Molecular mechanism for catalysis by a new zinc-enzyme, dopachrome tautomerase

Dopachrome tautomerase (DCT; EC 5.3.3.12) catalyses the conversion of L-dopachrome into 5,6-dihydroxyindole-2-carboxylic acid in the mammalian eumelanogenic biosynthetic pathway. This enzyme, also named TRP2, belongs to a family of three metalloenzymes termed the tyrosinase-related proteins (TRPs). It is well known that tyrosinase has copper in its active site. However, the nature of the metal ion in the active site of DCT is under discussion. Whereas theoretical predictions based on similarity between the protein sequences of the TRPs suggest the presence of copper, the different inhibition pattern of DCT with some metal chelators compared with that of tyrosinase suggests that the nature of the metal ion could differ. Direct estimations of the metal content in purified DCT preparations show the presence of around 1.5 Zn atoms/molecule and the absence of copper. Apoenzyme preparation by treatment of DCT with cyanide or o-phenanthroline followed by reconstitution experiments of tautomerase activity in the presence of different ions confirmed that the metal cofactor for the DCT active site is zinc. Our results are consistent with Zn2+ chelation by the highly conserved histidine residues homologous to the histidines at the classical copper-binding sites in tyrosinase. This finding accounts for the reaction catalysed by DCT, i.e. a tautomerization, versus the copper-mediated oxidations catalysed by tyrosinase. Based on the predicted tetrahedrical coordination of the zinc ions in the enzyme active site, a molecular mechanism for the catalysis of L-dopachrome tautomerization is proposed. From the present data, the existence of additional ligands for metal ions other than zinc in the DCT molecule, such as the proposed cysteine iron-binding sites, cannot be completely ruled out. However, if such sites exist, they could be subsidiary binding sites, whose function would be likely to stabilize the protein.