Cryo-EM captures a single-module peptide synthetase mid-reaction, revealing a built-in Pictet–Spengler domain

Researchers in Japan and Germany have reported the first structural view of a Pictet–Spengler cyclization domain sitting inside a nonribosomal peptide synthetase (NRPS), the enzyme class that builds many complex peptide natural products without a ribosome. The paper, "Cryo-EM Structure of a Monomeric Safracin B Nonribosomal Peptide Synthetase Enabled by an MbtH-like Protein and Covalent Probe Trapping," was published in the Journal of the American Chemical Society on October 20, 2026, by a team from the University of Tokyo, Tohoku University, Kitasato University, Hokkaido University and the University of Münster. [1]

Why these enzymes are hard to image

NRPSs work like assembly lines. Each module contains domains that select an amino acid building block (the adenylation, or A, domain), carry the growing chain on a flexible arm (the peptidyl carrier protein, or PCP), and join or modify the pieces. A 2017 review in Angewandte Chemie describes them as large multienzyme machineries whose products show great structural diversity. [2] That same flexibility is what makes them difficult to study: according to the new paper, NRPSs remain challenging targets for high-resolution structural analysis because of their conformational heterogeneity and dynamic catalytic cycles. [1]

An unusual one-module synthetase

The enzyme in this study, SacC, belongs to the pathway for safracin B, a bis-tetrahydroisoquinoline alkaloid. It is the counterpart of SfmC from the related saframycin pathway. Rather than linking a long chain of amino acids, this single-module NRPS reduces a peptidyl thioester to an aldehyde and then runs two sequential Pictet–Spengler reactions with a tyrosine-derived unit. The cascade forms two carbon–carbon bonds, sets several stereocenters and builds a pentacyclic scaffold. [1]

That chemistry was first pieced together biochemically. In 2010, a Nature Chemical Biology paper showed that SfmC converts simple dipeptidyl substrates into the saframycin core and proposed a mechanism with a single reduction (R) domain followed by iterative condensation-domain Pictet–Spengler reactions. [3] A 2018 study that included two of the new paper's authors, Ryo Tanifuji and Hiroki Oguri, later used recombinant SfmC in chemo-enzymatic syntheses, reporting that it could assemble the elaborated pentacyclic skeleton in a single day from two simple synthetic substrates. [4] According to the new paper, no earlier structure had shown a Pictet–Spengler cyclization domain embedded within an NRPS assembly line. [1]

How the team froze a moving enzyme

To capture one transient state, the researchers combined two stabilizing tools. The first was SacC's cognate MbtH-like protein, a small partner that many NRPSs need. Earlier work on the enterobactin system in E. coli showed that MbtH-like proteins can be required for solubility or for amino acid activation by the adenylation domain, and that their influence on NRPS function is more complex than previously appreciated. [5] The second was a custom covalent adenosine vinylsulfonamide probe that mimics the aminoacyl-adenylate intermediate and traps the carrier-protein-tethered species inside the adenylation domain. [1]

With both in place, the team solved the thioesterification-state structure of SacC at 2.76 Å resolution. The map directly shows an intermediate attached to the carrier protein and reveals how the Pictet–Spengler, adenylation, peptidyl carrier protein and reduction domains are organized relative to one another. [1] The group also developed nonreducible ester probes that mimic later PCP-bound intermediates, designed to let them capture further steps along SacC's reaction path. [1]

Why it matters for peptide research

The authors frame the result as a general strategy: rational chemical probe design can make state-resolved cryo-EM possible for highly dynamic single-module NRPSs, and that could support efforts to engineer noncanonical NRPS reactivity. [1] For laboratories working on peptide biosynthesis and enzyme engineering, a domain-level map of a working assembly line is a concrete reference point for asking how chain handling and ring formation are coordinated.

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