The crystal structure of the Fab‡‡ Fab, antigen-binding fragment, consisting of light chain and half of the heavy chain (VL, VH, CL, CH1);IgG, immuno-γ-globulin; Fc, C-terminal half of the heavy chain with the inter-heavy chain disulphide bond intact; VL, variable half of light chain; CL, constant half of light chain; VH, variable part of heavy chain; CH1, CH2 and CH3, the 3 constant homology regions of heavy chain. Hinge peptide, the segment connecting CH1 and CH2 and containing the disulphide linkage between the 2 heavy chains; switch peptide, the segments connecting V and C domains around residues H119 and L110, respectively; H, heavy and L, light chain; C face, the face in a C domain that is involved in the C-C type aggregation; V face, the face in a V domain that is involved in V-V type aggregation. The suffix κ or λ indicates a κ- or λ-type light chain. SDS, sodium dodecyl sulphate; r.m.s., root-mean-square fragment of the IgG protein Kol was analysed and an electron density map calculated at 3 Å resolution based on phases obtained from multiple isomorphous replacement. An atomic model was constructed but the lack of amino acid sequence data causes some uncertainty, in particular concerning the amino acid side-chains. Several cycles of constrained crystallographic refinement (Deisenhofer & Steigemann, 1975) produced an R value of 0.36. The resulting model was compared with the intact Kol IgG crystal structure (Colman et al., 1976), which had also been subjected to constrained crystallographic refinement, and two Fab fragments (Davies et al., 1975). The elbow angle was found open and only 8 ° more bent than in intact Kol, in contrast to the other Fab fragments, which show a strongly bent elbow angle. The intramolecular longitudinal V-C contacts are very similar in Kol Fab and Kol IgG and considerably fewer in number than in the other Fab fragments. The lateral V-V and C-C association is virtually identical in Kol Fab and Kol IgG. Although Kol Fab and Kol IgG crystallize in different lattices, they exhibit a nearly identical head to tail packing of the Fab parts, with the C modules touching the hypervariable segments of the V modules. This strongly conserved particular mode of aggregation might be reflected in the property of cold precipitation of the Kol cryoglobulin. © 1978.
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