We have so far noted that a substrate molecule normally forms a productive complex at the enzyme active site. However, there are cases where substrate also interacts with the enzyme (or the ES complex) in a nonproductive fashion. If this interaction is “kinetically silent,” it will not show up in the routine steady state kinetic analysis. Other methods (like equilibrium dialysis, fluorescence difference spectroscopy, or MALDI-TOF) may, however, be able to detect such binding phenomena. Most often nonproductive interactions of substrate are not considered at all—except when they also interact with the same enzyme as activators or inhibitors.