Structure of a single-chain Fv fragment of an antibody that inhibits the HIV-1 and HIV-2 proteases

The monoclonal antibody 1696, which was raised against the HIV-1 protease, inhibits the catalytic activity of the enzyme from both the HIV-1 and HIV-2 strains. The antibody cross-reacts with peptides containing the N-terminus of the enzyme, which is highly conserved between these strains. The crystal structure of a single-chain Fv fragment of 1696 (scFv-1696) in the non-complexed form, solved at 1.7 Å resolution, is compared with the previously reported non-complexed Fab-1696 and antigen-bound scFv-1696 structures. Large conformational changes in the third hypervariable region of the heavy chain and differences in relative orientation of the variable domains are observed between the different structures.