Mapping the catalytic conformations of an assembly-line polyketide synthase module

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Assembly-line polyketide synthases (PKS) are large enzymatic complexes responsible for synthesizing complex polyketide natural products.
• These enzymes utilize a modular organization, where each module contains multiple active sites that process substrates sequentially.
• Understanding the mechanism of substrate channeling within PKS is crucial for engineering novel compounds.

Purpose of the Study:

• To elucidate the structural basis of substrate channeling in polyketide synthases.
• To investigate the mechanism of 6-deoxyerythronolide B synthase (DEBS) module 1 using high-resolution cryo-electron microscopy.
• To characterize the structural features responsible for active site gating and substrate transfer.

Main Methods:

• High-resolution cryogenic electron microscopy (cryo-EM) was employed to determine the structures of DEBS module 1.
• Multiple structural states, including the intact module and a product-bound state, were analyzed.
• Analysis focused on domain-domain interfaces and structural asymmetry to understand channeling.

Main Results:

• 3.2- to 4.3-angstrom resolution cryo-EM structures of DEBS module 1 were obtained.
• Key domain-domain interfaces critical for substrate channeling were identified.
• An unexpected module asymmetry and a "turnstile" mechanism for active site gating were revealed in the product-bound state.

Conclusions:

• The study provides a structural model for substrate channeling in assembly-line polyketide synthases.
• The findings highlight the importance of module asymmetry and dynamic gating mechanisms in PKS function.
• This structural insight could enable the rational design of engineered PKS for novel polyketide synthesis.