyr-H100b. This group can stabilize a hydroxide ion, which, however, abstracts the C alpha proton only from D-alanine. Together with the absence of any residue capable of deprotonating C alpha of L-alanine, Tyr-H100b thus underlies the enantiomeric selectivity of 15A9. The reprotonation of C4' of PLP, the rate-limiting step of 15A9-catalyzed transamination, is most likely performed by a water molecule that, assisted by Lys-H96, produces a hydroxide ion stabilized by the anion-binding environment.

Pyridoxal-5'-phosphate-dependent catalytic antibodies

Strategies for expanding the catalytic scope of antibodies include the incorporation of inorganic or organic cofactors into their binding sites. An obvious choice is pyridoxal-5'-phosphate (PLP), which is probably the most versatile organic cofactor of enzymes. Monoclonal antibodies against the hapten N(alpha)-(5'-phosphopyridoxyl)-L-lysine, a stable analog of the covalent coenzyme-substrate adducts were screened by a competition ELISA for binding of the PLP-amino acid Schiff base adduct. The Schiff base with its C4'-N alpha double bond is, in contrast to the hapten, a planar compound and is an obligatory intermediate in all PLP-dependent reactions of amino acids. This highly discriminating screening step eliminated all but 5 of 24 hapten-binding antibodies. The five remaining antibodies were tested for catalysis of the PLP-dependent alpha,beta-elimination reaction of beta-chloroalanine. Antibody 15A9 complied with this selection criterion and catalyzed in addition the cofactor-dependent transamination reaction of hydrophobic D-amino acids and oxo acids (k(cat)'=0.42 min(-1) with D-alanine at 25 degrees C). Homology modeling together with alanine scanning yielded a 3D model of Fab 15A9. The striking analogy between antibody 15A9 and PLP-dependent enzymes includes the following features: (1) The binding sites accommodate the planar coenzyme-amino acid adduct. (2) The bond at C alpha to be broken lies together with the C alpha-N bond in a plane orthogonal to the plane of coenzyme and imine bond. (3) The alpha-carboxylate group of the substrate is bound by an arginine residue. (4) The coenzyme-substrate adduct assumes a cisoid conformation. (5) PLP markedly contributes to catalytic efficiency, being a 10(4) times more efficient amino group acceptor than pyruvate. The protein moiety, however, ensures reaction as well as substrate specificity, and further accelerates the reaction (in 15A9 k(cat (Ab x PLP))'/k(cat (PLP))'=5 x 10(3)). The analogies of antibody 15A9 with PLP-dependent enzymes suggest that the selection criteria in the screening protocol were similar to those that have been operative in the molecular evolution of enzyme-assisted pyridoxal catalysis.