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Figure 3.
FIGURE 3. The pH-dependent conformational switch observed
in M-ficolin. A, the active binding conformation of site S1
observed at neutral pH involves residues contributed by four
external loops. B, nonbinding conformation of site S1 observed
at pH 5.6. The structure determined in this study (cyan, lacking
the disordered segment 278–285) and subunit C of the structure
reported previously by Tanio et al. (18) (orange) are
superposed. The acidic pH destabilizes the four loops and
dislocates the S1 site. C, superposition of the binding
(magenta) and nonbinding (cyan) conformations determined in this
study, illustrating the essential histidine-mediated
stabilizations of the binding conformation (magenta) that are
lost at acidic pH. The red dashed lines represent hydrogen bonds.
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