Escherichia coli beta-galactosidase responds enzymatically to antiviral antibodies when a viral antigenic peptide, acting as receptor, is conveniently displayed in the vicinity of the active site. The allosteric response of a beta-galactosidase molecular sensor containing a B-cell epitope from HIV has been finely dissected upon binding of an effector monoclonal antibody, within a wide range of standard concentrations of both enzyme and substrate. The topography of the enzymatic activation reveals a wide set of conditions in which the enzymatic response renders a signal over threefold the background, that is suitable for analytical biosensing. Moreover, at discrete enzyme-substrate coordinates, the effector antibody promotes an enhanced activation factor up to fivefold. The insertion of the 37-mer viral peptide between beta-galactosidase residues 795 and 796 is observed as inducer of the structural flexibility required for molecular sensing, whose dynamics and efficiency are intimately associated with the concentrations of enzyme and substrate, the two partners in the signal transduction event.