Polyketide Synthases and Modifications

  • Polyketide Synthases (PKS): Enzymes that synthesize polyketides using various building blocks, primarily malonyl-CoA, but can also utilize methylmalonyl-CoA, which originates from propionyl-CoA.

    • Difference Between Propionyl-CoA and Acetyl-CoA:
      • Propionyl-CoA has an ethyl group whereas Acetyl-CoA has only a methyl group.
      • Use of propionyl-CoA leads to the incorporation of extra methyl groups and an ethyl group in the polyketide.

  • Other Building Blocks:

    • Example: Hydroxycinnamic acid can be activated as a CoA thioester for condensation reactions to extend carbon chains.
    • Hydroxycinnamic acid is derived from tyrosine, a standard amino acid in proteins, which can lead to alkaloid metabolites.
    • Identifying Tyrosine Derivatives: Look for the 1,4 substitution pattern on aromatic rings (OH and carbon arrangement).

  • Curly Arrow Mechanisms:

    • Enolization and proton transfer can begin the mechanism for synthesizing orsalinic acid.
    • Familiarize with condensation mechanisms for forming rings.
    • Conversion of phenols to carbonyls can help in deducing the original structure from the end product.

  • Types of Polyketide Synthases:

    • Type 1: Multifunctional enzymes, similar to fatty acid synthases, involved in producing macrocyclic polyketides like erythromycin.
      • Consists of various active sites within a single protein.
    • Type 2: Produces smaller phenolic compounds through separate proteins that operate non-covalently, each performing specific actions.
    • Type 3: Specific to plants, producing chalcone; not discussed in depth as they are less common.

  • Post-PKS Modifications:

    • Common modifications include oxidation (often through cytochrome P450 enzymes) and glycosylation.
    • Cytochrome P450 facilitates the oxidation of CH bonds to alcohols, requiring NADPH for electron supply.
    • Glycosylation involves transferring activated sugar groups to alcohols, frequently using uridine diphosphate (UDP) sugars.

  • Example: Erythromycin is formed from 6-Deoxyerythronolide through several oxidation and glycosylation reactions.

    • Identifying the source of OH groups can help discern which modifications are post-PKS.
    • Technique: Examine the location of the OH groups; those that don't fit a one, three pattern are often post-synthetic modifications.

  • Actinorhodin Example: An illustration of type 2 metabolites; discussions include specific genes coding for PKS enzymes and their orientation in DNA.

    • Gene Structure in DNA: Read 5’ to 3’ direction, understanding the different gene orientations that dictate protein functions.

  • DEBS Enzymes: Responsible for producing 6-Deoxyerythronolide B, consisting of three mega enzymes with multiple functional modules for polyketide synthesis.

    • Detailed analysis of individual modules reveals the flow of synthetic steps governed by enzyme arrangements and active sites.
    • Predictions of product structures possible by understanding gene sequences and enzyme mechanisms.
    • Final modification by thioesterase allows closure of the polyketide structure into a functional form.

  • Study Focus:

    • Distinctions between type 1 and type 2 PKSs, understanding modular versus iterative enzyme functions, and recognizing modification patterns in polyketides.
    • Clarify which metabolites are linked to specific types of PKS based on their structures.

  • Exam Preparation Problem:

    • Identify metabolites to classify them as type 1, type 2, or type 3 based on observed structural characteristics.