Figure 5.

Magnesium binding measured by intrinsic tryptophan fluorescence of wild-type and mutant ETOP enzymes. Decrease of tryptophan fluorescence signal at 334 nm (excitation at 295 nm) with increasing concentration of MgCl₂ was monitored. Fraction of maximal decrease of fluorescence signal was determined and curve fitted for binding of two Mg²⁺/enzyme molecule with the GraphPad Prism program. The KDs shown in the insert represent the average from two sets of data for ETOP-G116S/M320V and three sets of data for wild-type ETOP and ETOP-G116S.