The Hantaan orthohantavirus (genovariant Amur–AMRV) is a rodent-borne zoonotic virus; it is the causative agent of haemorrhagic fever with renal syndrome in humans. The currently limited
therapeutic options require the development of effective anti-orthohantavirus drugs. The ability of
native fucoidan from Fucus evanescens (FeF) and its enzymatically prepared high-molecular-weight
(FeHMP) and low-molecular-weight (FeLMP) fractions to inhibit different stages of AMRV infection
in Vero cells was studied. The structures of derivatives obtained were determined using nuclear
magnetic resonance (NMR) spectroscopy. We found that fucoidan and its derivatives exhibited significant antiviral activity by affecting the early stages of the AMRV lifecycle, notably virus attachment
and penetration. The FeHMP and FeLMP fractions showed the highest anti-adsorption activity
by inhibiting AMRV focus formation, with a selective index (SI) > 110; FeF had an SI of ~70. The
FeLMP fraction showed a greater virucidal effect compared with FeF and the FeHMP fraction. It
was shown by molecular docking that 2O-sulphated fucotetrasaccharide, a main component of the
FeLMP fraction, is able to bind with the AMRV envelope glycoproteins Gn/Gc and with integrin β3
to prevent virus–cell interactions. The relatively small size of these sites of interactions explains the
higher anti-AMRV activity of the FeLMP fraction.