Xylanase XynA from the hyperthermophilic bacterium Thermotoga maritima: Structure and stability of the recombinant enzyme and its isolated cellulose‐binding domain

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Abstract
The hyperthermophilic bacterium Thermotoga maritima is capable of gaining metabolic energy utilizing xylan. XynA, one of the corresponding hydrolases required for its degradation, is a 120‐kDa endo‐l,4‐D‐xylanase exhibiting high intrinsic stability and a temperature optimum —90 °C. Sequence alignments with other xylanases suggest the enzyme to consist of five domains. The C‐terminal part of XynA was previously shown to be responsible for cellulose binding (Winterhalter C, Heinrich P, Candussio A, Wich G, Liebl W. 1995. Identification of a novel cellulose‐binding domain within the multi‐domain 120 kDa Xylanase XynA of the hyperthermophilic bacterium Thermotoga maritima. Mol Microbiol 15:431‐444).In order to characterize the domain organization and the stability of XynA and its C‐terminal cellulose‐binding domain (CBD), the two separate proteins were expressed in Escherichia coli. CBD, because of its instability in its ligand‐free form, was expressed as a glutathione S‐transferase fusion protein with a specific thrombin cleavage site as linker. XynA and CBD were compared regarding their hydrodynamic and spectral properties. As taken from analytical ultracentrifugation and gel permeation chromatography, both are monomers with 116 and 22 kDa molecular masses, respectively. In the presence of glucose as a ligand, CBD shows high intrinsic stability. Denaturation/renaturation experiments with isolated CBD yield >80% renaturation, indicating that the domain folds independently. Making use of fluorescence emission and far‐UV circular dichroism in order to characterize protein stability, guanidine‐induced unfolding of XynA leads to biphasic transitions, with half‐concentrations c1/2(GdmC1) ∼4 M and >5 M, in accordance with the extreme thermal stability. At acid pH, XynA exhibits increased stability, indicated by a shift of the second guanidine‐transition from 5 to 7 M GdmC1. This can be tentatively attributed to the cellulose‐binding domain. Differences in the transition profiles monitored by fluorescence emission and dichroic absorption indicate multi‐state behavior of XynA. In the case of CBD, a temperature‐induced increase in negative ellipticity at 217 nm is caused by alterations in the environment of aromatic residues that contribute to the far‐UV CD in the native state.