Structural Models for Carcinoembryonic Antigen and Its Complex with the Single-chain Fv Antibody Molecule MFE23

MFE23 is a single chain Fv antibody that has a high affinity for carcinoembryonic antigen (CEA). A full homology model for CEA based on V-type, I-type and C2-type immunoglobulin folds, 28 oligosaccharides and the interdomain angle of CD2 was validated using solution scattering data. The superimposition of the intermolecular contacts observed in our recent crystal structure of MFE23 with the N-terminal domain of CEA permitted the MFE23-CEA complex to be modelled. Good surface and electrostatic complementarity and carbohydrate-unhindered access of MFE23 with the indentation between the first two CEA domains was observed. The model is supported by biochemical data and provides insight on the high affinity of MFE23 for CEA.