Abstract
We have previously described anti‐acetylcholinesterase antibodies that display acetylcholinesterase‐like catalytic activity. No evidence of contaminating enzymes was found, and the antibodies are kinetically and apparently structurally distinct from both acetylcholinesterase (AChE) and butyrylcholinesterase. We have also mimicked the antibody catalytic sites in anti‐anti‐idiotypic (Ab3) antibodies. Independently from us, similar acetylcholinesterase‐like antibodies have been raised as anti‐idiotypic (Ab2) antibodies against a non‐catalytic anti‐acetylcholinesterase antibody, AE‐2. In this paper, we describe an epitope analysis, using synthetic peptides in ELISA and competition ELISA, and a peptide array, of five catalytic anti‐acetylcholinesterase antibodies (Ab1s), three catalytic Ab3s, as well as antibody AE‐2 and a non‐catalytic Ab2. The catalytic Ab1s and Ab3s recognized three Pro‐ and Gly‐containing sequences (^40^PPMGPRRFL, ^78^PGFEGTE, and ^258^PPGGTGGNDTELVAC) on the AChE surface. As these sequences do not adjoin in the AChE structure, recognition would appear to be due to cross‐reaction. This was confirmed by the observation that the sequences superimpose structurally. The non‐catalytic antibodies, AE‐2 and the Ab2, recognized AChE's peripheral anionic site (PAS), in particular, the sequence ^70^YQYVD, which contains two of the site's residues. The crystal structure of the AChE tetramer (Bourne et al., 1999) shows direct interaction and high complementarity between the ^257^CPPGGTGGNDTELVAC sequence and the PAS. Antibodies recognizing the sequence and the PAS may, in turn, be complementary; this may account for the apparent paradox of catalytic development in both Ab1s and Ab2s. The PAS binds, but does not hydrolyze, substrate. The catalytic Ab1s, therefore, recognize a site that may function as a substrate analog, and this, together with the presence of an Arg‐Glu salt bridge in the epitope, suggests mechanisms whereby catalytic activity may have developed. In conclusion, the development of AChE‐like catalytic activity in anti‐AChE Ab1s and Ab2s appears to be the result of a combination of structural complementarity to a substrate‐binding site, charge complementarity to a salt bridge, and specific structural peculiarities of the AChE molecule. Copyright © 2008 John Wiley & Sons, Ltd.