Abstract
IgG New binds ligands such as orceine, menadione, and uridine with a low affinity (K0 about 1 × 103 liter/mol) and a γ-hydroxy derivative of vitamin K1 with a higher affinity (K0 = 1.7 × 105 liter/mol). Binding studies indicate that both the 2-methylnaphthoquinone rings and the phytyl tail of the vitamin K1 hapten contribute to the total binding energy.
The binding of these ligands in the crystalline state has been investigated by difference Fourier maps of Fab′ New-ligand complexes at 6-Å resolution. A 3.5-Å resolution difference Fourier map obtained for the γ-hydroxy derivative of the vitamin K1-Fab′ complex shows that this hapten is bound in a shallow groove or crevice between the light and the heavy chains, in close proximity to the polypeptide segments containing the hypervariable regions. At least 12 amino-acid residues from both the light and the heavy chains appear to be in close contact with the ligand. No major conformational changes were detected in the Fab′ fragment after ligand binding.
Keywords: antibody-combining region, hypervariable positions, myeloma protein, vitamin K1
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