Combined with an efficient and flexible regeneration system, cofactor-dependent enzymes can be used to selectively introduce modifications in complex molecules.

S-Adenosylmethionine (SAM)-dependent enzymes have great potential for selective alkylation processes. In this study we investigated the regiocomplementary O-methylation of catechols. Enzymatic methylation is often hampered by the need for a stoichiometric supply of SAM and the inhibitory effect of the SAM-derived byproduct on most methyltransferases. To counteract these issues we set up an enzyme cascade. Firstly, SAM was generated from l-methionine and ATP by use of an archaeal methionine adenosyltransferase. Secondly, 4-O-methylation of the substrates dopamine and dihydrocaffeic acid was achieved by use of SafC from the saframycin biosynthesis pathway in 40-70 % yield and high selectivity. The regiocomplementary 3-O-methylation was catalysed by catechol O-methyltransferase from rat. Thirdly, the beneficial influence of a nucleosidase on the overall conversion was demonstrated. The results of this study are important milestones on the pathway to catalytic SAM-dependent alkylation processes.

Chorismatases are a class of chorismate-converting enzymes involved in the biosynthetic pathways of different natural products, many of them with interesting pharmaceutical characteristics. So far, three subfamilies of chorismatases are described that convert chorismate into different (dihydro-)benzoate derivatives (CH-FkbO, CH-Hyg5, and CH-XanB2). Until now, the detailed enzyme mechanism and the molecular basis for the different reaction products were unknown. Here we show that the CH-FkbO and CH-Hyg5 subfamilies share the same protein fold, but employ fundamentally different reaction mechanisms. While the FkbO reaction is a typical hydrolysis, the Hyg5 reaction proceeds intramolecularly, most likely via an arene oxide intermediate. Two nonconserved active site residues were identified that are responsible for the different reaction mechanisms in CH-FkbO and CH-Hyg5. Further, we propose an additional amino acid residue to be responsible for the discrimination of the CH-XanB2 subfamily, which catalyzes the formation of two different hydroxybenzoate regioisomers, likely in a single active site. A multiple sequence alignment shows that these three crucial amino acid positions are located in conserved motifs and can therefore be used to assign unknown chorismatases to the corresponding subfamily.

The macrocyclic polyketides FK506, FK520, and rapamycin are potent immunosuppressants that prevent T-cell proliferation through initial binding to the immunophilin FKBP12. Analogs of these molecules are of considerable interest as therapeutics in both metastatic and inflammatory disease. For these polyketides the starter unit for chain assembly is (4 R ,5 R )-4,5-dihydroxycyclohex-1-enecarboxylic acid derived from the shikimate pathway. We show here that the first committed step in its formation is hydrolysis of chorismate to form (4 R ,5 R )-4,5-dihydroxycyclohexa-1,5-dienecarboxylic acid. This chorismatase activity is encoded by fkbO in the FK506 and FK520 biosynthetic gene clusters, and by rapK in the rapamycin gene cluster of Streptomyces hygroscopicus . Purified recombinant FkbO (from FK520) efficiently catalyzed the chorismatase reaction in vitro, as judged by HPLC-MS and NMR analysis. Complementation using fkbO from either the FK506 or the FK520 gene cluster of a strain of S. hygroscopicus specifically deleted in rapK (BIOT-4010) restored rapamycin production, as did supplementation with (4 R ,5 R )-4,5-dihydroxycyclohexa-1,5-dienecarboxylic acid. Although BIOT-4010 produced no rapamycin, it did produce low levels of BC325, a rapamycin analog containing a 3-hydroxybenzoate starter unit. This led us to identify the rapK homolog hyg5 as encoding a chorismatase/3-hydroxybenzoate synthase. Similar enzymes in other bacteria include the product of the bra8 gene from the pathway to the terpenoid natural product brasilicardin. Expression of either hyg5 or bra8 in BIOT-4010 led to increased levels of BC325. Also, purified Hyg5 catalyzed the predicted conversion of chorismate into 3-hydroxybenzoate. FkbO, RapK, Hyg5, and Bra8 are thus founder members of a previously unrecognized family of enzymes acting on chorismate.

No abstract