Topographical characterization of the ubiquinone reduction site of glucose dehydrogenase in Escherichia coli using depth-dependent fluorescent inhibitors

PQQ-GDH - Structure, function and application in bioelectrochemistry

This review summarizes the basic features of the PQQ-GDH enzyme as one of the sugar converting biocatalysts. Focus is on the membrane -bound and the soluble form. Furthermore, the main principles of enzymatic catalysis as well as studies on the physiological importance are reviewed. A short overview is given on developments in protein engineering. The major part, however, deals with the different fields of application in bioelectrochemistry. This includes approaches for enzyme-electrode communication such as direct electron transfer, mediator-based systems, redox polymers or conducting polymers and holoenzyme reconstitution, and covers applied areas such as biosensing, biofuel cells, recycling schemes, enzyme competition, light-directed sensing, switchable detection schemes, logical operations by enzyme electrodes and immune sensing.

Menaquinone as well as ubiquinone as a bound quinone crucial for catalytic activity and intramolecular electron transfer in Escherichia coli membrane-bound glucose dehydrogenase

Escherichia coli membrane-bound glucose dehydrogenase (mGDH), which is one of quinoproteins containing pyrroloquinoline quinone (PQQ) as a coenzyme, is a good model for elucidating the function of bound quinone inside primary dehydrogenases in respiratory chains. Enzymatic analysis of purified mGDH from cells defective in synthesis of ubiquinone (UQ) and/or menaquinone (MQ) revealed that Q-free mGDH has very low levels of activity of glucose dehydrogenase and UQ2 reductase compared with those of UQ-bearing mGDH, and both activities were significantly increased by reconstitution with UQ1. On the other hand, MQ-bearing mGDH retains both catalytic abilities at the same levels as those of UQ-bearing mGDH. A radiolytically generated hydrated electron reacted with the bound MQ to form a semiquinone anion radical with an absorption maximum at 400 nm. Subsequently, decay of the absorbance at 400 nm was accompanied by an increase in the absorbance at 380 nm with a first order rate constant of 5.7 x 10(3) s(-1). This indicated that an intramolecular electron transfer from the bound MQ to the PQQ occurred. EPR analysis revealed that characteristics of the semiquinone radical of bound MQ are similar to those of the semiquinone radical of bound UQ and indicated an electron flow from PQQ to MQ as in the case of UQ. Taken together, the results suggest that MQ is incorporated into the same pocket as that for UQ to perform a function almost equivalent to that of UQ and that bound quinone is involved at least partially in the catalytic reaction and primarily in the intramolecular electron transfer of mGDH.

Characterisation of the PQQ cofactor radical in quinoprotein ethanol dehydrogenase of Pseudomonas aeruginosa by electron paramagnetic resonance spectroscopy

The binding pocket of the pyrroloquinoline quinone (PQQ) cofactor in quinoprotein alcohol dehydrogenases contains a characteristic disulphide ring formed by two adjacent cysteine residues. To analyse the function of this unusual structural motif we have investigated the wild-type and a double cysteine:alanine mutant of the quinoprotein ethanol dehydrogenase from Pseudomonas aeruginosa by electron paramagnetic resonance (EPR) spectroscopy. Thus, we have obtained the principal values for the full rhombic g-tensor of the PQQ semiquinone radical by high-field (94 GHz) EPR necessary for a discrimination of radical species in dehydrogenases containing PQQ together with other redox-active cofactors. Our results show that the characteristic disulphide ring is no prerequisite for the formation of the functionally important semiquinone form of PQQ.

Occurrence of a bound ubiquinone and its function in Escherichia coli membrane-bound quinoprotein glucose dehydrogenase

The membrane-bound pyrroloquinoline quinone (PQQ)-containing quinoprotein glucose dehydrogenase (mGDH) in Escherichia coli functions by catalyzing glucose oxidation in the periplasm and by transferring electrons directly to ubiquinone (UQ) in the respiratory chain. To clarify the intramolecular electron transfer of mGDH, quantitation and identification of UQ were performed, indicating that purified mGDH contains a tightly bound UQ(8) in its molecule. A significant increase in the EPR signal was observed following glucose addition in mGDH reconstituted with PQQ and Mg(2+), suggesting that bound UQ(8) accepts a single electron from PQQH(2) to generate semiquinone radicals. No such increase in the EPR signal was observed in UQ(8)-free mGDH under the same conditions. Moreover, a UQ(2) reductase assay with a UQ-related inhibitor (C49) revealed different inhibition kinetics between the wild-type mGDH and UQ(8)-free mGDH. From these findings, we propose that the native mGDH bears two ubiquinone-binding sites, one (Q(I)) for bound UQ(8) in its molecule and the other (Q(II)) for UQ(8) in the ubiquinone pool, and that the bound UQ(8) in the Q(I) site acts as a single electron mediator in the intramolecular electron transfer in mGDH.

Membrane-bound D-sorbitol dehydrogenase of Gluconobacter suboxydans IFO 3255--enzymatic and genetic characterization

Gluconobacter strains effectively produce L-sorbose from D-sorbitol because of strong activity of the D-sorbitol dehydrogenase (SLDH). L-sorbose is one of the important intermediates in the industrial vitamin C production process. Two kinds of membrane-bound SLDHs, which consist of three subunits, were reportedly found in Gluconobacter strains [Agric. Biol. Chem. 46 (1982) 135,FEMS Microbiol. Lett. 125 (1995) 45]. We purified a one-subunit-type SLDH (80 kDa) from the membrane fraction of Gluconobacter suboxydans IFO 3255 solubilized with Triton X-100 in the presence of D-sorbitol, but the cofactor could not be identified from the purified enzyme. The SLDH was active on mannitol, glycerol and other sugar alcohols as well as on D-sorbitol to produce respective keto-aldoses. Then, the SLDH gene (sldA) was cloned and sequenced. It encodes the polypeptide of 740 residues, which contains a signal sequence of 24 residues. SLDH had 35-37% identity to those of membrane-bound quinoprotein glucose dehydrogenases (GDHs) from Escherichia coli, Gluconobacter oxydans and Acinetobacter calcoaceticus except the N-terminal hydrophobic region of GDH. Additionally, the sldB gene located just upstream of sldA was found to encode the polypeptide consisting of 126 very hydrophobic residues that is similar to the one-sixth N-terminal region of the GDH. Development of the SLDH activity in E. coli required co-expression of the sldA and sldB genes and the presence of PQQ. The sldA gene disruptant showed undetectable oxidation activities on D-sorbitol in growing culture, and resting-cell reaction (pH 4.5 and 7); in addition, they showed undetectable activities on D-mannitol and glycerol. The disruption of the sldB gene by a gene cassette with a downward promoter to express the sldA gene resulted in formation of a larger size of the SLDH protein and in undetectable oxidation of the polyols. In conclusion, the SLDH of the strain 3255 functions as the main polyol dehydrogenase in vivo. The sldB polypeptide possibly has a chaperone-like function to process the SLDH polypeptide into a mature and active form.

Molecular cloning and functional expression of D-sorbitol dehydrogenase from Gluconobacter suboxydans IF03255, which requires pyrroloquinoline quinone and hydrophobic protein SldB for activity development in E. coli

The sldA gene that encodes the D-sorbitol dehydrogenase (SLDH) from Gluconobacter suboxydans IFO 3255 was cloned and sequenced. It encodes a polypeptide of 740 residues, which contains a signal sequence of 24 residues. SLDH had 35-37% identity to the membrane-bound quinoprotein glucose dehydrogenases (GDHs) from E. coli, Gluconobacter oxydans, and Acinetobacter calcoaceticus except the N-terminal hydrophobic region of GDH. Additionally, the sldB gene located just upstream of sldA was found to encode a polypeptide consisting of 126 very hydrophobic residues that is similar in sequence to the one-sixth N-terminal region of the GDH. For the development of the SLDH activity in E. coli, co-expression of the sldA and sldB genes and the presence of pyrrloquinolone quinone as a co-factor were required.

C-terminal periplasmic domain of Escherichia coli quinoprotein glucose dehydrogenase transfers electrons to ubiquinone

Membrane-bound quinoprotein glucose dehydrogenase (GDH) in Escherichia coli donates electrons directly to ubiquinone during the oxidation of d-glucose as a substrate, and these electrons are subsequently transferred to ubiquinol oxidase in the respiratory chain. To determine whether the specific ubiquinone-reacting site of GDH resides in the N-terminal transmembrane domain or in the large C-terminal periplasmic catalytic domain (cGDH), we constructed a fusion protein between the signal sequence of beta-lactamase and cGDH. This truncated GDH was found to complement a GDH gene-disrupted strain in vivo. The signal sequence of the fused protein was shown to be cleaved off, and the remaining cGDH was shown to be recovered in the membrane fraction, suggesting that cGDH has a membrane-interacting site that is responsible for binding to membrane, like peripheral proteins. Kinetic analysis and reconstitution experiments revealed that cGDH has ubiquinone reductase activity nearly equivalent to that of the wild-type GDH. Thus, it is likely that the C-terminal periplasmic domain of GDH possesses a ubiquinone-reacting site and transfers electrons directly to ubiquinone.

Pyrroloquinoline quinone (PQQ) and quinoprotein enzymes

This review summarises the characteristics, identification, and measurement of pyrroloquinoline quinone, the prosthetic group of bacterial quinoprotein dehydrogenases whose structures, mechanisms, and electron transport functions are described in detail. Type I alcohol dehydrogenase includes the "classic" methanol dehydrogenase; its x-ray structure and mechanism are discussed in detail. It is likely that its mechanism involves a direct hydride transfer rather than a mechanism involving a covalent adduct. The x-ray structure of a closely related ethanol dehydrogenase is also described. The type II alcohol dehydrogenase is a soluble quinohaemoprotein, having a C-terminal extension containing haem C, which provides an excellent opportunity for the study of intraprotein electron transfer processes. The type III alcohol dehydrogenase is similar but it has two additional subunits (one of which is a multihaem cytochrome c) bound in an unusual way to the periplasmic membrane. One type of glucose dehydrogenase is a soluble quinoprotein whose role in energy transduction is uncertain. Its x-ray structure (in the presence and absence of substrate) is described together with the detailed mechanism, which also involves a direct hydride transfer. The more widely distributed glucose dehydrogenases are integral membrane proteins, bound to the membrane by transmembrane helices at the N-terminus.