Germicidin

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Germicidins are a family of natural compounds made by Streptomyces bacteria. They act as autoregulatory inhibitors of spore germination: at very low concentrations (about 200 picomolar) they stop the organism’s own spores from germinating, and at higher concentrations they can inhibit a pig enzyme (Na+/K+-ATPase). They can also affect germination in plants.

There are four known germicidins: A, B, C, and D (surugapyrone A is the same as germicidin D). All of them block spore germination; germicidin A can also cause reversible inhibition and may promote hyphal elongation.

Biosynthesis and structure: Germicidins are made by a type III polyketide synthase called germicidin synthase (Gcs). This enzyme can use different starter units and works best when the acyl group is carried by ACP (acyl carrier protein) rather than CoA. The enzyme’s crystal structure shows a typical type III PKS framework with a distinctive 40‑amino‑acid insertion at the dimer interface.

How they’re built in cells: The proposed pathway starts with fatty acid–like starter units. ACP is converted to its holo form, combined with CoA, and transformed by AcpS. FabD makes malonyl‑AcpP, and FabH condenses this with an isobutyryl‑CoA or 2‑methylbutyryl‑CoA to form an acyl‑AcpP. Finally, Gcs fuses this with methylmalonyl‑CoA or ethylmalonyl‑CoA to produce germicidin. The germicidins are of interest for potential pharmaceutical uses, including antibiotic or antifungal activity.