Discovery of a parallel family of euglenatide analogs in Euglena gracilis

Abstract

The euglenatides are a family of hybrid polyketide-nonribosomal peptides produced by the unicellular algae Euglena gracilis. These compounds have antiproliferative activity against fungal pathogens and mammalian cancer cell lines. Analysis of E. gracilis extracts revealed that the algae produce not only the euglenatides, but also a corresponding family of analogs that have the same molecular weights as the euglenatides, but are lacking the characteristic triene chromophore. In comparison to the euglenatides, the activity of these analogs is greatly reduced in a mammalian cytotoxicity assay, indicating that the triene is critical to the biological activity of the euglenatides.

Keywords: Euglena gracilis; Euglenatide; Natural products; Nonribosomal peptide; Polyketide.

Fig. 1

Previously reported structures of euglenatides A–E. The unusual amino acids found in the euglenatides include β-aminoisobutyric acid (β-Aib), 4,5-dihydroxynorvaline (Dnv), and 4,5-dihydroxynorleucine (Dnl)

Fig. 2

E. gracilis produces the euglenatide analogs, euglenatides A₂, B₂, and C₂. a Molecular networking analysis of crude E. gracilis extracts analyzed in positive (ESI+) mode. The orange node represents the [M-CH₄O + H]⁺ ion for A₂, the yellow nodes represent the [M-CH₄O + H]⁺, [M-CH₆O₂ + H]⁺, and fragment ions of B, the green nodes represent the [M + H]⁺, [M-CH₄O + H]⁺ and fragment ions of B₂, the pink node represents the [M-CH₄O + H]⁺ ion of C, and the purple node represents the [M-CH₄O + H]⁺ ion of D. b Extracted ion chromatograms (EICs) of euglenatides A–C and their analogs A₂–C₂ analyzed in negative (ESI-) mode, along with the absorbance at 280 nm. The fraction analyzed here has roughly equal amounts of the different euglenatides and their analogs. However, in the crude extracts, euglenatide B and B₂ are much more abundant than euglenatides A, A₂, C, and C₂. c UV–Vis spectrum of euglenatide B (top) and euglenatide B₂ (bottom)

Fig. 3

MS–MS spectra of the euglenatides and their analogs. Mirrored MS–MS spectra for euglenatides A and A₂ (a), euglenatides B and B₂ (b), euglenatides C and C₂ (c)

Fig. 4

Key 2D NMR correlations for the euglenatide B analog, euglenatide B₂

Fig. 5

Chemical structures of euglenatides A₂, B₂, and C₂ (a) and the antiproliferative activity of euglenatides B and B₂ against A549 cells (b)