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  • The co occurrence of the AA and

    2020-11-19

    The co-occurrence of the AA12 and the AA8 domains in CcPDH would allow for electron transfer between these domains. Such electron transfer is known to occur in CDH, and the AA8 family in CAZy is in fact largely comprised of cytochrome domains of multi-domain CDHs. One notable exception concerns the carbohydrate-binding cytochrome b562 from Phanerochaete chrysosporium, a two-domain structure consisting of an AA8 and a CBM1 but not containing a dehydrogenase domain [22]. AA8 domains contain a 6-coordinated low-spin heme b group within an ellipsoidal antiparallel β-sandwich fold [23]. The heme is bound in a hydrophobic pocket at one face, with one heme edge being exposed to the solvent, and being axially ligated by Met and His. The amino 8351 sale sequence of the AA8 domain in CcPDH has 32–42% sequence identity with the AA8 domains of basidiomycete CDHs. The UV–vis and resonance Raman spectra of CcPDH in the oxidized and reduced forms are in good agreement with spectra obtained for the AA8 domain of P. chrysosporium CDH (PcCDH) []. Our electrochemical studies indicated that the redox potential of the heme is around +130 mV versus NHE at pH 7.0, which is almost identical to the value obtained for PcCDH []. Plant cell wall degrading enzymes often contain a non-catalytic CBM. The majority of CBMs attached to fungal cellulolytic enzymes belongs to family 1. The presence of a CBM1 indicates a role in plant cell wall degradation, as the main function of CBM1s is to facilitate binding of the enzyme to the surface of cellulose, which is thought to enhance catalytic efficiency by increasing the effective enzyme concentration near the substrate. Some ascomycetous CDHs possess a CBM1 at their C-terminus, whereas basidiomycetous CDHs lack this domain [24]. The C-terminal CBM1 of CcPDH contains three conserved aromatic residues that likely contribute to binding on cellulose, as suggested by in-depth studies of the functionality of other CBM1 domains [16]. Because CcPDH shows high affinity toward microcrystalline and amorphous celluloses [], in its natural environment, the enzyme presumably is localized on the surface of cellulose.
    Substrate specificity CcPDH is special; in that, it has broad substrate specificity and, thus, is a versatile biocatalyst. In our first report on this enzyme [], it was named sugar dehydrogenase (CcSDH) because of its oxidizing activity toward various sugars. It was renamed pyranose dehydrogenase (CcPDH) when the enzyme was found to act on a wide variety of pyranoses []. CcPDH shows significant activity toward 2KG, l-fucose, and rare sugars such as d-arabinose, l-galactose, d-talose, and l-gulose, and substrate oxidation at the C-1 position was confirmed by using l-fucose as a substrate []. Thus, CcPDH seems to prefer the 1C4 conformation of aldose sugars with C-2 and C-3 hydroxyl groups in an equatorial configuration and the C-4 hydroxyl group in an axial configuration, as suggested in a previous study []. Regarding 2KG, the 1C4 conformation is one of the more plausible tautomers, at 21% abundance in aqueous solution [26], but the C-1 position of this conformation is clearly different from those of other pyranoses such as l-fucose, d-arabinose, and l-galactose. Moreover, it is worth noting that the affinity of CcPDH for the substrates is quite low, with Km values in the mM range. Therefore, while there is a clear biocatalytic potential here, the mechanism of substrate recognition in this enzyme is still unclear, and more studies are needed to clarify the details of its catalytic action. As described above, CcPDH acts on various pyranoses including rare sugars, and recently we found that the enzyme can oxidize the C-1 position of l-guluronate, which is an acidic sugar of l-gulose [27]. The substrate specificity of CcPDH is clearly different from those of previously characterized prokaryotic PQQ-dependent enzymes, as shown in Table 1. The unique broad substrate specificity of CcPDH can be used in biorefinery processes, especially to convert rare sugars and uronates to corresponding oxidized forms. In fact, we are currently exploring the recently discovered ability of CcPDH to convert l-guluronate to d-glucarate [27] to establish a large-scale production system for hexarates, which are bio-based platform chemicals.