David R. Houck

PQQ: Biosynthetic studies inMethylobacterium AM1 andHyphomicrobium X using specific13C labeling and NMR

Using13C labeling and NMR spectroscopy we have determined biosynthetic precursors of pyrroloquinoline quinone (PQQ) in two closely related serine-type methylotrophs,Methylobacterium AM1 andHyphomicrobium X. Analysis of the13C-labeling data revealed that PQQ is constructed from two amino acids: the portion containing N-6,C-7,8,9 and the two carboxylic acid groups,C-7′ and 9′, is derived-intact-from glutamate. The remaining portion is derived from tyrosine; the phenol side chain provides the six carbons of the ring containing the orthoquinone, whereas internal cyclization of the amino acid backbone forms the pyrrole-2-carboxylic acid moiety. This is analogous to the cyclization of dopaquinone to form dopachrome. Dopaquinone is a product of the oxidation of tyrosine (via dopa) in reactions catalyzed by monophenol monooxygenase (EC 1.14.18.1). Starting with tyrosine and glutamate, we will discuss possible biosynthetic routes to PQQ.

Stereochemical Course of the Transmethylation Catalyzed by Histamine N-Methyltransferase'

The stereochemical course of the methyl group transfer catalyzed by histamine N-methyltransferase was studied using S-adenosylmethionine (AdoMet), which carried a chiral methyl group. The incubation of these AdoMet samples and histamine with a partially purified enzyme obtained from guinea pig brains gave the corresponding methylated histamine. The N-methylhistamine samples were degraded to convert the N-methyl group into the methyl group of acetate using a reaction sequence of known stereochemistry. The results of the configurational analysis of these acetate samples indicated that the enzymatic transfer of the methyl group from the sulfur of AdoMet to the nitrogen of histamine occurs with inversion of configuration.