B-type natriuretic
peptide (BNP) is a naturally
secreted regulatory
hormone that influences
blood pressure and vascular
water retention in human physiology. The plasma BNP concentration is a clinically recognized biomarker for various
cardiovascular diseases. Quantitative detection of BNP can be achieved in
immunoassays using the high-affinity
monoclonal IgG1 antibody 106.3, which binds an
epitope spanning residues 5-13 of the mature
bioactive peptide. To understand the structural basis of this molecular recognition, we
crystallized the
Fab fragment complexed with the
peptide epitope and determined the three-dimensional structure by
X-ray diffraction to 2.1 A resolution. The structure reveals the detailed interactions that five of the
complementarity-determining regions make with the partially folded
peptide. Thermodynamic measurements using
fluorescence spectroscopy suggest that the interaction is enthalpy driven, with an overall change in free energy of binding, DeltaG = -54 kJ/mol, at room temperature. The parameters are interpreted on the basis of the structural information. The kinetics of binding suggest a diffusion-limited mechanism, whereby the
peptide easily adopts a bound conformation upon interaction with the
antibody. Moreover, comparative analysis with alanine-scanning results of the
epitope explains the basis of selectivity for BNP over other related natriuretic
peptides. Proteins 2009. (c) 2009 Wiley-Liss, Inc.
