  The activities and interactions of proteins that govern the cellular response
to a signal generate a multitude of protein phosphorylation states and
heterogeneous protein complexes. Here, using a computational model that
accounts for 307 molecular species implied by specified interactions of four
proteins involved in signalling by the immunoreceptor Fc$\epsilon$RI, we
determine the relative importance of molecular species that can be generated
during signalling, chemical transitions among these species, and reaction paths
that lead to activation of the protein tyrosine kinase (PTK) Syk. By all of
these measures and over 2- and 10-fold ranges of model parameters--rate
constants and initial concentrations--only a small portion of the biochemical
network is active. The spectrum of active complexes, however, can be shifted
dramatically, even by a change in the concentration of a single protein, which
suggests that the network can produce qualitatively different responses under
different cellular conditions and in response to different inputs. Reduced
models that reproduce predictions of the full model for a particular set of
parameters lose their predictive capacity when parameters are varied over
2-fold ranges.
