INVESTIGATIONS OF AN ANTIBODY LIGASE

Further investigation of the monoclonal antibody 16G3 has revealed tha t it not only couples activated amino acids to form dipeptides with hi gh turnover rates but also couples an activated amino acid with a dipe ptide to form a tripeptide, as well as an activated dipeptide with ano ther dipeptide to give a tetrapeptide. The catalytic rates for these r eactions greatly exceed the background rate of ester hydrolysis provid ing average yields of 80% within the assay time of 20 min. Importantly , the amount of product inhibition is low, allowing for high yields of products using multiple addition of substrates to the same antibody r eaction mixture. A sequential mechanism is employed by 16G3 for dipept ide coupling, and this mechanism appears to hold for the formation of the other peptides. High catalytic selectivity is observed for the nuc leophilic alpha-amino group of an alpha,beta-diamino nucleophile and f or the para substituent on the activated ester, traits that are consis tent with hapten design. The former chemoselectivity is crucial for th e condensation of fragments which are unprotected at the epsilon-amino group of lysine.