  Specific interactions between receptors and their target ligands in the
presence of non-target ligands are crucial for biological processes such as T
cell ligand discrimination. To discriminate between the target and non-target
ligands, cells have to increase specificity by amplifying the small differences
in affinity among ligands. In addition, sensitivity to ligand concentration and
quick discrimination are also important to detect low amounts of target ligands
and facilitate fast cellular decision making after ligand recognition. In this
work, we find that ultraspecificity is naturally derived from a well-known
mechanism for zero-order ultrasensitivity to concentration. We also show that
this mechanism can produce an optimal balance of specificity, sensitivity, and
quick discrimination. Furthermore, we show that a model for insensitivity to
large number of non-terget ligands can be naturally derived from the
ultraspecificity model. Zero-order ultraspecificity may provide alternative way
to understand ligand discrimination from the viewpoint of the nonlinear
properties of biochemical reactions.
