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The slope of $\log [\theta /(1-\theta )]$ against $\log [L]$ in a Hill equation description. A value above one supports positive cooperative binding under the model's assumptions. It does not by itself count occupied sites or identify a molecular mechanism.

The positive concentration giving half of the maximal response in a Hill equation. This effective threshold need not equal an individual binding site's microscopic dissociation constant.

Yes: the binding data support strong positive cooperativity. On the linear concentration axis, occupancy rises sigmoidally, with reported Hill coefficient $n=3.97\pm 0.13$, far above the noncooperative value one. The Hill equation gives the useful empirical descriptionThe steep response is consistent with the independent evidence for a four-subunit assembly. The fitted half-occupancy concentration is not automatically the microscopic dissociation constant of one isolated site.

Binding alone does not demonstrate which conformational change is coupled to occupancy. Here the microscopy supplies the complementary structural evidence, so together the observations support a cooperative ligand-dependent switch. A Hill coefficient near four is consistent with strongly coupled binding in a tetramer, but is not an independent exact count of ligands.