Dissertation on cross-linking enzymes for designing food properties
VTT’s Research Scientist Chiara Gasparetti presents her doctoral dissertation “Biochemical and structural characterisation of the copper containing oxidoreductases catechol oxidases, tyrosinase, and laccase from ascomycete fungi” for public examination and debate at the Aalto University School of Chemical Technology in Espoo (Auditorium KE2 Komppa) on the 26th of October 2012, at 12.
Food microstructure has an important role in the sensory perception of texture, water-binding, and rheological properties as well as in the digestibility and absorption of food components. Cross-linking enzymes may be used in food technology as specific tools for designed food properties and new products.
This work was aimed at characterising copper-containing oxidative enzymes that can be utilised for cross-linking of proteins in food technology. The work focused on examination of the properties of catechol oxidase (EC 22.214.171.124), tyrosinase (EC 126.96.36.199), and laccase (EC 188.8.131.52) enzymes.
A novel family of short-tyrosinase sequences was identified in the available fungal genomes. A novel catechol oxidase from the ascomycete fungus Aspergillus oryzae belonging to the novel family was characterised from biochemical and structural point of view. Since generally catechol oxidases of fungal origin are not well-characterised enzymes, the characterisation of A. oryzae catechol oxidase could provide a new contribution to the knowledge of these enzymes.
A detailed characterisation of tyrosinase from Trichoderma reesei was performed and the well characterised commercial tyrosinase from Agaricus bisporus was used as a reference enzyme. T. reesei tyrosinase was found to suffer of dopachrome end product inhibition. Thus far, this is the first report on a tyrosinase end product inhibition.
The oxidation capacity of laccases was elucidated by using a set of laccases with different redox potential and a set of substituted phenolic substrates with different redox potential. For this purpose, a novel laccase 1 from the ascomycete fungus Thielavia arenaria and the laccases from Melanocarpus albomyces and Trametes hirsuta were used. The laccase-catalysed oxidation of phenolic substrates was found to be dependent on the difference in redox potential between the enzyme and the substrate and the pH.
Finally, an evaluation of the protein cross-linking ability of catechol oxidase from A. oryzae, tyrosinases from T. reesei and A. bisporus and laccases from T. hirsuta, T. arenaria, and M. albomyces was performed. T. reesei tyrosinase was found to be the best enzyme for cross-linking of α-caseins, whereas A. bisporus tyrosinase and A. oryzae catechol oxidase were found to be the best enzymes for cross-linking α-caseins in the presence of catechol as a cross-linking agent. As a novel finding, A. oryzae catechol oxidase was also found to cross-link α-caseins in the presence of catechol as a cross-linking agent.
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