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  • br Electron transfer pathway Intramolecular

    2020-02-26


    Electron transfer pathway Intramolecular (or inter-domain) electron transfer (IET) in CDH has been studied in much depth [], and similar IET occurs in CcPDH, from the PQQ cofactor in the AA12 domain to the heme b in the AA8 domain. When two kinds of electron acceptors are used for measuring enzymatic activity, phenazinemethosulfate (PMS) reacts directly with the PQQ domain, whereas cytochrome c (cyt c) reacts via the AA8 domain of CcPDH (Figure 4). Comparison of the pH-dependence of the activity between the isolated PQQ domain and the full length CcPDH suggested that the IET reaction is the rate-limiting step and the pH-dependent process in the full length CcPDH []. Interestingly, since the AA8 can act as ‘a built-in mediator’ to shuttle electrons between the active site in the catalytic domain and an electrode, CcPDH is capable of direct electron transfer (DET) to an electrode []. Indeed, a CcPDH-immobilized glassy carbon electrode shows catalytic currents during oxidation of l-fucose without the use of redox mediators []. For the development of biosensors and biofuel prexasertib cells, DET has important advantages compared to mediated electron transfer (MET), which needs a redox mediator [30]. Thus, CcPDH is an excellent candidate for making DET-based bioelectronics devices as is CDH [31].
    Biological functions For CcPDH, the occurrence of the non-catalytic, prexasertib binding domain indicates a role in plant cell wall degradation, a complex process carried out by the synergistic action of hydrolases and oxidoreductases []. Interestingly, CcPDH shows the highest oxidative activity toward 2KG, which is the product of glucose oxidation by pyranose-2-oxidase and pyranose dehydrogenase, which are produced by several fungi. It is thus conceivable that CcPDH is a part of an alternative pathway in glucose metabolism. However, this hypothesis has yet to be confirmed and several observations argue against it: the affinity of CcPDH for 2KG (and all the other substrates tested) is low and genes encoding 2KG-producing enzymes have not been identified in the genome of C. cinerea. The interplay between the fungal PQQ-dependent dehydrogenase and LPMOs is of major interest. LPMOs are copper-dependent metalloenzymes that cleave polysaccharide chains oxidatively and act synergistically with hydrolases to degrade plant cell walls [,32,33,]. LPMO action requires exogenous electron donors, which may be enzymatic, such as CDH [35,36], or non-enzymatic, such as l-ascorbic acid [37] and various hydroquinones (e.g. diphenols) []. It was recently shown that CcPDH can activate two Neurospora crassa LPMOs (belonging to the AA9 family) and that the AA8 cytochrome domain is indispensable for such an activation to occur []. Furthermore, removal of the CBM domain from CcPDH has a negative impact on the efficiency of the CcPDH-LPMO system. Since the cellulose binding CBM keeps CcPDH associated with cellulose [], this observation suggests that electron transfer in the vicinity of the substrate is advantageous for the CcPDH-LPMO system []. The important roles of the AA8 domain and CBM1 may be taken to suggest that CcPDH is tailored for activating LPMOs. It should be noted, however, that a considerable fraction of AA12 enzymes come without a CBM. Interestingly, 2KGA, the oxidation product of the most favorable CcPDH substrate, 2KG, is a precursor of erythorbic acid, which is known as isoascorbic acid (i.e. a stereoisomer of ascorbic acid) [39]. It is thus conceivable that CcPDH plays a role in the biosynthesis of l-ascorbic acid, which is a known good electron donor for LPMOs. CcPDH requires a PQQ molecule for its catalytic activity. It has been known that only a limited number of prokaryotic species can synthesize PQQ, while there is no report on PQQ synthesis in fungi or in other eukaryotes. Therefore, the supply route of PQQ to fungal AA12 enzymes is still unclear. It is well known that bacteria and fungi frequently co-occur in plant cell wall degradation and their symbiotic interactions have also been suggested [40]. Therefore, one plausible possibility is that the PQQ cofactor of the fungal quinoproteins is provided from the PQQ-producing bacteria. Of note, it has been predicted that PQQ plays various physiological roles in prokaryotic and eukaryotic organisms that do not synthesize PQQ [41]. The high affinity of CcPDH to PQQ (Kd of 1.1 nM) indeed enables the enzyme to capture PQQ from the natural environment. However, at present, there is no clear evidence for a supply route of PQQ to fungal AA12 enzymes, and it thus remains a mystery.