Interaction of Chicken Liver Basic Fatty Acid-Binding Protein with Fatty Acids: A 13C NMR and Fluorescence Study†
journal contributionposted on 29.09.2001, 00:00 by Tiziana Beringhelli, Luca Goldoni, Stefano Capaldi, Alessandra Bossi, Massimiliano Perduca, Hugo L. Monaco
Two different groups of liver fatty acid-binding proteins (L-FABPs) are known: the mammalian type and the basic type. Very few members of this second group of L-FABPs have been characterized and studied, whereas most of the past studies were concerned with the mammalian type. The interactions of chicken liver basic fatty acid-binding protein (Lb-FABP) with 1-13C-enriched palmitic acid (PA) and oleic acid (OA) were investigated by 13C NMR spectroscopy. Samples containing fatty acids (FA) and Lb-FABP at different molar ratios exhibited only a single carboxylate resonance corresponding to bound FA, and showed a binding stoichiometry of 1:1 both for PA and for OA. Fluorescence spectroscopy measurements yielded the same binding stoichiometry for the interaction with cis-parinaric acid [Kd = 0.38(4) μM]. Competition studies between cis-parinaric acid and the natural ligands indicated a decreasing affinity of chicken Lb-FABP for PA, OA, and retinoic acid (RA). 13C NMR proved that pH and ionic strength affect complex stability. The carboxyl signal intensity reversibly decreased upon lowering the pH up to 5. The pH dependence of the bound carboxyl chemical shift yielded an apparent pKa of 4.8. A decrease of the integrated intensity of the bound carboxylic signal in the NMR spectra was observed while increasing the chloride ion concentration up to 200 mM. This body of evidence indicates that the bound FA is completely ionized at pH 7.4, that its polar head is positioned in a solvent-accessible region, that a FA−protein strong ionic bond is not present, and that high ionic strength causes the release of the bound FA. The reported results show that, insofar as the number of bound ligands and its relative affinity for different FAs are concerned, chicken Lb-FABP is remarkably different from the mammalian liver FABPs, and, within its subfamily, that it is more similar to catfish Lb-FABP while it behaves quite differently from shark or axolotl Lb-FABPs.