New insight on β‐lactoglobulin binding sites by 1‐anilinonaphthalene‐8‐sulfonate fluorescence decay

Abstract

The fluorescence time decay parameters of the b-lactoglobulin-1-anilinonaphthalene-8-sulfonate complex have been investigated under physical and chemical perturbations~2 Ͻ pH Ͻ 8 and added electrolyte 0 Ͻ NaCl Ͻ 0.5 M! to obtain new insight on the nature of the protein binding interactions. A double exponential decay of the bound probe lifetime has been confirmed by the presence of a longer component, 11 to 14.5 ns, and a shorter component, 2.5 to 3.5 ns. The two lifetimes are ascribed to different binding modes associated also with different exposure to the solvent; in particular, the longer component is attributed to binding inside the hydrophobic beta barrel, while a "surface" site is suggested for the shorter component. A detailed analysis of the lifetime fractional intensities correlates the binding constants with ionic strength and supports the presence of electrostatic effects at both sites. A Debye-Hückel approach, applied to extrapolate the electrostatic free energy contribution vs. pH at vanishing ionic strength, gives interesting clues on the effective charge felt by the ANS ligands in the proximity of each site. In particular, binding is found to parallel the aspartate and glutamate titrations between pH 3 and pH 4.5; the "surface" site mainly responds to the presence of these local titrating charges while the "internal" site more closely follows the overall protein net charge.Keywords: ANS; b-lactoglobulin; binding sites; electrostatic interactions b-Lactoglobulin, a 162 residues globular protein whose proper functionality has not been clarified yet, in spite of the great number of studies performed on this protein, is present in very large quantities in the milk of several mammals, thus suggesting its nutritional role. Its interaction with a great variety of hydrophobic ligands, such as retinol~Futterman & Heller, 1972;Dufour et al., 1990;Dufour & Haertlé, 1991;Cho et al., 1994;Narayan & Berliner, 1997; Lange et al., 1998!, fatty acids and triglycerides~Du-four et al., 1990;Frapin et al., 1993;Narayan & Berliner, 1997;Qin et al., 1998b;Wu et al., 1999!, has led to its inclusion in the lipocalin superfamily~Brownlow et al., 1997!. This family includes transport proteins, such as the retinol binding protein, the odorant binding protein, and the major urinary protein, which all share the common structural feature of a b-barrel calyx, built from eight antiparallel b-sheets, arranged as an ideal site for hydrophobic ligands~Brownlow et al., 1997!.Since b-lactoglobulin appears to lack specificity for particular ligands and since several studies suggest that more than one binding site exists, it seems interesting to further investigate the nature and the general features of the protein binding sites. A thorough study is carried out here with a "test" ligand, ANS, a molecule that shares both hydrophilic and hydrophobic characters and whose fluorescence response appears as a convenient and sensitive tool apt to investigate the nature of b-lactoglobulin binding regions. While ANS is only weakly fluor...

ANS fluorescence: Potential to augment the identification of the external binding sites of proteins

“…ANS have been distinguished [1][2][3]. Studies of the binding energetics and crystal structure of ANS-protein complexes show that for some proteins, ANS binding depends primarily on ion pairing with positively charged side chains [4][5][6][7].…”

Evidence for internal and external binding sites on human tear lipocalin

“…In this case, a small enhancement of ANS fluorescence intensity upon binding does not necessarily reflect a low binding affinity to proteins [3][4][5].…”

“…In fact, studies of lipocalin binding sites and interpretation of function are usually predicated on the assumption that the fold or cavity is the only ligand binding site in the molecule. However, external binding sites with unique characteristics were discovered for two lipocalins, α1-acid glycoprotein and β-lactoglobulin [2][3][4][5]. The careful delineation of all binding sites in lipocalins is relevant because a new classification scheme is based on comparative binding affinity analysis using retinol, retinoic acid, as well as the generic molecular probes, 11-(5-(dimethylamino)-1-naphthalene-sulfonylamino)undecanoic acid (DAUDA) and 8-anilino-1-naphtalenesulfonic acid (ANS) [6].…”

“…These potential differences have not been thoroughly investigated. Study of external binding sites requires improved resolution of ligand binding that is possible with the detection of multiple fluorescent lifetimes for ANS [3][4][5].…”

Molecular mobility and oxygen permeability in amorphous β-lactoglobulin films

“…β-Lg has a protein fold composed of two antiparallel β-sheets and shows a remarkable similarity to plasma retinol-binding protein ; it is a member of the lipocalin super family of transporters of hydrophobic molecules. Its physiological role is still unclear and, it has been demonstrated that β-Lg binds a variety of hydrophobic molecules such as retinol, fatty acids, triglycerides, protoporphyrin IX and ANS Fugate and Song, 1980;Collini et al, 2000;D'Alfonso et al, 1999;Andrade and Costa, 2002) β-Lg film properties such as transparency, water vapor permeability, oxygen permeability, and mechanical properties are identical to the properties of whey protein isolate films (Mate and Krochta, 1996;Anker et al, 1998). Heat denaturation opens the β-Lg globular structure, exposes sulfhydryl and hydrophobic groups, induces oxidation of free sulfhydryls, and promotes disulfide bond interchange and hydrophobic bonding thereby forming water-insoluble edible films Mate and Krochta, 1996).…”