The role of PQQ and quinoproteins in methylotrophic bacteria

Abstract

Quinoprotein dehydrogenases play a non-exclusive role in the dissimilation of C 1 compounds. Methanol and methylamine oxidation occur by covalent catalysis while the reduction equivalents are transferred to the respiratory chain in one-electron steps. Cytochrome c L is an excellent electron acceptor for methanol dehydrogenase at pH 7.0 and a bad one at pH 9.0. Efficient methanol oxidation (with NH 3 as activator) occurs at pH 9.0, but (due to the failure of NH 3) not at pH 7.0. Since stimulation occurred at the latter condition with a compound prepared from Hyphomicrobium X, most probably methanol oxidation in vivo requires the presence of a natural activator. The finding of pro-PQQ in methylamine dehydrogenase implicates that certain quinoproteins may have a modified tyrosine as cofactor. This type of quinoprotein is involved in assimilation routes which also occur in methylotrophs. l-Tyrosine and l-glutamate are the precursors of PQQ biosynthesis. Free intermediates in the route of biosynthesis have not been found. Most probably the whole process occurs on a protein matrix. In view of the significant amounts found in their culture fluid, methylotrophic bacteria seem particularly well suited for the fermentative production of PQQ.