Structure of the recombinant antibody Fab fragment f3p4

The structure of the antibody Fab fragment f3p4, which was selected from a subset of the synthetic HuCAL antibody library to bind the sodium citrate symporter CitS, is described at 1.92 Å resolution. Comparison with computational models revealed deviations in a few framework positions and in the binding loops. The crystals belong to space group P2₁2₁2 and contain four molecules in the asymmetric unit, with unit-cell parameters a = 102.77, b = 185.92, c = 102.97 Å. These particular unit-cell parameters allowed pseudo-merohedral twinning; interestingly, the twinning law relates a twofold screw axis to a twofold axis.