Figure 1. Basic features of the Ig fold. (a) Schematic illustration of the Greek keymotif. Conserved strands are indicated by letters A to G, beginning at the N-terminus (N) and proceeding through to the C-terminus (C term). The definitive disulfide bond linking strand B with strand F is shown. (b) Three-dimensional illustration depicting the -barrel resulting from the ‘four on three’ sandwich. Here, the disulfide bond is not shown. (c) A ‘top view’ illustration in which each of the strands, shown by a triangle, is lettered, its orientation is indicated by the upward or downward direction in which the triangle is pointing, and the strand associations are indicated by the juxtaposition of the sides of the triangles. The disulfide bond connecting strands B and F is indicated by a line.

Figure 2. Schematic and ribbon diagram of C-type Ig domains. The C1 domain is the CH domain from 1dfb. One interesting variation of this particular molecule is that the region joining the E and F strands is a short helix. The C2 domain is from the CD2 structure, 1hnf. The strand switch, in which C¢ remains as part of the G, F, C, C¢ layer of the sandwich rather than associating with the A, B, E layer is apparent.

Figure 3. Schematic and ribbon illustrations of V domains. As an example, the heavy chain V region from 7Fab is illustrated. Because the C¢¢ strand is very short in this molecule, it is not indicated in the ribbon diagram.

Figure 4. Diagram of the structure of an intact antibody. This is an illustration of the IgG2a monoclonal antibody 231 (Harris et al., 1997). The heavy chains are shown in cyan and blue; the light chains in red and violet. The variable heavy (V_H) and variable light (V_L) domains are labelled as are the constant C domain, which are labelled from the N to C termini as C_H1, C_H2, and C_H3. The carbohydrate moiety is shown in a stick representation.