HOW THE ANTI-(METAL CHELATE) ANTIBODY CHA255 IS SPECIFIC FOR THE METAL-ION OF ITS ANTIGEN - X-RAY STRUCTURES FOR 2 FAB' HAPTEN COMPLEXES WITH DIFFERENT METALS IN THE CHELATE
Ra. Love et al., HOW THE ANTI-(METAL CHELATE) ANTIBODY CHA255 IS SPECIFIC FOR THE METAL-ION OF ITS ANTIGEN - X-RAY STRUCTURES FOR 2 FAB' HAPTEN COMPLEXES WITH DIFFERENT METALS IN THE CHELATE, Biochemistry, 32(41), 1993, pp. 10950-10959
Antibodies with bound metal-chelate haptens provide new means for expl
oiting the diverse properties of metallic elements. The murine monoclo
nal antibody CHA255 (IgG1lambda) binds the metal-chelate hapten I)-4-[
N'-(2-hydroxyethyl)thioureido]-L-benzyl-EDTA (designated In-EOTUBE) wi
th high affinity (K(a) = 1.1 x 10(10) M-1). Antibody binding, is highl
y specific for the indium chelate; the affinity decreases as much as 1
0(4) with other metals, even those, having ionic radii close to indium
. To better understand this selectivity, the crystal structure of the
antigen-binding fragment (Fab') of CHA255 complexed with its hapten, I
n(III)-EOTUBE, was determined by molecular replacement and refined at
2.2-angstrom resolution. The structure of CHA255 Fab' complexed with F
e(III)-EOTUBE was also determined and refined at 2.8-angstrom resoluti
on. In both structures, the hapten's EDTA moiety is half-buried near t
he center of the complementarity-determining regions (CDR's). Five of
the six CDR's on the Fab' interact with the hapten through protein sid
e-chain atoms (but not main-chain atoms). A novel feature of the In-EO
TUBE/Fab' complex is coordination of the indium by Nepsilon of one his
tidine from the heavy chain's third CDR (distance = 2.4 angstrom). The
histidine coordination is not observed in the Fe-EOTUBE/Fab' complex,
due mainly to a slightly different hapten conformation that reduces m
etal accessibility; this may partially explain the 20-fold lower affin
ity of CHA255 for iron hapten. An unexpected feature of the Fab' overa
ll is an elbow angle of 193-degrees (the angle between the pseudodyad
axes of the Fab's constant and variable domains).