Fig. 2.

Carbohydrate structures in the four crystal forms. (A) Mass spectrometry trace showing carbohydrate present in IgE-Fc. The molecular mass of the polypeptide dimer is 72,604 Da, and with two N-acetylglucosamine (GlcNAc) residues per chain is 73,416 Da. The first peak in the spectrum corresponds to one additional mannose (Man) residue in each chain. (B) Mass spectrometry trace showing carbohydrate present in Fcε3–4. The molecular mass of the polypeptide dimer is 49,756 Da, and with two GlcNac residues per chain is 50,568 Da. (C) Blue squares represent GlcNAc and green circles, Man units. i) Shows the arrangement of the high-mannose N-linked carbohydrate modelled in IgE-Fc chain B, Fcε3–4 P2₁ ‘large’ chains B and D, and Fcε3–4 P1 chain D. ii) Shows the carbohydrate modelled in Fcε3–4 P2₁ ‘small’ chains A and B, and in Fcε3–4 P1 chains C and E. iii) Shows the carbohydrate modelled in IgE-Fc chain A and Fcε3–4 P1 chain B. iv) Shows the carbohydrate modelled in Fcε3–4 P2₁ ‘large’ chain A. v) Shows the carbohydrate modelled in Fcε3–4 P2₁ ‘large’ chain C and Fcε3–4 P1 chain A. vi) Shows the carbohydrate modelled in Fcε3–4 P1 chain F.