  This work presents a general chemical reaction network theory for olfactory
sensing processes that employ G-protein-coupled receptors as olfactory
receptors (ORs). The theory is applicable to general mixtures of odorants and
an arbitrary number of ORs. Reactions of ORs with G-proteins, both in the
presence and the absence of odorants, are explicitly considered. A unique
feature of the theory is the definition of an odor activity vector consisting
of strengths of odorant-induced signals from ORs relative to those due to
background G-protein activity in the absence of odorants. It is demonstrated
that each component of the odor activity defined this way reduces to a
Michaelis-Menten form capable of accounting for cooperation or competition
effects between different odorants. The main features of the theory are
illustrated for a two-odorant mixture. Known and potential mixture effects,
such as suppression, shadowing, inhibition, and synergy are quantitatively
described. Effects of relative values of rate constants, basal activity, and
G-protein concentration are also demonstrated.
