Potential Bioactivities, Chemical Composition, and Conformation Studies of Exopolysaccharide-Derived Aspergillus sp. Strain GAD7

Abstract

This research identified a marine fungal isolate, Aspergillus sp. strain GAD7, which produces an acidic and sulfated extracellular polysaccharide (EPS) with notable anticoagulant and antioxidant properties. Six fungal strains from the Egyptian Mediterranean Sea were screened for EPS production, with Aspergillus sp. strain GAD7 (EPS-AG7) being the most potent, yielding ~5.19 ± 0.017 g/L. EPS-AG7 was characterized using UV-Vis and FTIR analyses, revealing high carbohydrate (87.5%) and sulfate (24%) contents. HPLC and GC-MS analyses determined that EPS-AG7 is a heterogeneous acidic polysaccharide with an average molecular weight (Mw¯) of ~7.34 × 10³ Da, composed of mannose, glucose, arabinose, galacturonic acid, galactose, and lyxose in a molar ratio of 6.6:3.9:1.8:1.3:1.1:1.0, linked through α- and β-glycosidic linkages as confirmed by NMR analysis. EPS-AG7 adopted a triple helix-like conformation, as evidenced by UV-Vis (Congo Red experiment) and circular dichroism (CD) studies. This helical arrangement demonstrated stability under various experimental conditions, including concentration, ionic strength, temperature, and lipid interactions. EPS-AG7 exhibited significant anticoagulant activity, doubling blood coagulation time at a concentration of 3.0 mg/mL, and showed significant antioxidant activity, with scavenging activities reaching up to 85.90% and 58.64% in DPPH and ABTS⁺ assays at 5.0 mg/mL, and EC50 values of 1.40 mg/mL and 3.80 mg/mL, respectively. These findings highlight the potential of EPS-AG7 for therapeutic applications due to its potent biological activities.

Keywords: Aspergillus sp.; biological activities; circular dichroism; exopolysaccharide; helix-like conformation; structural analysis.

Figure 1

Sampling sites for marine algae samples and the corresponding fungi isolates along the Abu-Qir shore in Egypt during September 2022.

Figure 2

(a) Mycelium dry weights and EPS yields of fungal isolates, (b) macrographs showing macroscopic features on SD agar (b-left) and microscopy details of Aspergillus sp. strain GAD7 (b-right), and (c) phylogenetic tree of Aspergillus sp. strain GAD7.

Figure 3

(a) FTIR spectrum, and (b) SEC chromatogram of the EPS-AG7 produced by Aspergillus sp. GAD7.

Figure 4

(a) Plot of ${\lambda }_{max}$ of CR-EPS against [NaOH] using UV-Vis, (b) VUV-CD spectrum of EPS-AG7 by Aspergillus sp. GAD7 (2.5 w/v% in DW, 25 °C); (c) VUV-CD concentration dependent of EPS-AG7 (0.5–6.0 w/v% in DW); (d) VUV-CD spectra in absence and presence of NaF (50 mmol/L); and (e) VUV-CD temperature-dependent spectra of EPS-AG7 (6.0 w/v%, 20 °C–65 °C in DW).

Figure 5

VUV-CD spectra of EPS-AG7/lipid at different weight ratios (w/w%): (a) EPS-AG7/DOPE; (b) EPS-AG7/DOPS; and (c) EPS-AG7/DOPC, in Tris-HCl buffer of pH 7.6 at 25 °C, and EPS-AG7 concentration of 2.5 w/v%.

Figure 6

DPPH and ABTS⁺ free radicals scavenging activity (%) of (a) EPS-AG7 at a final concentration range of 0.25–5.0 mg/mL, versus (b) ascorbic acid at a final concentration range of 0.005–1.0 mmol/L (0.88–176.12 µg/mL).