**8. References**

58 Biochemistry

bind into the gp41 NHR hydrophobic pocket. A carboxyl group provides electrostatic interaction with K574 in the binding pocket and is critical for the activity of these small molecules; methylation of the carboxyl group resulted in loss of activities of the compounds in both the biochemical assay and cell-cell fusion assay. Three-unit compounds are prone to form aggregates under the assay conditions used and showed no activity, while single-unit compounds, such as M1 (**56**), display submillimolar inhibitory activity (Cai et al. 2009). Compound 1 (**57**), based on M1, was developed, which displayed an IC50 value of 4.5 ± 0.5 and 3.2 ± 0.5 μM in a fluorescence biochemical assay and a cell-cell fusion assay, respectively (Zhou et al. 2010). Compound 1 (**57**) showed very low cytotoxcity (IC50 > 500 μM); with a

Others have reported well-characterized small molecule fusion inhibitors targeting gp41, including SDS-J1 (**50**) (Debnath et al. 1999), NB64 (**51**), NB2 (**52**) (Jiang et al. 2004), and 4M041 (**53**) (Frey et al. 2006). These fusion inhibitors were selected from an active compound library by visual screening, then identified by high-throughput screening, and finally verified by a cell-cell fusion assay or HIV-1 infection assay. They usually showed low micromolar IC50 values for fusion inhibition; however, the following work to optimize the structures to obtain more potent fusion inhibitors were less fruitful, resulting in the identification of more small molecules with similar activity (Jiang et al. 2011). Also, their

Peptides and peptidomimetics are efficient tools to study the HIV-1 gp41 NHR-CHR interaction, a key protein-protein interaction for HIV-1 gp41 mediated virus-cell membrane fusion, which enables HIV-1 enters and ultimately infects host cells. Peptides derived from wild-type HIV-1 gp41 sequences have been used to model the HIV-1 gp41 fusogenic core, a 6-HB formed by the NHR trimer as the inner core, and anti-parallel bind with three CHRs. Crystallographic structure analysis of the 6-HB has uncovered structure details for the gp41 NHR-CHR interaction. A deep pocket in the surface of NHR is a hot spot for the NHR-CHR interaction and a potential target for small molecule fusion inhibitors. N-peptides can be efficient targets for screening fusion inhibitors targeting the gp41 deep pocket by adding

relatively small size, it is a promising lead for fusion inhibitor design.

exact binding model with the gp41 NHR deep pocket still needs to be verified.

Fig. 8. Small molecule fusion inhibitors.

structural modulators to promote the trimeration of N-peptide.

**6. Conclusion** 


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