Page 226 - Advances in Textile Biotechnology

Functionalisation of wool and silk fi bres using enzymes 207

during the subsequent reaction in which diphenols are oxidised and
+
+
o-quinones are released, is converted into the deoxy form (Cu -Cu )
able to bind reversibly with molecular oxygen, producing the oxy form
2+
2+
(Cu -O 2 -Cu ), which can act on both monophenols and diphenols.
Selinheimo et al. (2006) purified and characterised a novel extracellular

tyrosinase from the fi lamentous fungus Trichoderma reesei, whose substrate
specifi city, stereospecificity, inhibition, and ability to crosslink model protein

substrates was compared with that of other fungal and plant tyrosinases
(Selinheimo et al., 2007). Mattinen et al. (2008) reported that the T. reesei
tyrosinase displays better ability to oxidise Tyr-containing model peptides
and to crosslink random coil protein substrates such as α-casein than A.
bisporus tyro s inase. The substrate specifi city of T. reesei and A. bisporus
tyrosinases was further compared by using mono- and diphenolic com-
pounds bearing different substituents and tripeptides containing Tyr in the
amine, middle, or carboxyl terminal position (Selinheimo et al. 2009). The
presence of an amine group in the structure of the substrate was found to
have negative effects on T. reesei tyrosinase, and phenol substrates with a
carboxylic group were not effectively oxidised by A. bisporus tyrosinase.
Differences in the structure of the catalytic site were hypothesised to
explain these results. With reference to the ability to oxidise tripeptides, T.
reesei tyrosinase showed higher K m and V max values but the oxidation prod-

ucts did not differ significantly between the two enzymes with the same
substrate.
The catalytic activity of tyrosinase from three different sources, including
one from A. bisporus, was also investigated by Martin et al. (2008). The
results showed that the catalytic efficiency towards each substituted phenol

substrate is different for each enzyme source and that large differences in
affinity and turnover are observed for different enzyme sources towards the

same substrate. These results demonstrate how important it is to understand

the substrate specificity of tyrosinases as well as their capability to oxidise
different substrates in view of selecting the best enzyme for the targeted
application.
Tyrosinases are widely distributed enzymes in nature. They are found in
mammals, invertebrates, plants, and in prokaryotic and eukaryotic microbes
(Claus and Decker, 2006; van Gelder et al., 1997; Garcia-Borron and Solano,
2002). Most of the reported tyrosinases are intracellular enzymes, whereas
bacterial tyrosinases, like the one found in Streptomyces, are secreted.
Tyrosinases are involved in several biological functions. In mammals, pig-
mentation of skin, eye, and hair is a tyrosinase-mediated process called
melanogenesis which contributes to protection against absorption of UV
radiation (Ito and Wakamatsu, 2008). Abnormal increase or decrease in
tyrosinase activity is the cause of hyperpigmentation and albinism, respec-
tively. The control of abnormal skin pigmentation by inhibiting tyrosinase

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