3.1. Direct Binding Inhibition (Virucidal Effect)

The binding to host cells through interaction with functional receptors is the initial trend in viral infection. Once the viral particle attaches to a host cell, its genetic material is inserted into the cell during this initial attachment and penetration stage. Regarding direct virion inhibition, the AVP papuamide A has shown an immediate virucidal effect of HIV-1 inhibition through a viral-membrane-targeting mechanism ^[34][35][43,44]. This marine peptide bears a tyrosine hydroxy and an aliphatic moiety tail that have been suggested to interact with the cholesterol membrane of the virus target and assist with insertion into the viral membrane, respectively, resulting in the subsequent viral membrane disruption and, ultimately, viral inactivation ^[35][44].

3.2. Viral Attachment (Cellular Association) and Entry Inhibition

The indirect binding, as evidenced by the HIV-1-targeting α-defensin human neutrophil peptide HNP-1, has been found to bind viral envelope Env and host CD4 glycoproteins/co-receptors in a glycan- and serum-independent manner. In addition, the oligomerization or refolding of HNP-1 could block the viral fusion. Hence, this peptide requires a multifaceted mode of action for HIV entry inhibition ^[36][37][45,46]. Moreover, several studies ^{[32][38][39][40][41]}[41,47,48,49,50] have reported that two mechanisms of the cellular association or viral attachment may prevent the entry of the enveloped viruses. In the instance of the activity towards the influenza virus, firstly, the AVP competes with sialic acid to bind with the envelope glycoprotein—namely, hemagglutinin (HA)—and clogs its receptor site, inhibiting the influenza virion from interacting with the host cell membrane. The peptide can imitate sialic acid’s behavior to be recognized by the receptor-binding site of viral HA. Furthermore, the binding of AVP to the other main component of the influenza virus—namely, N-acetylneuraminic acid—on the host cell surface can also prevent viral attachment to host cells. Secondly, the conformation of HA is inhibited; thus, intracellular entry is prohibited, leading to endosomal escape and viral genome release ^[32][41]. A similar antiviral activity targeting primary attachment has also been found to inhibit HSV and hepatitis viruses. The AVP with a positive charge and good hydrophobicity binds to a cellular glycan moiety to prevent HSV from attaching to the host cell surface ^[42][51]. This glycan moiety is heparan sulfate, which is known to be a negatively charged glycosaminoglycan that favors HSV viral particles through its basic positively charged binding pocket in the virion glycoprotein to attach to the host cell surface. Therefore, the AVP can prevent HSV virion invasion by binding to glycosaminoglycan molecules as a receptor, hampering the interaction of the virus and the receptor in a host cell. With respect to hepatitis viruses—particularly HCV—some AVPs have also been demonstrated to interact with the virion receptors and co-receptors. AVPs that resemble the cellular protein apolipoprotein E (ApoE) can break up the glycan-dependent interaction or attachment of HCV, hindering entry and infection of target host cells (hepatocytes) ^[43][44][52,53]. As observed by Chi et al. (2016) ^[45][54], the HCV fusion-inhibitory peptide could block viral envelope glycoproteins’ E1/E2-mediated membrane fusion by interfering with E1 and E2 heterodimerization. Moreover, the peptide was likely to provoke the dimer E1/E2 glycoproteins’ conformational changes, impairing HCV membrane fusion. Additionally, another AVP (CL58) seemed to inhibit viral entry, possibly after initial binding (post-binding) of the co-receptor cellular membrane protein CD81, and just prior to the final intracellular fusion in endosomes ^[46][47][55,56]. Furthermore, these mentioned inhibition mechanisms of the viral membrane fusion stage by peptides have been further comprehensively discussed elsewhere ^[48][57].

3.3. Viral Enzymes and Replication Inhibition

In addition, AVPs have also been reported to inhibit the viral replication of HSV-2, influenza viruses, and HCV. In the case of HSV-2, the AVPs can block the transport of a primary viral protein named VP16 into the nucleus. This transcriptional protein regulator induces immediate expression of viral genes required for survival at the initial cellular response. Some AVPs that act on influenza viruses also target viral-RNA-dependent RNA polymerase, comprising PB1, PB2, and PA subunits. These subunits control polymerization and endonuclease cleavage, recognition, binding to the host mRNAs, and endonuclease activity towards host pre-mRNA. Binding those subunits, AVPs can also prohibit the assembly of the influenza polymerase complex ^[13][32][49][13,41,58]. For HCV, AVPs can act on NS3-4A—a multifunctional enzyme with serine proteinase and helicase functions that are harnessed for HCV replication ^[50][51][59,60]. Furthermore, to inhibit the replication of the very recent virus SARS-CoV-2, Tonk et al. (2021) ^[52][61] and Heydari et al. (2021) ^[53][62] gathered findings on a number of antiviral activities of AVPs that could be involved. Some AVPs directly act on the viral envelope (virucidal effect), binding to the viral spike glycoprotein that blocks the interaction with angiotensin-converting enzyme-2 (ACE2) in host cells, hampering endosomal acidification for uncoating throughout the initial viral life cycle, or escorting the host ACE2 receptor ^{[54][55][56][57][58]}[63,64,65,66,67]. Moreover, several approaches have been deployed to evaluate the antiviral activity of marine peptides, including neutralization, viral titration, cell viability, and virus plaque-reduction assays ^[59][27]. Some selected examples of marine AVPs that possess antiviral activity against important human enveloped viruses—such as HIV, influenza, HSV, HCV, and even SARS-CoV-2—are summarized in Figure 1, listed in Table 1, and featured below.