We report the first account of a comparative analysis of the binding affinities of nine FDA-approved drugs against subtype B as well as the South African subtype C HIV PR (C-SA). A standardized protocol was used to generate the inhibitor/C-SA PR complexes with the relative positions of the inhibitors taken from the corresponding X-ray structures for subtype B complexes. The dynamics and stability of these complexes were investigated using molecular dynamics calculations. Average relative binding free energies for these inhibitors were calculated from the molecular dynamics simulation using the molecular mechanics generalized Born surface area method. The calculated energies followed a similar trend to the reported experimental binding free energies. Postdynamic hydrogen bonding and electrostatic interaction analysis of the inhibitors with both subtypes reveal similar interactions. Most inhibitors show slightly weaker binding affinities for C-SA PR. Molecular dynamics studies demonstrated increased flap movement for C-SA PR, which can perhaps explain the weaker affinities. This study serves as a standardized platform for optimizing the design of future more potent HIV C-SA PR inhibitors.
Phosphopeptide enrichment is an essential step in large-scale, quantitative phosphoproteomics by mass spectrometry. Several phosphopeptide affinity enrichment techniques exist, such as Immobilized Metal ion Affinity Chromatography (IMAC) and Metal Oxide Affinity Chromatography (MOAC). We compared Zirconium(IV) IMAC (Zr-IMAC) magnetic microparticles to more commonly used Titanium(IV) IMAC (Ti-IMAC) and TiO2 magnetic microparticles for phosphopeptide enrichment from simple and complex protein samples prior to phosphopeptide sequencing and characterization by mass spectrometry (LC-MS/MS). We optimized sample-loading conditions to increase phosphopeptide recovery for Zr-IMAC, Ti-IMAC and TiO2 based workflows by 22%, 24% and 35% respectively. The optimized protocol resulted in improved performance of Zr-IMAC over Ti-IMAC and TiO2 as well as HPLC-based Fe(III)-IMAC with up to 23% more identified phosphopeptides. The different enrichment chemistries showed a high degree of overlap but also differences in phosphopeptide selectivity and complementarity. We conclude that Zr-IMAC improves phosphoproteome coverage and recommend that this complementary and scalable affinity enrichment method is more widely used in biological and biomedical studies of cell signaling and the search for biomarkers. Data are available via ProteomeXchange with identifier PXD018273.
The HIV protease plays a major role in the life cycle of the virus and has long been a target in antiviral therapy. Resistance of HIV protease to protease inhibitors (PIs) is problematic for the effective treatment of HIV infection. The South African HIV-1 subtype C protease (C-SA PR), which contains eight polymorphisms relative to the consensus HIV-1 subtype B protease, was expressed in Escherichia coli, purified, and crystallized. The crystal structure of the C-SA PR was resolved at 2.7 Å, which is the first crystal structure of a HIV-1 subtype C protease that predominates in Africa. Structural analyses of the C-SA PR in comparison to HIV-1 subtype B proteases indicated that polymorphisms at position 36 of the homodimeric HIV-1 protease may impact on the stability of the hinge region of the protease, and hence the dynamics of the flap region. Molecular dynamics simulations showed that the flap region of the C-SA PR displays a wider range of movements over time as compared to the subtype B proteases. Reduced stability in the hinge region resulting from the absent E35-R57 salt bridge in the C-SA PR, most likely contributes to the increased flexibility of the flaps which may be associated with reduced susceptibility to PIs.
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