## HIV-1 vs HIV-2: Structural Differentiation ### Key Structural Differences **Key Point:** The V3 loop of gp120 is the primary variable region that distinguishes HIV-1 from HIV-2 at the structural level. This loop differs significantly in length, amino acid sequence, and conformational properties between the two species. ### Comparative Analysis | Feature | HIV-1 | HIV-2 | |---------|-------|-------| | **V3 loop length** | ~35 amino acids | ~40 amino acids | | **V3 loop sequence** | Highly divergent from HIV-2 | Highly divergent from HIV-1 | | **Tropism determinant** | V3 loop determines co-receptor usage | V3 loop determines co-receptor usage | | **Phylogenetic origin** | SIVcpz (chimpanzee) | SIVsm (sooty mangabey) | **High-Yield:** The V3 loop is used in serological differentiation assays and is the basis for HIV-1/HIV-2 discriminatory tests. Mutations in this region directly affect CCR5 vs CXCR4 tropism. ### Why V3 Loop Matters 1. **Co-receptor binding specificity** — The V3 loop determines whether the virus binds CCR5 (R5-tropic) or CXCR4 (X4-tropic) 2. **Immunological recognition** — V3 is a major target of neutralizing antibodies 3. **Diagnostic utility** — V3 sequences are used in genotypic tropism testing **Clinical Pearl:** V3 loop sequencing is essential for predicting maraviroc (CCR5 antagonist) efficacy before treatment initiation. ### Shared Structural Elements Both HIV-1 and HIV-2 possess: - Reverse transcriptase (essential for replication) - Vpr, Vif, Nef accessory proteins - gp120/gp41 envelope glycoprotein complex - Integrase and protease enzymes These shared features make them both retroviruses but do NOT distinguish them.
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