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Figure 4.
Fig. 4. Electron density maps and models illustrating the
structure of the DHBD:3-methylcatechol-bound and substrate-free
forms of DHBD. Each part is a (divergent) stereo drawing
prepared with the program MolView (42). The identical observed
structure factors were used in all maps, which demonstrates the
presence of both forms of the enzyme in the same crystal. All
maps are at 1.9-Å resolution and are contoured at two
standard deviations above the mean of the map. In the models,
the carbon atoms are more darkly shaded than the nitrogen and
oxygen atoms. A, F[o]  F[c]
electron density representing the iron, 3-methylcatechol, and
two water ligands. The F[c] 's and phases are from the
structure of the substrate-free enzyme (3). The model is the
initial fit to this density. B, residual F[o] F[c]
electron density following refinement of a model that included
the iron, 3-methylcatechol, two water ligands, and a t-butanol
bound in the auxiliary site, as shown. The F[c] 's and phases
are from this model. The density features arise from the
fraction of the crystal that is in the substrate free-form, as
demonstrated by C, which shows the refined model of this form
(3) in conjunction with the same density. Note that the
t-butanol binds in the substrate-binding site in this form of
the enzyme.
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