Active Site-directed Side Chain Modification

Abstract

Two families of reagents have served for active site mapping of proteins. Many experiments make use of reagents suited to modify a certain kind of amino acid side chains, if possible without side reactions. Acylation of ɛ-amino-groups of lysines by acid anhydrides or acid chlorides may be named as examples, or the reaction of tetranitromethane with tyrosines. It is usually not possible, however, to guide the reaction to an unequivocal modification of only one kind of functional group, and a careful evaluation of the number of side chains derivatized will be necessary. Thus, iodoacetic acid derivatives, although entering reaction with SH groups much faster, will also alkylate imidazoles and amino groups, and the reaction conditions for one given side chain may be changed considerably by its environment on the protein molecule. Usually, experiments of this kind are evaluated by comparing the amount of reagent introduced into the enzyme monomer with the decrease of catalytic activity. A major problem is posed by the difficulty of excluding an unspecific denaturation or conformational change by physical or chemical means.