Pictet-Spengler reaction is one of the main ways to synthesize spiroindolones, which widely exists in natural products and pharmaceutical agents. We accidentally discovered bovine serum albumin (BSA) had catalytic activity for Pictet-Spengler reaction previously. After a series of exploratory experiments around the catalytic process, it was found that some amino acids played a key role. It is worth noting that, Pictet-Spengler reaction catalyzed by amino acids has been rarely described. Herein, we report the cysteine-catalyzed Pictet-Spengler reaction between tryptamine and isatin under metal-free and convenient conditions. The catalytic performance of amino acid was optimized through screening reaction conditions such as time, temperature, solvent, molar ratio and catalyst amount and a series of spiroindolones can be obtained in moderate to excellent yields. The present investigation featured relatively mild reaction conditions with good functional group tolerance and provided an alternative method for preparation of spiroindolones. Graphical Abstract
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.
Pictet–Spengler (PS) reactions are pivotal in the biosynthesis of bioactive alkaloids and pharmaceuticals, yet key structural details underlying their enzymatic catalysis remain insufficiently understood. We identified AsKslB from Actinosynnema sp. ALI-1.44 as a Pictet-Spenglerase with broad substrate scope that catalyzes the stereoselective condensation of l -tryptophan ( l -Trp) and α-ketoglutarate (α-KG) to form kitasetalic acid (KA), a tetrahydro-β-carboline (THβC). High-resolution crystal structures of apo, substrate-, intermediate-, and product-bound forms elucidate the full catalytic trajectory and key residues. Crucially, the elusive iminium ion intermediate (IM-1) and a synchronously released water molecule are captured, providing direct structural evidence for the initiating cyclization step of Pictet-Spengler reaction. Glu276 undergoes conformational changes essential for catalysis. These findings offer detailed mechanistic insights into Pictet-Spenglerase function and establish AsKslB as a promising biocatalyst for stereoselective N -heterocycle synthesis.
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