Biomolecular Mass Spectrometry: Applications to Proteins and Peptides - Molecules: Nucleation, Aggregation and Crystallization - page 63

as collisional cooling, which allows efficient focusing of macromolecular
ions in the mass spectrometer, very large non-covalent complexes can be
maintained and analyzed (Sobott et al ., 2002; Chernushevich et al ., 2004).
It turns out to be an excellent complementary tool for X-ray crystallography.
When the molecules exceed the size of typical peptides, such as pro-
teins and their complexes, the energies absorbed by ions in the collision
cell are not sufficient to cause fragmentation but only simple dissociation
(of complexes). Although this cannot give sequence information, it pro-
vides useful data in terms of structural organization and stoichiometry.
Given a macromolecular complex, through dissociation it is possible to
detect heterogeneous composition (van den Heuvel et al ., 2004; Ilag et al .,
2005; Hernandez et al ., 2006). Finally, it could also set the stage for com-
paring relative binding strengths of complexes opening avenues, for exam-
ple, in screening drug binding (Tjenberg et al ., 2004; Ilag et al ., 2004). One
should bear in mind, however, that in any MS analysis the interactions
being studied are gas-phase interactions. Though interactions existing in
solution may still be reflected in the gas phase, one should not assume that
one is still looking at exactly native solution structures or interactions.
Fibroblast growth factor (FGF) and fibroblast growth
factor receptor (FGFR)
The fibroblast growth factor (FGF) is a heparin binding protein which
requires this interaction for its signal transduction. Crystal structures of
FGF complexed with its receptor (FGFR) and heparin have been deter-
mined. However, two markedly different structures (Pellegrini et al ., 2000;
Schlessinger et al ., 2000) have been elucidated, reflecting two different
stoichiometries: one with two molecules of heparin bound and another
with only one heparin bound. The question is: which stoichiometry (and
therefore structure) is most likely to occur in solution? This is a straight-
forward matter for MS to resolve because the difference in the number of
bound heparin molecules would be clearly reflected in the mass of the
complex. In this particular case, it was shown that the measured mass is
consistent with 2:2:1 stoichiometry (Harmer et al ., 2004) (Fig. 4). Further