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Cationic Chain-Growth Polymerization: Mechanism00:57

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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the generated carbocation,...
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Bacterial protein maturation is a tightly regulated process that ensures newly synthesized polypeptides achieve correct functional conformations. This maturation involves a series of modifications, folding events, and quality control steps, often assisted by specialized chaperone proteins.N-Terminal ModificationsThe maturation of bacterial polypeptides begins cotranslationally as the polypeptide exits the ribosome. The first amino acid, N-formylmethionine (fMet), is typically modified at the...
Anionic Chain-Growth Polymerization: Mechanism01:04

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Related Experiment Video

Updated: May 10, 2026

From a Natural Product to Its Biosynthetic Gene Cluster: A Demonstration Using Polyketomycin from Streptomyces diastatochromogenes Tü6028
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C-terminal dimerization motifs control asynchronous chain elongation during modular polyketide biosynthesis.

Chengli Liu1, Ryan C West1, Muyuan Chen2

  • 1Department of Pharmacology and Pharmaceutical Sciences, University of Southern California, Los Angeles California, USA.

The Journal of Biological Chemistry
|May 8, 2026
PubMed
Summary

Amycolatopsis mediterranei rifamycin synthetase (RIFS) uses C-terminal dimerization to coordinate polyketide assembly. This conformational asymmetry is crucial for antibiotic biosynthesis, guiding substrate carrier protein movement within ketosynthase active sites.

Keywords:
antibioticscryogenic electron microscopyenzymologynatural product biosynthesis

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Published on: June 24, 2016

Area of Science:

  • Biochemistry
  • Molecular Biology
  • Structural Biology

Background:

  • The rifamycin synthetase (RIFS) from Amycolatopsis mediterranei is a large, multi-domain enzyme responsible for synthesizing rifampicin precursors.
  • RIFS functions as a modular assembly line, catalyzing numerous chemical reactions.
  • Coordination of catalytic activities within RIFS remains poorly understood.

Purpose of the Study:

  • To investigate the spatial and temporal coordination of catalytic activities in RIFS.
  • To elucidate the role of conformational asymmetry in polyketide chain elongation.
  • To understand the function of C-terminal dimerization motifs in bacterial polyketide synthases.

Main Methods:

  • Thiol-selective crosslinking to probe protein conformation.
  • Cryogenic electron microscopy (cryo-EM) for structural analysis of the first PKS module (M1).
  • Single-turnover kinetic analysis to assess enzyme activity and interactions.

Main Results:

  • C-terminal dimerization motifs induce conformational asymmetry by directing substrate carrier protein (CP) domains.
  • CP domains co-migrate between equivalent ketosynthase (KS) active site chambers.
  • Mutations reducing CP dimerization increased KS:CP interactions but did not fully resolve product accumulation issues.
  • Cryo-EM revealed a unique architecture for the RIFS M1 module.

Conclusions:

  • C-terminal dimerization is a key factor in regulating asymmetry during polyketide biosynthesis.
  • Understanding these mechanisms provides insights into antibiotic production pathways.
  • Findings can inform future engineering of megasynth(et)ase enzymes.