(a) Representation of the contents of the asymmetric unit of the Fc/TM crystal. The only visible mutation comprising `TM', P331S, is indicated in red. One zinc ion is shown chelated by two spatially close histidine residues. The conventional `horseshoe'-shaped homodimeric Fc fragment would be achieved by invoking a crystallographic twofold symmetry operator. The carbohydrate residues attached to Asn297 were modeled according to their electron density. (b) Stereographic representation of three unliganded human Fc regions separately superimposed through their respective CH2 and CH3 domains. Polypeptides are color-coded as follows: blue, 2ql1
(Oganesyan et al., 2008); red, 1h3w
(Krapp et al., 2003); green, Fc/TM.