Pyrroloquinoline quinone dependent enzymes

The other less common type of phenol biosensor involves enzymes like PQQ (pyrroloquinoline quinone)-dependent glucose dehydrogenase (GDH) (9-12) or... 

Shimao M, Ninomiya K, Kuno O, Kato N, Sakazawa C (1986), Existence of a novel enzyme, pyrroloquinoline quinone-dependent polyvinyl alcohol dehydrogenase, in a bacterial symbiont. Pseudomonas sp. strain VM15C , Appl Environ Microbiol, 51, 268-275. 

Pyrroloquinoline quinone (PQQ) (or methoxatin) 6 is a coenzyme, responsible for the oxidation of methanol [7]. It has been found that cyclopropanol 4 inactivates the enzyme from M. methanica [8], the dimeric methanol dehydrogenase and the monomeric enzyme from a Pseudomonas PQQ-dependent methanol dehydrogenase [9] by forming adducts such as 7, through a one-electron oxidation process and the ready ring opening of a cyclopropyloxonium radical, Eq. (3) [8,9]. 

An alternative biosensor system has been developed by Hart et al. [44] which involves the use of the NAD+-dependent GDH enzyme. The first step of the reaction scheme involves the enzymatic reduction of NAD+ to NADH, which is bought about by the action of GDH on glucose. The analytical signal arises from the electrocatalytic oxidation of NADH back to NAD+ in the presence of the electrocatalyst Meldola s Blue (MB), at a potential of only 0Y. Biosensors utilising this mediator have been reviewed elsewhere [1,17]. Razumiene et al. [45] employed a similar system using both GDH and alcohol dehydrogenase with the cofactor pyrroloquinoline quinone (PQQ), the oxidation of which was mediated by a ferrocene derivative. 

Isolated oxidoreductases always depend on cofactors for the transfer of electrons. Enzyme groups which are well characterized with respect to their biochemistry are those requiring the nicotinamide coenzymes NAD or NADP, the flavins FAD or FMN and the ortho-quinoids such as pyrroloquinoline quinone (PQQ) or trihydroxy-phenylalanine (TOPA). 

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. ... 

Zhao, C., Wittstock, G. Scanning electrochemical microscopy for detection of biosensor and biochip surfaces with immobilized pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase as enzyme label. Biosens Bioelectron 2005, 20, 1277-1284. 

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