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V
IV
III
RH + {O=Fe } → R + {HO-Fe }→ R-OH + Fe

OH 2 RH RH
N III N RH N III N e - N II N
Fe Fe Fe
N N N N N N

S(Cys) S(Cys) S(Cys)

O
ROH 2
H O
2
RH RH O O RH O O
O +
N V N 2H N N N III N
III
Fe Fe e - Fe
N N N N N N
S(Cys) S(Cys) S(Cys)
H O
2

Tyrosinase and catecholoxidase: These two closely related enzymes contain type III copper
centres (see inset on p. 20) and thus resemble the haemocyanins (ch. 3). The homology of the
amino acid sequence is, however, restricted to the direct surroundings of the copper centres.
Activation of oxygen by tyrosinase and catecholoxidase compares to that of haemocyanin,
except that is irreversible:
II
II
2-
I
2Cu + O 2 → Cu (O 2 )Cu

One of the histidine ligands on one of the Cu centres in tyrosinase can be weakened to allow
for attachment of the substrate tyrosine. Tyrosinase catalyses the oxygenation of tyrosine to
dopa (o- dihydroxyphenylalanine; precursor for the neurotransmitter dopamine, and for
adrenaline), catecholoxidase further oxidises dopa to the respective quinone (Fig. 18). These
reactions are followed by further dehydrogenation to form indolquinone and finally melanin.
Melanin, a very complex compound, is the dark pigment formed as freshly broken fruits (like
apples or bananas) or vegetable (like potatoes) are exposed to air. Melanin is also the dark
pigment responsible for the suntan, or present in the brown beauty patches and in melanomas.

NH
NH
OC + -
OH + O + 2H + 2e OC OH + H O
2
2
Tyrosin
Tyrosine Dopa OH
- 2[H]

N O NH O
Melanin OC
- 4[H] OC
O
O
Indolchinon
Indolquinone

Figure 18. Reactions which are catalysed by tyrosinase/catecholase.