6C5K

S25-23 Fab in complex with Chlamydiaceae LPS (Crystal form 2)

Summary for 6C5K

• Entry DOI: 10.2210/pdb6c5k/pdb
• Related: 6C5H 6C5I 6C5J
• Descriptor: IgG1 Fab Heavy Chain, IgG1 Fab Light Chain (Kappa), 2-acetamido-2-deoxy-beta-D-glucopyranose-(1-4)-[alpha-L-fucopyranose-(1-6)]2-acetamido-2-deoxy-beta-D-glucopyranose, ... (6 entities in total)
• Functional Keywords: carbohydrate, lipopolysaccharide, antibody, antigen, complex, chlamydia, sugar binding protein
• Biological source: Mus musculus (Mouse); More
• Total number of polymer chains: 4
• Total formula weight: 98801.40

Authors

Haji-Ghassemi, O.,Evans, S.V. (deposition date: 2018-01-16, release date: 2019-01-09, Last modification date: 2024-10-16)

Primary citation

Haji-Ghassemi, O.,Muller-Loennies, S.,Brooks, C.L.,MacKenzie, C.R.,Caveney, N.,Van Petegem, F.,Brade, L.,Kosma, P.,Brade, H.,Evans, S.V.
Subtle Changes in the Combining Site of the Chlamydiaceae-Specific mAb S25-23 Increase the Antibody-Carbohydrate Binding Affinity by an Order of Magnitude.
Biochemistry, 58:714-726, 2019

PubMed Abstract: Murine antibodies S25-23, S25-26, and S25-5 derive from a common germ-line origin, and all bind the Chlamydiaceae family-specific epitope αKdo(2→8)αKdo(2→4)αKdo (where Kdo is 3-deoxy-α-d- manno-oct-2-ulosonic acid) with high affinity and specificity. These antibodies recognize the entire trisaccharide antigen in a linkage-dependent manner via a groove composed largely of germ-line residues. Despite sharing identical heavy and light chain genes, S25-23 binds the family-specific epitope with nanomolar affinity, which is an order of magnitude higher than that of S25-26, while S25-5 displays an affinity between those of S25-23 and S25-26. We determined the high-resolution crystal structures of S25-23 and S25-5 antigen binding fragments in complex with a pentasaccharide derived from the LPS of Chlamydia and measured the affinity of S25-5 for chlamydial LPS antigens using isothermal titration microcalorimetry. The 1.75 Å resolution structure of S25-23 shows how subtle conservative mutations Arg(L)-27E to lysine and Ser(H)-56 to threonine lead to an order of magnitude increase in affinity. Importantly, comparison between previous S25-26 structures and the 1.99 and 2.05 Å resolution liganded and unliganded structures of S25-5, respectively, shows how a Ser(L)-27E mutation results in an intermediate affinity due to the reduced enthalpic penalty associated with complex formation that would otherwise be required for arginine in this position. This strategy allows for subtle adjustments in the combining site via affinity maturation that have dramatic consequences for the affinity of an antibody for its antigen.

PubMed: 30571096
DOI: 10.1021/acs.biochem.8b00318

Experimental method

X-RAY DIFFRACTION (1.75 Å)