MICROCALORIMETRIC STUDY OF BINDING BY LYSOZYME OF N‐ACETYL GLUCOSAMINE OLIGOMERS

Binding heats of monomer, dimer, trimer, tetramer, pentamer and hexamer of N‐acetyl glucosamine to hen's egg white lysozyme (E.C. 2.3.1.17) were measured using a Tian‐Calvet conduction‐type microcalorimeter. Monomer and oligomers of N‐acetylglucosamine were prepared by molecular sieve chromatography from the acid hydrolysate of crab shell chitin. Lysozyme activity was evaluated by a chemical method based upon reducing group activity. When plotted as log Q/(Qm‐Q)/vs. log I, the binding phenomena fall into two distinct classes with regard to energetic character, while measurements for the hexamer are a departure from both classes. Data has been interpreted in terms of the six hypothesized lysozyme‐binding‐sites. We have concluded that the mode of binding of the monomer and dimer is different from that of the trimer, tetramer, pentamer and hexamer. The monomer and dimer comprise one group (utilizing sites C and CB, respectively) while the trimer, tetramer and pentamer comprise the second group deriving additional stability principally from site A, with the hexamer falling between them.