[Research progress of Pictet-Spenglerases]

Pictet-Spenglerases (P-Sases) catalyze the Pictet-Spengler (P-S) reactions and exhibit high stereoselectivity and regioselectivity under mild conditions. The typical P-S reaction refers to the condensation and recyclization of β-arylethylamine with aldehyde or ketone under acidic conditions to form tetrahydroisoquinoline and β-carboline alkaloid derivatives. The related enzymatic products of P-Sases are the backbones of various bioactive compounds, including clinical drugs: morphine, noscapine, quinine, berberine, ajmaline, morphine. Furthermore, the activity of P-Sases in stereoselective and regioselective catalysis is also valuable for chemoenzymatic synthesis. Therefore, this review summarizes the research progress in the discovery, functional identification, biological characteristics and catalytic applications of P-Sases, which provide the useful theoretical reference in future P-Sases research and development.

Alkaloid and iridoid glucosides from Palicourea luxurians (Rubiaceae: Palicoureeae) indicate tryptamine- and tryptophan-iridoid alkaloid formation apart the strictosidine pathway

The first phytochemical examination of extracts from leaves and stem bark of Palicourea luxurians (Rusby) Borhidi yielded two undescribed and one known alstrostine derivative together with the oxindole alkaloid javaniside as well as with 5α-carboxystrictosidine. Additionally, five iridoids and four secologanin derived isolation artifacts have been isolated. Lack of strictosidine and its follow-up metabolization products suggested that the Pictet-Spenglerase in P. luxurians does barely or not catalyze the formation of strictosidine. Against this background the biosynthesis of javaniside and 5α-carboxystrictosidine is discussed with regard to possible reaction mechanisms. Similarly, P. luxurians used an independent biosynthetic pathway to produce alstrostine type structures from secologanin and tryptamine in a 2:1 ratio. The structure of isoalstrostine A, which was isolated for the first time, allowed the refinement of a previously reported pathway to the alstrostine type carbon skeleton as well as to some follow-up metabolization products. In spite of various biosynthetic pathways incorporating secologanin to gain different types of tryptophan- and tryptamine-iridoid alkaloids, P. luxurians accumulates this compound as well a couple of further metabolized iridoids deriving from loganin and secologanin.

Characterization of a new microbial Pictet-Spenglerase NscbB affording the β-carboline skeletons from Nocardiopsis synnemataformans DSM 44143

The enzymatic Pictet-Spengler (PS) reaction, catalyzed by Pictet-Spenglerase (PSase), is the feature of β-carboline (βC) alkaloid biosynthesis. NscbB is a rare microbial PSase discovered from a cryptic β-carboline alkaloid biosynthetic gene cluster (BGC) in the Nocardiopsis synnemataformans DSM 44143 (kidney transplant patient derived) by homologous alignment with its well-characterized counterpart McbB. The biochemical analysis showed that NscbB could catalyze l-tryptophan (K_M = 89.64 ± 8.69 μM) and methylglyoxal (K_M = 147.70 ± 16.38 μM), without cofactors, to form the two βC skeletons 1-acetyl-3-carboxy-β-carboline and 1-acetyl-β-carboline in vitro. Additionally, the heterologous expression of nscbB in E. coli BL21 (DE3) revealed the efficient bioproductivity of NscbB to bioproduce two βC skeletons, 1-acetyl-3-carboxy-β-carboline (5.5 mg/L) and 1-acetyl-β-carboline (3.1 mg/L), within 16 h fermentation. These results demonstrate NscbB is a novel and practical microbial PSase, which is useful for future bioactive βC alkaloid exploitation and development.