Location of substitutions introduced into the b12 antibody molecule. (A) Single substitutions (G236A, S239A, S267G, D270E, R292A, S298A, I332E, and K338A). (B) Double substitutions (S239D/I332E). (C) Triple substitutions (S239D/I332E/G236A). (D) Triple substitutions (S239D/I332E/A330L). The structure of b12 is described in the work of Saphire et al. (42) (PDB accession code 1HZH).