Crystal structure of a camel single-domain V-H antibody fragment in complex with lysozyme

The Camelidae is the only taxonomic family known to possess functional heavy-chain antibodies, lacking light chains. We report here the 2.5 Angstrom resolution crystal structure of a camel V-H in complex with its antigen, lysozyme. Compared to human and mouse V-H domains, there are no major backbone rearrangements in the V-H framework. However, the architecture of the region of V-H that interacts with a V-L in a conventional Fv is different from any previously seen. Moreover, the CDR1 region, although in sequence homologous to human CDR1, deviates fundamentally from the canonical structure. Additionally, one half of the CDR3 contacts the V-H region which in conventional immunoglobulins interacts with a V-L, whereas the other half protrudes from the antigen binding site and penetrates deeply into the active site of lysozyme.