The rate of hen egg-white lysozyme (mucopeptide N-acetylmuramoylhydrolase, EC 22.214.171.124), catalysis was determined in the presence of various metal ions (CO2+, Zn2+ and eight of the trivalent lanthanide ions). In the assay system employed, the lanthanides were found to inhibit more strongly than either Zn2+ or Co2+. The inhibition data was fitted to several models of the interactions of the metal ion with the enzyme. These models ranged in complexity from a single inhibitory metal binding site on the enzyme (two-parameter fit) to the presence of two non-independent and non-equivalent inhibitory metal binding sites (five-parameter fit). The more complicated models did not fit the data more precisely than the simplest one-site model, suggesting that the adoption of the simpler model is warranted. The fact that association constants obtained from the simplest analysis for Co2+ (1.3 ± 1.9 · 102 M-1) and Gd3+ (7.0 ± 2.6 · 103 M-1) are consistent with literature values determined from spectroscopic measurements further supports the validity of the simplest model.
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