Expression and characterization of recombinant single-chain Fv and Fv fragments derived from a set of catalytic antibodies

Abstract

The generation of catalytic antibodies should enable the catalysis of reactions for which no enzymatic or chemical catalyst is currently available. In previous studies, we established a series of catalytic antibodies capable of hydrolysing p-nitrobenzyl (pNB) and p-nitrophenyl (pNP) esters. A group of these catalytic antibodies exhibited high reactivity and substrate specificity, yet each individual antibody demonstrated different kinetic parameters. In order to study the molecular basis for these differences, we have cloned, sequenced and expressed the variable regions of this group of antibodies as functional scFv and Fv in bacteria. The variable region of the heavy chain is derived from a novel germline gene of the J558 family whereas the light chain comes from a germline gene previously found in our catalytic antibodies catalysing the hydrolysis of only nitrophenyl esters, demonstrating that the heavy chain determines the specificity for the nitrobenzyl esters. Several different expression systems were examined for their ability to produce catalytically active antibodies. When expressed as an scFv, both refolded and secreted scFvs exhibited catalytic activity although yields of expressed protein were low. The secreted scFvs had higher specific activity. On the other hand, Fv fragments were expressed in sufficient quantities to allow kinetic analysis. Levels of expression were dependent on the sequence of VL used. Using this expression system, the relative contributions of the individual light and heavy chains to catalysis and binding could be evaluated. Both original VH and VL regions are required for hapten binding, although the VH is more crucial for catalysis. By replacing the CDR3 of the heavy chain with a random sequence, it was shown to be essential for both binding and catalysis. This expression system together with site-directed mutagenesis should enable a more detailed study of the catalytic mechanism of this set of antibodies.