Neurospora tyrosinase: structural, spectroscopic and catalytic properties

Abstract

Tyrosinase is a copper containing monooxygenase catalyzing the formation of melanin pigments and other polyphenolic compounds from various phenols. This review deals with the recent progress on the molecular structure of the enzyme from Neurospora crassa and the unique features of the binuclear active site copper complex involved in the activation of molecular oxygen and the binding of substrates. The results of the spectroscopic properties of Neurospora tyrosinase will also be discussed in the light of the structural similarity of the copper complex in the oxygen binding hemocyanins.

Identification of copper ligands in Aspergillus oryzae tyrosinase by site-directed mutagenesis

“…Mutation positions were selected on the basis of a sequence comparison of the tyrosinase from A. oryzae [7] with other tyrosinases from N. crassa [8][9][10][11], S. antibioticus [13] and H. sapiens [12] (Figure 2). For the cysteine residue it is also assumed that a covalent Cys)#-Xaa-His)% might be important for enzymic activity, as described for the thioether bridge between Cys-94 and His-96 in the Cys*%-Xaa-His*' peptide of N. crassa tyrosinase [9].…”

“…On the basis of the study of Neurospora tyrosinase with a molecular mass of 46 kDa including 2 g-atoms of copper\mol [8][9][10][11], it is conceivable that the activation of protyrosinase takes place by the active conformation of the thioether linkage between Cys-94 and His-96 [9]. However, the absence of a similar structure in the tyrosinase from S. antibioticus [13] makes such a function rather unlikely.…”

“…However, the absence of a similar structure in the tyrosinase from S. antibioticus [13] makes such a function rather unlikely. After dye-sensitized photo-oxidation of the apoenzyme of Neurospora tyrosinase, three histidine residues (His-188, His-193 and His-289) were destroyed [10]. Copper measurements of photo-oxidized, reconstituted apoenzyme demonstrated the loss of binding of one copper\mol of enzyme as a consequence of the photo-oxidation of histidine residues [10].…”

“…After dye-sensitized photo-oxidation of the apoenzyme of Neurospora tyrosinase, three histidine residues (His-188, His-193 and His-289) were destroyed [10]. Copper measurements of photo-oxidized, reconstituted apoenzyme demonstrated the loss of binding of one copper\mol of enzyme as a consequence of the photo-oxidation of histidine residues [10]. Lerch suggested that the three histidine residues destroyed might be ligands of one of the two active-site copper ions ; His-306 of Neurospora tyrosinase was indicated as an active site by active-site-directed modification with catechol [10].…”

“…The 1671-bp nucleotide sequence of the protyrosinase gene, melO, from A. oryzae was determined [7]. The predicted amino acid sequence of the melO gene products has 17 % and 15 % identity with those from Neurospora crassa [8][9][10][11] and Homo sapiens [12] respectively. The protyrosinase from A. oryzae was assumed to contain 2 g-atoms of copper\mol of the subunits with a molecular mass of 67 kDa [7].…”