PQQ-Dependent Enzymes

FIGURE 4.8 Soluble quinoprotein glucose dehydrogenase ftom A. calcoaceticus in complex with PQQH2 and glucose. (Reprinted with permission from Ref. [110]. Copyright 2003, Elsevier.) Structure retrieved from NCBI (PDB ID ICQl), viewed in Cn3D 4.3. Protein homodimer colored in blue/purple tube worm strucmre with calcium ions colored yellow for clarity PQQ cofactor represented as space fill model. 

PQQred are the oxidized, semiquinone, and redueed forms of PQQ, respectively [114,115]  

Similarly, in 2005, Eerapontova and Gorton demonstrated DET of o-fructose dehydrogenase from Gluconobacter industrius by immobUizalion on gold electrodes modified with an alkanethiol self-assembled monolayer (SAM). A formal redox potential of -0.158 and -0.089 V was observed at pH 5.0 and 4.0, respectively. It was found that a positively charged SAM was essential for DET and the potential for fructose oxidation could be shifted from -0.080 to -0.120 V by simply varying the functional groups of the SAM [118]. 

ANODIC CATALYSTS FOR OXIDATION OF CARBON-CONTAINING FUELS 

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Evidence for a hydride transfer mechanism (Scheme 29) for the PQQ-dependent enzyme methanol dehydrogenase (MDH) was obtained by a theoretical analysis combined with an improved refinement of a 1.9 A resolution crystal structure of MDH from Methylophilus methylotrophus in the presence of CH3OH <2001PNA432>. The alternative mechanism proceeding via a hemiketal intermediate was discounted when the observed tetrahedral configuration of the C-5 atom of PQQ in that crystal structure was shown to be the C-5-reduced form of the cofactor 198, a precursor to the more common reduced form of PQQ 199. 

Redox proteins that include quinone cofactor units play important roles in biological ET processes. Some of the quinoproteins include the quinone cofactor in a non-covalently linked configuration, such as the pyrroloquinoline quinone, PQQ, dependent enzymes, whereas other quinoproteins include the quinone cofactor covalently-linked to the protein, for example topaquinone (2,4,6-trihydroxyphenylalanine quinone, TPQ) dependent enzymes. A number of quinoproteins include in addition to the quinone cofactor an ET cofactor unit in another protein subunit. These cofactors may be metal ions or a cytochrome-type heme cofactor such as D-fructose dehydrogenase that is a heme containing PQQ-dependent enzyme. ... 

PQQ dependent enzymes are dehydrogenases that catalyze the oxidations of a variety of alcohols and sugars. A feature of PQQjdependent dehydrogenases that distinguishes them is that all are pyridine nucleotide-independent dehydrogenases. They do not use NAD or NADP as electron acceptors. Instead, in their oxidative half-reactions the soluble PQQ dependent dehydrogenases donate electrons to other soluble redox proteins and the membrane-bound PQQ dependent dehydrogenases donate electrons to ubiquinone. 

Ferrocene and its derivatives even mediate the enzymatic redox reactions of PQQ-dependent enzymes. As an example, the mediation of the enzymatic glucose dehydrogenation by PQQ-glucose dehydrogenase has been reported [140] the same enzyme also can use phenazines as electron acceptors [141]. 

PQQ-dependent enzymes, such as PQQ-GDH, have been investigated as potential enzymes that could undergo DET, due to their proximally located PQQ prosthetic group. Although many studies report that PQQ-GDH can undergo DET with electrodes, recent studies indicate that efficient DET is hard to obtain in the absence of an additional heme group (quinohemoprotein). ... 

In 2009, Treu et al. purified PQQ-dependent ADH from Gluconobacter and immobilized the enzyme with PQQ-dependent ALDH and high surface area nanotube supports on Toray paper with a hydrophobieaUy modified Nafion layer to bind the mixture to the surface [36]. With this type of eleetrode fabrication, the PQQ-dependent enzymes were able to achieve better eleetrieal eontact on the electrodes. This biologieal fuel anode with earboxylated single-waUed CNTs (SWCNTs) achieved the best result with a power density of 1.25mAem , a current density of 0.252 mAem , and an OCP of 1.15 V. Short and long earboxylated multiwalled nanotubes were also studied and all nanotube anodes showed inereased performance compared with earbon paper anodes without nanotubes. 

Laurinavicius V, Razumiene J, Kurtinaitiene B, Lapenaite I, Bachmatova L Marcinke-viciene L, Meskys R, Ramanavicius A. Bioelectrochemical application of some PQQ-dependent enzymes. Bioelectrochemistry 2002 55 29-32. 

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