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A method for evaluating the association and dissociation rate constants of interaction between a lysozyme and its substrate analogue, an immobilized p-aminophenyl-tri-N-acetyl-beta-chitotrioside, by means of surface plasmon resonance has been developed. Site-specific immobilization of p-aminophenyl-tri-N-acetyl-beta-chitotrioside, which is a product of p-nitrophenyl-tri-N-acetyl-beta-chitotrioside, on carboxymethyldextran linked to the surface of the cuvette of the instrument, IAsys, was carried out by catalysis with EDC/NHS. The kinetic parameters of the interaction between hen or human lysozyme and the immobilized substrate analogue indicated that a larger dissociation constant of the human lysozyme-immobilized substrate analogue complex depended on a smaller association rate constant. The kinetic parameters of the interaction between the immobilized substrate analogue and a mutant hen lysozyme, in which Arg14 and His15 are deleted, with higher activity than the wild type hen lysozyme were measured. It was suggested that the higher activity of the mutant lysozyme was due to faster removal of the substrate from the active site cleft and/or the formation of a stabler and better complex as to hydrolysis.

We prepared two dissected fragments of hen lysozyme and examined whether or not these two fragments associated to form a native-like structure. One (Fragment I) is the peptide fragment Asn59-homoserine-105 containing Cys64-Cys80 and Cys76-Cys94. The other (Fragment II) is the peptide fragment Lys1-homoserine-58 connected by two disulfide bridges, Cys6-Cys127 and Cys30-Cys115, to the peptide fragment Asn106-Leu129. It was found that the Fragment I immobilized in the cuvette formed an equimolar complex with Fragment II (K(d) = 3.3x10(-4) M at pH 8 and 25 degrees C) by means of surface plasmon resonance. Moreover, from analyses by circular dichroism spectroscopy and ion-exchange chromatography of the mixture of Fragments I and II at pH 8 under non-reducing conditions, it was suggested that these fragments associated to give the native-like structure. However, the mutant Fragment I in which Cys64-Cys80 and Cys76-Cys94 are lacking owing to the mutation of Cys to Ala, or the mutant fragment in which Trp62 is mutated to Gly, did not form the native-like species with Fragment II, because the mutant Fragment I derived from mutant lysozymes had no local conformation due to mutations. Considering our previous results where the preferential oxidation of two inside disulfide bonds, Cys64-Cys80 and Cys76-Cys94, occurred in the refolding of the fully reduced Fragment I, we suggest that the peptide region corresponding to Fragment I is an initiation site for hen lysozyme folding.