Considerations about the kinetic mechanism of tyrosinase in its action on monophenols: A review

The mechanism of action of tyrosinase on monophenols is complex, several processes overlap in time, such as the hydroxylation of monophenols to o-diphenols, the oxidation of these to o-quinones and the evolution of the latter towards melanin. The enzyme's mechanism of action is unique but depending on the chemical nature of the substrate it may show different exceptions. In this review we want to dissect the kinetic mechanism for the action of the enzyme on: a) L-tyrosine, the physiological substrate for mammalian tyrosinase, and related compounds, whose o-quinones in their chemical evolution accumulate o-diphenol in the medium (Type A). b) Substrates that cannot accumulate o-diphenol in the medium because it is easily oxidized and they need the presence of hydrogen peroxide for the enzyme to show activity, such as hydroquinone and related compounds (Type B). c) Substrates that release o-diphenol into the medium and the enzyme oxidizes it generating a stable o-quinone and therefore does not generate more o-diphenol in the medium, as is the case of 4‑tert-butylphenol and related compounds (Type C). d) Substrates that do not release or generate o-diphenol in the medium, as is the case with deoxyarbutin, which produces a stable o-quinone (Type D). The different mechanisms that explain the enzymatic activity are proposed, a kinetic analysis is established for each mechanism and by means of numerical integration results are obtained that are discussed and compared with experimental data. To help and support the results and discussion, molecular docking for substrates (L-tyrosine, hydroquinone, 4‑tert-butylphenol, and deoxyarbutin) to both the oxy and met forms of tyrosinase was carried out.