I can direct you to PDB entry 1EWY, where the average isotropic
temperature factor for the major component of the complex is ca. 47 A**2
and that for the smaller component is ca. 69 A**2. Similar values than
the ones you are reporting. I am assuming some sort of "disorder", or if
you prefer, "wobbling" of the smaller component at the lever of the
binding site.
Fred.
Sebastiano Pasqualato wrote:
Hi all,
I have a crystallographical/biochemical problem, and maybe some of you guys can
help me out.
We have recently crystallized a protein:protein complex, whose Kd has been
measured being ca. 10 uM (both by fluorescence polarization and surface plasmon
resonance).
Despite the 'decent' affinity, we couldn't purify an homogeneous complex in
size exclusion chromatography, even mixing the protein at concentrations up to
80-100 uM each.
We explained this behavior by assuming that extremely high Kon/Koff values
combine to give this 10 uM affinity, and the high Koff value would account for
the dissociation going on during size exclusion chromatography. We have partial
evidence for this from the SPR curves, although we haven't actually measured
the Kon/Koff values.
We eventually managed to solve the crystal structure of the complex by mixing
the two proteins (we had to add an excess of one of them to get good
diffraction data).
Once solved the structure (which makes perfect biological sense and has been
validated), we get mean B factors for one of the component (the larger) much
lower than those of the other component (the smaller one, which we had in
excess). We're talking about 48 Å^2 vs. 75 Å^2.
I was wondering if anybody has had some similar cases, or has any hint on the
possible relationship it might (or might not) exist between high a Koff value
and high B factors (a relationship we are tempted to draw).
Thanks in advance,
best regards,
ciao
s
