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Pyrroquinoline Quinone as an In-vivo Biofactor in Polyol Pathway

Dr. Dhrubo Jyoti Sen and Dr. R. Badmanaban

Pyrroquinoline quinone-dependent alcohol dehydrogenase (PQQ-ADH) of acetic acid bacteria is a membrane-bound enzyme involved in the acetic acid fermentation by oxidizing ethanol to acetaldehyde coupling with reduction of membranous ubiquinone (Q), which is, in turn, reoxidized by ubiquinol oxidase, reducing oxygen to water. PQQ-ADHs seem to have co-evolved with the organisms fitting to their own habitats. The enzyme consists of three subunits and has a pyrroloquinoline quinone, 4 heme c moieties and a tightly bound Q as the electron transfer mediators. Biochemical, genetic and electrochemical studies have revealed the unique properties of PQQ-ADH since it was purified in 1978. The enzyme is unique to have ubiquinol oxidation activity in addition to Q reduction. This focuses on the molecular properties of PQQ-ADH, such as the roles of the subunits and the cofactors, particularly in intramolecular electron transport of the enzyme from ethanol to Q. Biotechnological applications of PQQ-ADH as to enantiospecific oxidations for production of the valuable chemicals and bioelectrocatalysis for sensors and fuel cells using indirect and direct electron transfer technologies and discuss unsolved issues and future prospects related to this elaborate enzyme. PQQ with ABCDE gene cluster from Gluconobacter oxydans, which is involved in pyrroloquinoline quinone (PQQ) biosynthesis, in Escherichia coli, resulting in PQQ accumulation in the medium. Since the gene cluster does not include the tldD gene needed for PQQ production, this result suggests that the E. coli tldD gene, which shows high homology to the G. oxydans tldD gene, carries out that function. The synthesis of PQQ activated d-glucose dehydrogenase in E. coli and the growth of the recombinant DNA has been improved. In an attempt to increase the production of PQQ, which acts as a vitamin or growth factor, E. coli has been transferred with various recombinant plasmids, resulting in the overproduction of the PQQ synthesis enzymes and, consequently, PQQ accumulation--up to 6 mM--in the medium.