RECOMBINANT PEROXIDASES AND XYLANASES: I. Cloning and production of a peroxidase from horseradish II. Characterisation of functional domains of thermostable xylanases from Rhodothermus marinus

Abstract: Part I: Plant peroxidases are useful in many areas of application e.g. as tracer enzymes for clinical applications and in biosensors as well as for industrial purposes like in waste water treatment and bleaching processes. A gene encoding a new, neutral horseradish peroxidase isoenzyme, HRP-n, was isolated. The encoded protein shows only 50% sequence identity with the classical HRP-C and lacks the C-terminal propeptide found in HRP-C. The cDNA encoding HRP-n was expressed in <i>Escherichia coli</i> but was found to be growth inhibiting and under certain circumstances also toxic. HRP-n was then successfully produced in the Baculovirus/insect cell system. Preliminary results indicate that HRP-n differs from the classical HRP-C with regard to catalytic properties. Differences between HRP-n and HRP-C are found in the peripheral Phe residues which are reported to guard the entrance to the exposed heme, suggesting difference in their properties with respect to substrate specificity. Part II: Xylanases are enzymes involved in plant cell wall hydrolysis. Many of them have a modular structure, with functional domains separated by linker sequences. The most common domains are the catalytic domains (CDs), and the cellulose or carbohydrate binding domains (CBDs). CBDs are proposed to enhance the efficiency of hydrolysis of crystalline and complex substrates. Xylanases have a recognised potential as modifying enzymes in the pulp and paper, food and feed, and textile industries. Some of these processes are harsh and require robust enzymes that can withstand both high temperatures and extremes of pH. Such requirements can be fulfilled by enzymes from thermophiles. The <i>xyn1</i> encoded xylanase (Xyn1) from the thermophilic bacterium <i>Rhodothermus marinus</i>, is a modular multidomain enzyme, containing two putative CBDs repeated in tandem. The full length xylanase and the two CBDs were produced separately in <i>E. coli</i> and were subsequently characterised. Our results show that Xyn1 represents a new, unique combination of thermostable binding domains, classified into CBD family IV, with affinity for xylan, in addition to amorphous cellulose and soluble glycans.

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