Question

Which configuration D-DOPA or L-DOPA facilitates the enzyme-catalyzed oxidation? Explain please.

For two inhibitors thiourea and cinnamic acid, which substance is competitive inhibitor for tyrosinase enzyme and which substance is noncompetitive inhibitor? Explain please.

Answer 1

1) L-DOPA or levodopa is the configuration that facilitates the enzyme catalyzed oxidation.

Explanation: L configuration of DOPA ( dihydroxyphenylalanine) is the molecule or substrate upon which tyrosinase (an oxidase) works to produce the reactive intermediate dopaquinone by oxidation, which reacts further to produce skin pigment melanin. Tyrosinase enzyme is stereospecific so it works only on L isomer not on D isomer.

2) For two inhibitors thiourea and cinnamic acid for tyrosinase enzyme, cinnamic acid will act as competitive inhibitor and thiourea will act as non-competitive inhibitor.

Explanation: For tyrosinase enzyme the substrate is tyrosine and product is DOPA. Tyrosinase oxidize tyrosine to do so and we know that competitive inhibitor compete with the substrate for enzyme as there is structural resembles. Here cinnamic acid have structural similarity with tyrosine so tyrosinase can bind cinnamic acid in its active site in place of its original substrate to bring the oxidation.

Thiourea also used as inhibitor of tyrosinase to decrease the production of melanin (skin pigment) but it act as noncompetitive inhibitor.