Abstract
The capability of Agaricus bisporus tyrosinase to catalyze the oxidation
of tyrosine residues of silk sericin was studied under homogeneous reaction
conditions, by using sericin peptides purified from industrial wastewater as
the substrate. Tyrosinase was able to oxidize about 57% of sericin-bound
tyrosine residues. The reaction rate was higher than with silk fibroin, but
lower than with other silk-derived model peptides, i.e. tryptic and
chymotryptic soluble peptide fractions of silk fibroin, suggesting that the
size and the molecular conformation of the substrate influenced the kinetics
of the reaction. The concentration of tyrosine in oxidized sericin samples
decreased gradually with increasing the enzyme-to-substrate ratio. The average
molecular weight of sericin peptides significantly increased by oxidation,
indicating that cross-linking occurred via self-condensation of o-quinones
and/or coupling with the free amine groups of lysine and, probably, with
sulfhydryl groups of cysteine. The high temperature shift of the main thermal
transitions observed in the differential scanning calorimetry curves confirmed
the formation of peptide species with higher molecular weight and higher
thermal stability. Fourier transform-infrared spectra of oxidized sericin
samples showed slight changes related to the loss of tyrosine and formation of
oxidation products. Oxidized sericin peptides were able to undergo
non-enzymatic coupling with chitosan. Infrared spectra provided clear evidence
of the formation of sericin–chitosan bioconjugates under homogeneous reaction
conditions. Spectral changes in the NH stretching region seem to support the
formation of bioconjugates via the Michael addition mechanism.
Original language | English |
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Pages (from-to) | 508-519 |
Journal | Journal of Biotechnology |
Volume | 127 |
Issue number | 3 |
DOIs | |
Publication status | Published - 2007 |
MoE publication type | A1 Journal article-refereed |
Keywords
- Silk sericin
- Chitosan
- Tyrosinase
- HP-SEC
- FT-IR
- DSC