The wonderful world of polyketides

From Senna leaf and fruit, Cassia angustifolia (Leguminosae/Fabaceae). Senna is a stimulant laxative that acts on the wall of the large intestine, increasingperistaltic movement.

This one helps you “go” Ansamycin antibiotics are produced by cultures of Amycolatopsis mediterranei Rifamycin SV (below ) is an anti-tuberculosis drug that works against Gram-positive bacteria by inhibiting bacterial RNA synthesis - it binds to DNA-dependent bacterial RNA polymerase.

Starter unit acetyl CoA +

Polyketide formation: folding is the key to ring formation:1. Aldol or Claisen reaction

forms the ring2. Enolization produces

phenol

Organisms that produce polyketides make highly reactive poly-β-keto chains, which must be stabilized by association with groups on the enzyme surface until chain assembly is complete and then cyclizations occur.

formation of a depside occurs through esterification of two phenolic acid units

Multiple aromaticrings can result fromlonger poly-b-ketochains

A depside fromcranberriesTurner, et al, 2007

a metabolite from the mold Alternaria tenuis

Formation of anthrones and anthraquinones:

1. chain folds in half2. aldol condensation, dehydration, enolizationsequence forms the rings3. oxidation of hydroxyl groups occurs after

“Emetics” or laxatives found in Senna (Cassia angustifolia), Cascara (Rhamnus purshianus)“Sennosides” have sugars attached to the ring skeleton

Assembly of hypericin (St. John’s Wort – Hypericum perforatum)

Senna (Cassia angustifolia)

St. John’s WortHypericum perforatum

Alkylations occur adjacent to oxygenated C andprior to cyclizationby aromatization

Radical coupling often joins phenolic rings. Ex: griseofulvin, anantifungal

Some common themes in polyketides

Produced by cultures of Penicillium griseofulvum and is effective for difficult skin infections. It’s absorbed from the gut and concentrated in keratin, so it is used orally to control dermatophytes. Works by disruption of the mitotic spindle, inhibiting fungal mitosis.

Oxidative cleavage, lactone and acetal formation results in major skeletal modifications

Patulin is a potent carcinogen produced by Penicillium patulum, acommon contaminant on apples. Products made from contaminated fruit may contain dangerous levels of patulin, so juices, etc. are routinely screened for patulin content(max of 50 μg kg−1).

Common reactions in polyketide biosynthesis

• Aldol/Claisen condensations cyclization• Enolization aromatic ring• Radical coupling join two pieces together• Aromatic cleavage open ring & allow for

twisting & reconfiguration of C skeleton• Lactone or acetal formation produces a new

ring

Poison Ivy / Oak (Toxicodendron radicans or Rhus radicans; T. toxicaria) contains urushiols – part fatty acid, part polyketide

Family:Anacardiaceae

Phenol getsoxidized to a quinone, which is theactive form, produces allergic rxn.

The origins of aflatoxins (Aspergillus flavus & Aspergillus parasiticus)

Aflatoxin – producing fungi grow mainly on peanuts, corn, rice, pistachio nutsTargets the liver, which becomes enlarged due to fat deposition, cell necrosis, etc.Recent dog food contamination resulted in several pet deathsAflatoxin B1 is carcinogenic - after epoxidization by CYP450 it can intercalate DNA, causing mutations.

alpha-linolenic acid (20%–25%)

Cannabis sativaCannabaceae

THC (tetrahydrocannibinol)works by mimicking thenatural molecules that bindto endogenous cannabinoid receptors (analgesia):CB1 in the brain (mood, memorymotor control, pain, appetite)CB2 in immune/reproductivesystem cells

heat, light

psychoactive inactive

Ether linkagenecessary

inactive

aromatic ring dec. activity

Structure-activity studies of natural products:Many plants produce a range of compounds having similar structures with a fewmodifications in substituents, side chains, sugars attached.Comparison of the relative bioactivities of these molecules allows determinationof which structural features are necessary for activity Important for turning drug leads into synthetic derivatives with increased activity

Two origins of methyl groups on polyketide skeletons

Actinomycetes (e.g. Streptomyces)are filamentous bacteria that gain methyls by incorporation of propionate via methylmalonyl-CoA as a building block.

There are a wide variety of macrolides produced by Streptomyces spp., most of which have some antibiotic activity. Many are readily identified by names ending in “mycin”.Note: Streptococcus are a separate genus

Methylation using SAM is more common in fungi

Tetracyclines: produced from polyketide chain through series of aldol condensations, enolizations, and oxidations, similar to biosynthesis of anthrones. They have broad-spectrum activity against a variety of Gram-positive and Gram-negative bacteria.

Bacterial resistance to tetracyclines has developed in pathogens such as Pneumococcus, Staphylococcus, Streptococcus, and E.coli. Mechanism of resistance: decreased cell permeability, and membrane-embedded transport proteins that export the tetracycline out of the cell before it can exert its effect. Still effective for infections caused by Chlamydia, Mycoplasma, Brucella, Rickettsia, and chronic bronchitis due to Haemophilus influenzae.

produced by Streptomyces

Macrolide ring formation and modification:

The macrolide antibiotics are a large family of compounds, many with antibiotic activity, - characterized by a macrocyclic lactone ring (12, 14, or 16 membered)- the poly-b-keto chain undergoes reductions, dehydrations, during chain extension, so the molecule will not undergo cyclizations to fully aromatic structure.

produced by fungiGibberella, Fusarium spp.that cause fruit & root rot

Macrolide antibiotics

Mode of action: “hijacks”replication by binding to 50S subunit of bacterial ribosome, blocking translocation of growing peptidyl RNA

Erythromycins(Saccharopolyspora erythreus)target Gram-positive bacteriasuch as MRSA(methicillin-resistantStaph. aureus)

from Streptomyces antibioticus

Spiramycins:Produced by Streptomyces ambofaciensUsed to treat toxoplasmosis,a feline disease caused byprotozoan Toxoplasma gondii

Several different linkersare used in building polyketide chain

Larger polyeneantifungals

Amphotericin iscommonly usedto treat Candidainfections (withazoles)

Works by bindingto ergosterol, a sterol in the fungalmembraneDisrupts membranecausing pores to form, cells leak vitalnutrients & electrolytes

Used to treat yeasts & Cryptococcus, andalso to reduce mold growth on surfaces

Compounds were isolated from a MeOH/CH2Cl2 extractof the sponge (3 kg) and found to kill HCT-116 coloncancer cells at sub uM concentrations.IC50 = 8 ng/mL for (2)Hurgholide A inhibited Candida at 31 ug/mL

How do they work?These macrolides kill cells by disrupting the actin cytoskeleton. One dimeric molecule binds simultaneously to two molecules of G-actin, forming a tertiary complex and inhibiting polymerization. The Swinholides also cause the filamentous actin strands to break. All caused loss of cellular microfilaments at nM concentrations. Cells treated with swinholide I collapsed and formed neuron-like structures.

Compounds 2 & 3 were isolated after several rounds of column chromatography in mg quantities.

Many marine toxins are complex polyethers with origins in the acetate pathway

The brevetoxins are potentneurotoxins that bind to sodium channels , keeping them open.

Brevenal (below) is a functionalantagonist to brevetoxin, inhibiting its activity in all assays.Brevenal reduces brevetoxinbinding to rat brain synaptosomes, blocks brevetoxin-inducedbronchoconstriction in sheep, and reduces brevetoxin toxicity in fish.

Biosynthetic studies have shown that fragmentsfrom the citric acid cycle and a 4C starter unit frommevalonate are involved in generating C skeleton. Some of the methyls originate from methionine.

Karenia brevis is a marine dinoflagellate known for production of several different families of bioactive ladder-frame polyether compounds. They contain similar trans-fused, ladder-shaped, cyclic ether ring systems; with ring sizes (5 to 9-membered rings), number of rings (4, 5, 6, 10, and 11), and side chains vary among the different families. Algal blooms by this organism causered tide in Florida.

Red tide off the coast ofSan Diego, caused by thedinoflagellate Lingulodiniumpolyedrum

Tamulamides A and B bind to the same sites as the toxins but do not causetoxic effects at nM concentrations.