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Crystal Structure of a Bivalent Antibody Fab Fragment



Salman Shahid, Mingming Gao, David Travis Gallagher, Steven Foung, Zhen-Yong Keck, Thomas Fuerst, Roy Mariuzza


We determined the crystal structure to 1.8 Å resolution of the Fab fragment of an affinity- matured human monoclonal antibody (HC84.26.5D) that recognizes the E2 envelope glycoprotein of hepatitis C virus (HCV). Unlike conventional Fabs, which are monovalent monomers, Fab HC84.26.5D assembles into a bivalent domain-swapped dimer in which the two VL/VH modules are separated by 25 Å. In solution, Fab HC84.26.5D exists predominantly as a dimer (80%) in equilibrium with the monomeric form of the Fab (20%). Dimerization is mediated entirely by deletion of a single residue, VHSer119, in the elbow region linking the VH and CH1 domains. In agreement with the crystal structure, dimeric Fab HC84.26.5D is able to bind two HCV E2 molecules in solution. This is only the second example of a domain-swapped Fab dimer from among >3,000 Fab crystal structures determined to date. Moreover, the architecture of the doughnut- shaped Fab HC84.26.5D dimer is completely different from that of the previously reported Fab 2G12 dimer. Bivalent domain-swapped Fab dimers engineered on the basis of HC84.26.5D may be useful in single-particle cryoEM as a means of doubling the size of conventional Fab–protein complexes.
Journal of Molecular Biology


antibody, Fab, domain swapping, structure


Shahid, S. , Gao, M. , Gallagher, D. , Foung, S. , Keck, Z. , Fuerst, T. and Mariuzza, R. (2020), Crystal Structure of a Bivalent Antibody Fab Fragment, Journal of Molecular Biology (Accessed April 17, 2024)
Created November 11, 2020, Updated March 1, 2023