Analysis of the internal motion of free and ligand-bound human lysozyme by use of ¹⁵N NMR relaxation measurement: A comparison with those of hen lysozyme

Human lysozyme has a structure similar to that of hen lysozyme and differs in amino acid sequence by 51 out of 129 residues with one insertion at the position between 47 and 48 in hen lysozyme. The backbone dynamics of free or (NAG)₃-bound human lysozyme has been determined by measurements of ¹⁵N nuclear relaxation. The relaxation data were analyzed using the Lipari-Szabo formalism and were compared with those of hen lysozyme, which was already reported (Mine S et al., 1999, J Mol Biol 286:1547-1565). In this paper, it was found that the backbone dynamics of free human and hen lysozymes showed very similar behavior except for some residues, indicating that the difference in amino acid sequence did not affect the behavior of entire backbone dynamics, but the folded pattern was the major determinant of the internal motion of lysozymes. On the other hand, it was also found that the number of residues in (NAG)₃-bound human and hen lysozymes showed an increase or decrease in the order parameters at or near active sites on the binding of (NAG)₃, indicating the increase in picosecond to nanosecond. These results suggested that the immobilization of residues upon binding (NAG)₃ resulted in an entropy penalty and that this penalty was compensated by mobilizing other residues. However, compared with the internal motions between both ligand-bound human and hen lysozymes, differences in dynamic behavior between them were found at substrate binding sites, reflecting a subtle difference in the substrate-binding mode or efficiency of activity between them.