BIOSYNTHESIS OF PYRROLOQUINOLINE QUINONE .2. BIOSYNTHETIC ASSEMBLY FROM GLUTAMATE AND TYROSINE

The biosynthesis of pyrroloquinoline quinone (PQQ), the prosthetic group of quinoproteins, was studied in the methylotrophic bacterium, Methylobacterium extorquens sp. AM1. Using C-13-labeled precursors and NMR spectroscopy, we have elucidated the biosynthetic origin of PQQ. In an initial series of feeding experiments, M. extorquens AM1 was grown on [1-C-13]- or [2-C-13]ethanol, and the resulting C-13 enrichments in PQQ were compared to the labeling patterns in amino acids. These data revealed that PQQ is biosynthesized from two amino acids: one molecule of glutamate and one molecule of either tyrosine or phenylalanine. Direct incorporation of tyrosine was observed by using [C-13]tyrosine labeled in the phenol side chain and at the methylene position. Moreover, a double-labeling experiment with [N-15, C-13]tyrosine demonstrated that the pyrrole nitrogen is derived from the alpha-amino group of tyrosine. Therefore, carbons 2', 2, 3, 3a, 4, 5, 5a, 9a, 1a and nitrogen 1 of PQQ are derived from tyrosine; the pyrrole ring forms via the intramolecular cyclization of the tyrosine. Carbons 7', 7, 8, 9, 9', and, in all probability, nitrogen 6 are derived from glutamate. Using this information, we have proposed biosynthetic pathways for the assembly of PQQ from tyrosine and glutamate.