Enzyme substrates bind, with various degrees of specificity, to enzymes to form an enzyme-substrate complex (ES). Within this complex the enzyme effects (catalyzes) a specific chemical transformation of the substrate. Products of this transformation are released and the active enzyme reconstitutes. This enzyme substrate cycle continues until available substrate is depleted. Most enzymatic transformations are reversible. For example: an esterase can catalyze ester hydrolysis as well as esterification.Enzyme substrates that produce a detectable signal upon transformation are used to measure enzymatic activity: detectable signals include changes in color, fluorescence or emission of light (luminescence). The corresponding substrates are called chromogenic, fluorogenic and luminogenic substrates. Upon enzymatic transformation these substrates yield their corresponding chromogens, fluorogens or luminogens. In most cases, secondary transformations, such as deprotonation, metal complexation or (air-) oxidation, are required to yield detectable chromophors, fluorophors or luminescence