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Multi-Step Reactions02:31

Multi-Step Reactions

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Chemical reactions often occur in a stepwise fashion involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs. Each of the steps in a reaction mechanism is called an elementary reaction. These...
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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
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Cooperative Allosteric Transitions01:58

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Cooperative allosteric transitions can occur in multimeric proteins, where each subunit of the protein has its own ligand-binding site. When a ligand binds to any of these subunits, it triggers a conformational change that affects the binding sites in the other subunits; this can change the affinity of the other sites for their respective ligands. The ability of the protein to change the shape of its binding site is attributed to the presence of a mix of flexible and stable segments in the...
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Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry01:29

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Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products.
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Diels–Alder Reaction Forming Cyclic Products: Stereochemistry01:28

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

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The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic movement of six π electrons: four π electrons from the diene and two π electrons from the dienophile.
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Related Experiment Video

Updated: Oct 14, 2025

From a Natural Product to Its Biosynthetic Gene Cluster: A Demonstration Using Polyketomycin from Streptomyces diastatochromogenes Tü6028
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From a Natural Product to Its Biosynthetic Gene Cluster: A Demonstration Using Polyketomycin from Streptomyces diastatochromogenes Tü6028

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Modular polyketide synthase contains two reaction chambers that operate asynchronously.

Saket R Bagde1,2, Irimpan I Mathews3, J Christopher Fromme2

  • 1Department of Chemistry and Biochemistry, The University of Texas at El Paso, El Paso, TX 79968, USA.

Science (New York, N.Y.)
|November 4, 2021
PubMed
Summary

Structural insights into Type I modular polyketide synthases (PKS) reveal how Lsd14 PKS uses two reaction chambers, with only one actively producing polyketide products at a time.

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Area of Science:

  • Biochemistry
  • Structural Biology
  • Enzymology

Background:

  • Type I modular polyketide synthases (PKS) are large, multi-domain enzymes crucial for synthesizing diverse polyketide natural products.
  • These enzymes function as assembly lines, catalyzing sequential chain extension and modification reactions.

Purpose of the Study:

  • To elucidate the structural mechanisms of Type I modular polyketide synthases.
  • To understand the domain positioning, rearrangements, and interactions within the Lsd14 PKS during its catalytic cycle.

Main Methods:

  • Determined the X-ray crystal structure of Lsd14 PKS at 2.4 angstrom resolution.
  • Determined the cryo-electron microscopy structure of Lsd14 PKS at 3.1 angstrom resolution, capturing distinct reaction states.

Main Results:

  • Revealed the precise positioning and dynamic rearrangements of domains within the Lsd14 PKS.
  • Identified specific inter-domain interactions critical for enzymatic function.
  • Showcased that Lsd14 PKS possesses two reaction chambers, but only one is catalytically active for product synthesis at any given time.

Conclusions:

  • The study provides a detailed structural understanding of Type I modular PKS function.
  • Lsd14 PKS exhibits a unique mechanism involving asymmetric utilization of its two reaction chambers.
  • These findings offer insights into polyketide biosynthesis and enzyme evolution.