New insights into intracellular lipid binding proteins:: The role of buried water

The crystal structures of most intracellular lipid binding proteins (LBPs) show between 5 and 20 internally bound water molecules, depending on the presence or the absence of ligand inside the protein cavity. The structural and functional significance of these waters has been discussed for several LBPs based on studies that used various biophysical techniques. The present work focuses on two very different LBPs, heart-type fatty acid binding protein (H-FABP) and ileal lipid binding protein (ILBP). Using high-resolution nuclear magnetic resonance spectroscopy, certain resonances belonging to side-chain protons that are located inside the water-filled lipid binding cavity were observed. In the case of H-FABP. the pH- and temperature-dependent behavior of selected side-chain resonances (Ser82 OgH and the imidazole ring protons of His93) indicated an unusually slow exchange with the solvent, implying that the intricate hydrogen-bonding network of amino-acid side-chains and water molecules in the protein interior is very rigid. In addition, holo H-FABP appeared to display a reversible self-aggregation at physiological pH. For ILBP, on the other hand, a more solvent-accessible protein cavity was deduced based on the pH titration behavior of its histidine residues. Comparison with data from other LBPs implies that the evolutionary specialization of LBPs for certain ligand types was not only because of mutations of residues directly involved in ligand binding but also to a refinement of the internal water scaffold.