Manganese peroxidase, lignin peroxides, and other peroxidases have also been used for pulp-bleaching reactions . The use of peroxidases requires the addition of hydrogen peroxide, which at high concentrations also inhibits the enzyme. One approach is to meter in the hydrogen peroxide at low concentrations, matching the consumption rate with the supply rate. Another is to use an enzyme like glucose oxidase or another hydrogen peroxide generating system at levels that produce the needed amount of substrate. The generation or metering of hydrogen peroxide alleviates the inhibition of the enzyme.
The requirement of an additional substrate such as manganese is a small problem for the marketable use of these enzymes compared to the lack of commercially available amounts of the enzymes. Laccases, even with the requirement for mediators, have been favored since they have been relatively easy to produce in heterologous expression systems. Peroxidases have long been available, but the use of manganese peroxidase and lignin peroxidase has been limited by the lack of commercial quantities of these enzymes.
Research on the use of manganese peroxidase has indicated a variety of potential applications. Manganese peroxidase has been used for the bleaching of chemical pulps , delignification of pulps , and treatment of pulps to lessen electrical refiner energy and improve handsheet strength . Lignin peroxidase is promising since it can react with non-phenolic components of lignin and does not require the addition of manganese. The commercialization of processes using lignin peroxidase has been hampered by the recalcitrant heterologous expression of the enzyme.
Textile applications of peroxidases for bleaching of cotton and of lignin-containing fibers, such as linen, are being explored, with limited success . Possible options are combining compatible oxidoreductases or applying the enzymes in suitable sequences of optimum pH and temperature. For example, the application of manganese peroxidase in the first step followed by glucose oxidase results in slightly higher whiteness levels than the application of glucose oxidase alone. Such process modifications might have a greater potential than does treatment with only one peroxidase. However, the process costs are still high and might not be economically justified at present.
Cellobiose dehydrogenase has also been explored to modify the structure of lignocellulosic materials. While under certain conditions cellobiose dehy-drogenase can catalyze lignin degradation , the cost and action of this enzyme will probably prohibit commercial application in pulp bleaching and delignification . The modification of carbohydrate by creating carboxylic acids is a potential cost-effective application ofcellobiose dehydrogenase .
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