Methoxatin

Pyrroloquinoline quinone (PQQ), also called methoxatin, is a redox cofactor. It is found in soil and foods such as kiwifruit, as well as human breast milk. Enzymes containing PQQ are called quinoproteins. Glucose dehydrogenase, one of the quinoproteins, is used as a glucose sensor. PQQ stimulates growth in bacteria. It was discovered by J.G. Hauge as the third redox cofactor after nicotinamide and flavin in bacteria (although he hypothesised that it was naphthoquinone). Anthony and Zatman also found the unknown redox cofactor in alcohol dehydrogenase. In 1979, Salisbury and colleagues as well as Duine and colleagues extracted this prosthetic group from methanol dehydrogenase of methylotrophs and identified its molecular structure. Adachi and colleagues discovered that PQQ was also found in Acetobacter. A novel aspect of PQQ is its biosynthesis in bacteria from a ribosomally translated precursor peptide, PqqA. A glutamic acid and a tyrosine in PqqA are cross-linked by the radical SAM enzyme PqqE in the first step of PqqA modification. Efforts to understand PQQ biosynthesis have contributed to broad interest in radical SAM enzymes and their ability to modify proteins, and an analogous radical SAM enzyme-dependent pathway has since been found that produces the putative electron carrier mycofactocin, using a valine and a tyrosine from the precursor peptide MftA. The scientific journal Nature published the 2003 paper by Kasahara and Kato that essentially stated that PQQ was a new vitamin and in 2005, an article by Anthony and Fenton that stated that the 2003 Kasahara and Kato paper drew incorrect and unsubstantiated conclusions. EC 1.2.1.31 – L-aminoadipate-semialdehyde dehydrogenase