Beta-lactoglobulin binds retinol and protoporphyrin IX at two different binding sites

Integrated spectroscopic and computational analyses unravel the molecular interaction of pesticide azinphos-methyl with bovine beta-lactoglobulin

Organophosphorus are typically hazardous chemicals used in the pharmaceutical, agricultural, and other industries. They pose a serious risk to human life and can be fatal upon direct exposure. Hence, studying the interaction between such compounds with proteins is crucial for environmental, health, and food safety. In this study, we investigated the interaction mechanism between azinphos-methyl (AZM) and β-lactoglobulin (BLG) at pH 7.4 using a combination of biophysical techniques. Intrinsic fluorescence investigations revealed that BLG fluorescence was quenched in the presence of increasing AZM concentrations. The quenching mechanism was identified as static, as evidenced by a decrease in the fluorescence quenching constant (1.25 × 104, 1.18 × 104, and 0.86 × 104 M-1) with an increase in temperatures. Thermodynamic calculations (ΔH > 0; ΔS > 0) affirmed the formation of a complex between AZM and BLG through hydrophobic interactions. The BLG's secondary structure was found to be increased due to AZM interaction. Ultraviolet -visible spectroscopy data showed alterations in BLG conformation in the presence of AZM. Molecular docking highlighted the significant role of hydrophobic interactions involving residues such as Val43, Ile56, Ile71, Val92, Phe105, and Met107 in the binding between BLG and AZM. A docking energy of -6.9 kcal mol-1, and binding affinity of 1.15 × 105 M-1 suggest spontaneous interaction between AZM and BLG with moderate to high affinity. These findings underscore the potential health risks associated with the entry of AZM into the food chain, emphasizing the need for further consideration of its impact on human health.

An Atomic and Molecular Insight into How PFOA Reduces α-Helicity, Compromises Substrate Binding, and Creates Binding Pockets in a Model Globular Protein

Per- and polyfluoroalkyl substances (PFAS) pose significant health risks due to their widespread presence in various environmental and biological matrices. However, the molecular-level mechanisms underlying the interactions between PFAS and biological constituents, including proteins, carbohydrates, lipids, and DNA, remain poorly understood. Here, we investigate the interactions between a legacy PFAS, viz. perfluorooctanoic acid (PFOA), and the milk protein β-lactoglobulin (BLG) obtained using a combination of experimental and computational techniques. Circular dichroism studies reveal that PFOA perturbs the secondary structure of BLG, by driving a dose-dependent loss of α-helicity and alterations in its β-sheet content. Furthermore, exposure of the protein to PFOA attenuates the on-rate constant for the binding of the hydrophobic probe 8-anilino-1-naphthalene sulfonic acid (ANS), suggesting potential functional impairment of BLG by PFOA. Steered molecular dynamics and umbrella sampling calculations reveal that PFOA binding leads to the formation of an energetically favorable novel binding pocket within the protein, when residues 129-142 are steered to unfold from their initial α-helical structure, wherein a host of intermolecular interactions between PFOA and BLG's residues serve to insert the PFOA into the region between the unfolded helix and beta-sheets. Together, the data provide a novel understanding of the atomic and molecular mechanism(s) by which PFAS modulates structure and function in a globular protein, leading to a beginning of our understanding of altered biological outcomes.

Energetics of β-lactoglobulin-flavor compounds interactions

Interaction of bovine β-lactoglobulin (BLG) with several flavor compounds (FC) (2-methylpyrazine, vanillin, 2-acetylpyridine, 2- and 3-acetylthiophene, methyl isoamyl ketone, heptanone, octanone, and nonanone) was studied by high-sensitivity differential scanning calorimetry. The denaturation temperature, enthalpy, and heat capacity increment were determined at different FC concentrations. It was found that the denaturation temperature and heat capacity increment do not depend on the FC concentration, while the denaturation enthalpy decreases linearly with the FC concentration. These thermodynamic effects disclose the preferential FC binding to the unfolded form of BLG. By the obtained calorimetric data, the free energies of FC binding vs. the FC concentrations were calculated. These dependences were shown to be linear. Their slope relates closely to the overall FC affinity for the unfolded BLG in terms of the Langmuir binding model. The overall BLG affinity for FC varies from 20 M-1 (2-methylpyrazine) up to 360 M-1(nonanone). The maximal stoichiometry of the BLG-FC complexes was roughly estimated as a ratio of the length of the unfolded BLG to the molecular length of FC. Using these estimates, the apparent BLG-FC binding constants were determined. They are in the range of 0.3-8.0 M-1 and correlated strictly with the FC lipophilicity descriptor (logP).

A Double Payload Complex between Hypericin and All-Trans Retinoic Acid in the β-Lactoglobulin Protein

Combined therapies are usually used to treat acne vulgaris since this approach can tackle various foci simultaneously. Using a combination of spectroscopic, computational, and microbiological techniques and methods, herein we report on the use of β-lactoglobulin as a double payload carrier of hypericin (an antimicrobial photodynamic agent) and all-trans retinoic acid (an anti-inflammatory drug) for S. aureus in vitro photodynamic inactivation. The addition of all-trans retinoic acid to hypericin-β-lactoglobulin complex renders a photochemically safe vehicle due to the photophysical quenching of hypericin, which recovers its photodynamic activity when in contact with bacteria. The ability of hypericin to photoinactivate S. aureus was not affected by retinoic acid. β-Lactoglobulin is a novel biocompatible and photochemically safe nanovehicle with strong potential for the treatment of acne.

Effect of Ligands on HP-Induced Unfolding and Oligomerization of β-Lactoglobulin

To probe intermediate states during unfolding and oligomerization of proteins remains a major challenge. High pressure (HP) is a powerful tool for studying these problems, revealing subtle structural changes in proteins not accessible by other means of denaturation. Bovine β-lactoglobulin (BLG), the main whey protein, has a strong propensity to bind various bioactive molecules such as retinol and resveratrol, two ligands with different affinity and binding sites. By combining in situ HP-small-angle neutron scattering (SANS) and HP-ultraviolet/visible absorption spectroscopy, we report the specific effects of these ligands on three-dimensional conformational and local changes in BLG induced by HP. Depending on BLG concentration, two different unfolding mechanisms are observed in situ under pressures up to ∼300 MPa: either a complete protein unfolding, from native dimers to Gaussian chains, or a partial unfolding with oligomerization in tetramers mediated by disulfide bridges. Retinol, which has a high affinity for the BLG hydrophobic cavity, significantly stabilizes BLG both in three-dimensional and local environments by shifting the onset of protein unfolding by ∼100 MPa. Increasing temperature from 30 to 37°C enhances the hydrophobic stabilization effects of retinol. In contrast, resveratrol, which has a low binding affinity for site(s) on the surface of the BLG, does not induce any significant effect on the structural changes of BLG due to pressure. HP treatment back and forth up to ∼300 MPa causes irreversible covalent oligomerization of BLG. Ab initio modeling of SANS shows that the oligomers formed from the BLG-retinol complex are smaller and more elongated compared to BLG without ligand or in the presence of resveratrol. By combining HP-SANS and HP-ultraviolet/visible absorption spectroscopy, our strategy highlights the crucial role of BLG hydrophobic cavity and opens up new possibilities for the structural determination of HP-induced protein folding intermediates and irreversible oligomerization.

Tri-substituted organotin compounds, but not retinoic acid, are potent ligands of complement component 8 γ

Complement component 8 γ (C8γ) is a subunit of complement protein 8 (C8), which itself is a subunit of the complement cytolytic membrane attack complex. However, C8γ is also suggested to be a carrier protein for the general clearance of endogenous and exogenous compounds because it belongs to the lipocalin family of small secreted proteins that have the common ability to bind small hydrophobic ligands. Although retinoic acid, a metabolite of vitamin A, has been suggested as a potential ligand of C8γ, it remains unclear which other substances are able to bind to C8γ as ligands. Here, we evaluated the binding affinity of several organotin compounds that are ligands of a receptor of retinoic acid, retinoid X receptor, by using radioligand binding assays. The amount of [¹⁴C]triphenyltin (TPT), a tri-substituted organotin, that bound to purified recombinant C8γ was increased with increasing protein concentration, whereas that of [³H]all-trans retinoic acid and [³H]9-cis retinoic acid was unchanged. Scatchard analysis revealed that [¹⁴C]TPT bound to C8γ with an equilibrium dissociation constant (K_d) of 56.2 ± 16.2 nM. Non-radiolabeled tributyltin (TBT), another tri-substituted organotin, blocked the binding of [¹⁴C]TPT to C8γ in a competitive manner, but non-radiolabeled mono- or di-substituted organotin compounds did not. Together, our present observations indicate that TBT and TPT, but not retinoic acid or mono- or di-substituted organotin compounds, are potent ligands of C8γ, suggesting that C8γ may be involved in the toxicities of these organotin compounds.

Development and Characterization of Whey Protein-Based Nano-Delivery Systems: A Review

Various bioactive compounds (BCs) often possess poor stability and bioavailability, which makes it difficult for them to exert their potential health benefits. These limitations can be countered by the use of nano-delivery systems (NDSs), such as nanoparticles and nanoemulsions. NDSs can protect BCs against harsh environments during food processing and digestion, and thereby, could enhance the bioavailability of BCs. Although various NDSs have been successfully produced with both synthetic and natural materials, it is necessary to fulfill safety criteria in the delivery materials for food applications. Food-grade materials for the production of NDSs, such as milk proteins and carbohydrates, have received much attention due to their low toxicity, biodegradability, and biocompatibility. Among these, whey proteins-from whey, a byproduct of cheese manufacturing-have been considered as excellent delivery material because of their high nutritional value and various functional properties, such as binding capability to various compounds, gelation, emulsifying properties, and barrier effects. Since the functional and physicochemical properties of whey protein-based NDSs, including size and surface charge, can be key factors affecting the applications of NDSs in food, the objectives of this review are to discuss how manufacturing variables can modulate the functional and physicochemical properties of NDSs and bioavailability of encapsulated BCs to produce efficient NDSs for various BCs.

Clinical or ATPase domain mutations in ABCD4 disrupt the interaction between the vitamin B12-trafficking proteins ABCD4 and LMBD1

Vitamin B₁₂ (cobalamin (Cbl)), in the cofactor forms methyl-Cbl and adenosyl-Cbl, is required for the function of the essential enzymes methionine synthase and methylmalonyl-CoA mutase, respectively. Cbl enters mammalian cells by receptor-mediated endocytosis of protein-bound Cbl followed by lysosomal export of free Cbl to the cytosol and further processing to these cofactor forms. The integral membrane proteins LMBD1 and ABCD4 are required for lysosomal release of Cbl, and mutations in the genes LMBRD1 and ABCD4 result in the cobalamin metabolism disorders cblF and cblJ. We report a new (fifth) patient with the cblJ disorder who presented at 7 days of age with poor feeding, hypotonia, methylmalonic aciduria, and elevated plasma homocysteine and harbored the mutations c.1667_1668delAG [p.Glu556Glyfs*27] and c.1295G>A [p.Arg432Gln] in the ABCD4 gene. Cbl cofactor forms are decreased in fibroblasts from this patient but could be rescued by overexpression of either ABCD4 or, unexpectedly, LMBD1. Using a sensitive live-cell FRET assay, we demonstrated selective interaction between ABCD4 and LMBD1 and decreased interaction when ABCD4 harbored the patient mutations p.Arg432Gln or p.Asn141Lys or when artificial mutations disrupted the ATPase domain. Finally, we showed that ABCD4 lysosomal targeting depends on co-expression of, and interaction with, LMBD1. These data broaden the patient and mutation spectrum of cblJ deficiency, establish a sensitive live-cell assay to detect the LMBD1-ABCD4 interaction, and confirm the importance of this interaction for proper intracellular targeting of ABCD4 and cobalamin cofactor synthesis.

Effect of supplementation of cows diet with linseed and fish oil and different variants of β-lactoglobulin on fatty acid composition and antioxidant capacity of milk

BACKGROUND

The aim of this study was to determine the influence of polymorphic variants of β-lactoglobulin in cows supplemented with linseed and fish oil on the fatty acid composition and antioxidant capacity of milk. From the herd of 320 Polish Holstein Friesian cows three groups of cows were selected according to the variants of β-LG (β-LGAA, β-LGBB, β-LGAB). During the first 7 days (the initial period) all the cows were fed the same total mixed ration (TMR) diet. From day 8 to 28,150 g fish oil and 250 g linseed (FOL) was added to the TMR diet of each cow.

RESULTS

The results showed that the diet supplemented with FOL was effective in reducing atherogenic and thrombogenic indices. Introducing supplementation improved the antioxidant capacity: higher concentration of C18:2cis-9 trans-11, C20:5 n-3, C22:6 n-3, bioactive whey proteins and vitamin soluble in fat has been recorded. The results showed that β-LGAA was associated with lower levels of atherogenic and thrombogenic indices and higher concentration of C22:5 n-6, phospholipids and β-carotene. β-LGBB favours a higher content of C18:1trans-11, C18:2cis-9 trans-11 and lactoferrin. β-LGAB was associated with higher concentrations of C20:5 n-3, Lysozyme, α-retinol, α-tocopherol and total antioxidant status.

CONCLUSION

Modification of the diet of cows with fish oil and linseed significantly influenced fatty acid composition and antioxidant properties of milk. The effect of β-LG phenotype on the fatty acid composition and antioxidant capacity of milk is variable, which could partly be the result of a β-LG phenotype × diet interaction.

An efficient synthesis of 1,3-dimethyl-5-(2-phenyl-4H-chromen-4-ylidene) pyrimidine-2,4,6(1H,3H,5H)-triones and investigation of their interactions with β-lactoglobiulin

An efficient method for synthesis of 1,3-dimethyl-5-(2-phenyl-4H-chromen-4-ylidene)pyrimidine-2,4,6(1H,3H,5H)-triones has been developed.

Multispectroscopic and molecular modeling studies on the interaction of two curcuminoids with β-lactoglobulin

This study demonstrates the binding properties of bisdemethoxycurcumin (BDMC) and diacetylbisdemethoxycurcumin (DABC) as bioactive curcuminoids with bovine β-lactoglobulin (BLG) variant B using fluorescence and circular dichroism (CD) spectroscopy; molecular docking, and molecular dynamics simulation methods. The estimated binding constants for BLG-BDMC and BLG-DABC complexes were (8.99±0.10)×10(4) M(-1) and (1.87±0.10)×10(2) M(-1), respectively. The distances between BLG and these curcuminoids were obtained based on the Förster's theory of non-radiative energy transfer. Molecular docking studies revealed the binding of BDMC and DABC to the protein surface cleft of protein by formation of four and one hydrogen bonds, respectively. Finally, molecular dynamics simulation results represent the conformational changes of BLG due to its interaction with BDMC. Also, the profiles of atomic fluctuations signified the rigidity of ligand binding site during the simulation.

NMR studies of the dynamics of high-spin nitrophorins: comparative studies of NP4 and NP2 at close to physiological pH

The β-barrel nitrophorin (NP) heme proteins are found in the saliva of the blood-sucking insect Rhodnius prolixus, which synthesizes and stores nitric oxide (NO) in the salivary glands. NO is bound to iron of the NPs and is released by dilution and an increase in pH when the insect spits its saliva into the tissues of a victim, to aid in obtaining a blood meal. In the adult insect, there are four nitrophorins, NP1–NP4, which have sequence similarities in two pairs, NP1 and NP4 (90% identical) and NP2 and NP3 (80% identical). The available crystal structures of NP4 have been used to propose that pH-dependent changes in the conformation of two loops between adjacent β-strands at the front opening of the protein, the A–B and G–H loops, determine the rate of NO release. At pH 7.3, NP4 releases NO 17 times faster than NP2 does. In this work, the aqua complexes of NP4 and NP2 have been investigated by nuclear magnetic resonance (NMR) relaxation measurements to probe the pico- to nanosecond and micro- to millisecond time scale motions at two pH values, 6.5 and 7.3. It is found that NP4-OH₂ is fairly rigid and only residues in the loop regions show dynamics at pH 6.5; at pH 7.3, much more dynamics of the loops and most of the β-strands are observed while the α-helices remain fairly rigid. In comparison, NP2-OH₂ shows much less dynamics, albeit somewhat more than that of the previously reported NP2-NO complex [Muthu, D., Berry, R. E., Zhang, H., and Walker, F. A. (2013) Biochemistry 52, 7910–7925]. The reasons for this major difference between NP4 and NP2 are discussed.

Bovine β-lactoglobulin/fatty acid complexes: binding, structural, and biological properties

Ligand-binding properties of β-lactoglobulin (β-lg) are well documented, but the subsequent biological functions are still unclear. Focusing on fatty acids/β-lg complexes, the structure-function relationships are reviewed in the light of the structural state of the protein (native versus non-native aggregated proteins). After a brief description of β-lg native structure, the review takes an interest in the binding properties of native β-lg (localization of binding sites, stoichiometry, and affinity) and the way the interaction affects the biological properties of the protein and the ligand. The binding properties of non-native aggregated forms of β-lg that are classically generated during industrial processing are also related. Structural changes modify the stoichiometry and the affinity of β-lg for fatty acids and consequently the biological functions of the complex. Finally, the fatty acid-binding properties of other whey proteins (α-lactalbumin, bovine serum albumin) and some biological properties of the complexes are also addressed. These proteins affect β-lg/fatty acids complex in whey given their competition with β-lg for fatty acids.

NMR studies of the dynamics of nitrophorin 2 bound to nitric oxide

The Rhodnius nitrophorins are β-barrel proteins of the lipocalin fold with a heme protruding from the open end of the barrel. They are found in the saliva of the blood-sucking insect Rhodnius prolixus, which synthesizes and stores nitric oxide (NO) in the salivary glands, where NO is bound to iron. NO is released by dilution and an increase in pH when the insect spits its saliva into the tissues of a victim, to aid in obtaining a blood meal. In the adult insect, there are four nitrophorins, NP1-NP4. At pH 7.3, NP4 releases NO 17 times faster than NP2 does, as measured by stopped-flow kinetics. A number of crystal structures of the least abundant protein, NP4, are available. These structures have been used to propose that two loops between adjacent β-strands at the front opening of the protein, the A-B and G-H loops, determine the rate of NO release. To learn how the protein loops contribute to the release of NO for each of the nitrophorins, the dynamics of these proteins are being studied in our laboratory. In this work, the NP2-NO complex has been investigated by nuclear magnetic resonance relaxation measurements to probe the picosecond-to-nanosecond and microsecond-to-millisecond time scale motions at three pH values, 5.0, 6.5, and 7.3. It is found that at pH 5.0 and 6.5, the NP2-NO complex is rigid and only a few residues in the loop regions show dynamics, while at pH 7.3, somewhat more dynamics, particularly of the A-B loop, are observed. Comparison to other lipocalins shows that all are relatively rigid, and that the dynamics of lipocalins in general are much more subtle than those of mainly α-helical proteins.

Interactions of β-lactoglobulin variants A and B with Vitamin A. Competitive binding of retinoids and carotenoids

β-Lactoglobulin (β-Lg) is the major whey protein of bovine milk present at a concentration of 2-3 g L(-1). Its biological role is still not well-known. However, many studies have suggested that β-Lg may play either nutritional or specific transporter role. The high affinity of β-Lg for retinol and other retinoids was reported. The results of interaction studies of β-Lg with carotenoids, that is, β-carotene, β-cryptoxanthin, and α-carotene, which display similar structures are reported in this study. The affinities of β-Lg for binding of retinoids and carotenoids were compared, providing more information about the binding site(s) of these molecules by β-Lg. Interactions were followed by the measurements of quenching of β-Lg tryptophan fluorescence and retinol fluorescence. The obtained results indicate that carotenoids are bound by β-Lg with high affinity of the order of 10(-8) M. Measurement of retinol competition with carotenoids for binding by β-Lg suggests that the binding of these two ligands occurs at two different sites of β-Lg.

Ostreopexin: a hemopexin fold protein from the oyster mushroom, Pleurotus ostreatus

Proteins with hemopexin repeats are widespread in viruses, prokaryotes and eukaryotes. We report here for the first time the existence of a protein in fungi with the four-bladed β-propeller fold that is typical for hemopexin-like proteins. This protein was isolated from the edible basidiomycetous fungus Pleurotus ostreatus and is named ostreopexin. It binds to Ni(2+)-NTA-agarose, and is structurally and functionally very similar to PA2 albumins isolated from legume seeds and the hemopexin fold protein from rice. Like these plant proteins, ostreopexin shows reversible binding to hemin with moderate affinity, but does not bind to polyamines. We suggest that ostreopexin participates in intracellular management of metal (II or III)-chelates.

Bioavailability of Oil-Based and β -Lactoglobulin-Complexed Vitamin A in a Rat Model

β -Lactoglobulin is capable of binding fat-soluble compounds including vitamin A palmitate and is suggested to specifically enhance intestinal uptake of retinol. In this study, bioavailability of a vitamin-A-retinyl palmitate complex in skim milk and in water-based liquids was investigated in vitamin-A-depleted rats. First, rats were fed a vitamin-A-free pellet diet for 6 wk and were thereafter gavage-fed with vitamin A in oil, vitamin-A- β -lactoglobulin complex, vitamin A in oil + skim milk, and vitamin-A- β -lactoglobulin + skim milk for 2 wk and 42 wk. Vitamin A repletion, as judged by vitamin A accumulation in serum and liver, occurred in all the treatments. Vitamin-A- β -lactoglobulin complex treatments had statistical equivalence with oil-based vitamin A treatments. In a second experiment, vitamin-A-depleted rats were fed UHT-processed skim milk fortified with either oil-based or freeze-dried β -lactoglobulin-complexed retinyl palmitate. Liver and serum vitamin A were analyzed by HPLC to indicate vitamin A status in the rats. Results showed no significant difference in bioavailability of retinyl palmitate from milk made with either regular oil-based or β -lactoglobulin-complexed fortifiers. The vitamin-A- β -lactoglobulin complex, being water soluble, may be useful for fortification of nonfat products.

LMBRD1: the gene for the cblF defect of vitamin B₁₂ metabolism

To date, only very few genetic disorders due to defects in lysosomal membrane transport are known. This paper reviews the identification of the underlying molecular defect causing an intriguing inborn error of vitamin B₁₂ metabolism, namely, defective lysosomal release of vitamin B₁₂ (cblF defect). Using microcell-mediated chromosome transfer of wild-type human chromosomes into immortalized fibroblasts from a cblF patient and genome-wide homozygosity mapping in 12 unrelated cblF patients, we identified LMBRD1 as a positional candidate gene on chromosome 6q13. Five different frameshift mutations leading to loss of function of both LMBRD1 alleles were detected in the affected patients. Transfection of the LMBRD1 wild-type construct into fibroblasts derived from cblF patients restored cobalamin coenzyme synthesis and function. LMBRD1 encodes a novel lysosomal membrane protein with significant homology to lipocalin membrane receptors. These studies give further insight into the intracellular transport of vitamins, challenge the views on lipocalin receptors, and add to our understanding of lysosomal diseases.

Parallel G-quadruplex-specific fluorescent probe for monitoring DNA structural changes and label-free detection of potassium ion

Here we demonstrate an anionic porphyrin, protoporphyrin IX (PPIX), as a parallel G-quadruplex-specific fluorescent probe for monitoring DNA structural changes and utilize it to develop a DNA-based K(+) sensor. The interactions of PPIX with different DNA structures in K(+) or Na(+) solution are investigated by using circular dichroism, fluorescence, and UV-vis spectroscopy. The observations reveal that PPIX has an ∼100-fold selectivity for parallel G-quadruplexes against duplexes and antiparallel G-quadruplexes. Meanwhile, the fluorescence intensity of PPIX increases by over 10-fold upon binding to parallel G-quadruplexes. On the basis of the selectivity and fluorescence property of PPIX, we introduce a facile, label-free approach to monitoring DNA structural changes via fluorescence signal readout that is tuned by PPIX binding and release. To illustrate it, we utilize PPIX and a G-rich DNA PS2.M to construct a fluorescent K(+) sensor based on an antiparallel-to-parallel conformation transition of the G-quadruplex. PS2.M adopts an antiparallel quadruplex structure in Na(+) solution, whereas it gradually converts into a parallel G-quadruplex upon addition of increasing K(+). This conformational change is indicated by a sharp increase in the fluorescence intensity of PPIX, owing to the good ability of PPIX to discriminate parallel G-quadruplexes from antiparallel ones. Even in the presence of 100 mM Na(+), such a "turn-on" fluorescent sensor can respond to low concentrations of K(+), with a limit of detection (0.5 mM) for K(+) analysis. In addition, this sensor exhibits a high selectivity for K(+) over other common metal ions, which ensures its practical applications to real samples. These results reveal that PPIX is promising for use as a specific DNA structural probe in sensing applications.

Bovine β-lactoglobulin acts as an acid-resistant drug carrier by exploiting its diverse binding regions

Binding of fluorine-containing drugs to bovine β-lactoglobulin, the most abundant whey protein in bovine milk, was investigated by means of 19F NMR and mass spectrometry. The stoichiometry of the binding and its stability in acidic medium, where β-lactoglobulin is folded and stable, were also studied, along with competition from molecules that can be regarded as analogs of physiological ligands to bovine β-lactoglobulin. Conditional binding data were combined with protein structural information derived from circular dichroism and limited proteolysis studies. Spectroscopic techniques were also used to assess whether the bound drugs stabilize the protein structure against denaturation by chaotropes or temperature at various pH values. The results obtained provide evidence for the presence of multiple binding regions on the protein, with a specific and different affinity for structurally different classes of hydrophobic drugs and, more generally, that bovine β-lactoglobulin can bind and protect against low pH values various classes of drugs of pharmaceutical relevance.

Interaction of curcumin and diacetylcurcumin with the lipocalin member beta-lactoglobulin

The binding of curcumin (CUR) and diacetylcurcumin (DAC) to bovine beta-lactoglobulin (BLG) genetic variant B was investigated by fluorescence and circular dichroism techniques. The binding parameters including number of substantive binding sites and the binding constants have been evaluated by fluorescence quenching method. The distance (r) between donor (BLG) and acceptor (CUR and DAC) was obtained according to the Förster's theory of non-radiative energy transfer. The far-UV circular dichroism spectra were used to investigate the possible changes in the secondary structure of BLG in the presence of CUR and DAC and showed that these two ligands change the alpha-helix and random coil contents of this protein to some extent. The visible circular dichroism spectra indicated that the optical activity during the ligand binding was observed due to the induced-protein chirality. All of the achieved results suggested the important role of the phenolic OH group of CUR in the binding process resulted in more affinity of CUR than DAC for binding to BLG.

Identification of a putative lysosomal cobalamin exporter altered in the cblF defect of vitamin B12 metabolism

Vitamin B(12) (cobalamin) is essential in animals for metabolism of branched chain amino acids and odd chain fatty acids, and for remethylation of homocysteine to methionine. In the cblF inborn error of vitamin B(12) metabolism, free vitamin accumulates in lysosomes, thus hindering its conversion to cofactors. Using homozygosity mapping in 12 unrelated cblF individuals and microcell-mediated chromosome transfer, we identified a candidate gene on chromosome 6q13, LMBRD1, encoding LMBD1, a lysosomal membrane protein with homology to lipocalin membrane receptor LIMR. We identified five different frameshift mutations in LMBRD1 resulting in loss of LMBD1 function, with 18 of the 24 disease chromosomes carrying the same mutation embedded in a common 1.34-Mb haplotype. Transfection of fibroblasts of individuals with cblF with wild-type LMBD1 rescued cobalamin coenzyme synthesis and function. This work identifies LMBRD1 as the gene underlying the cblF defect of cobalamin metabolism and suggests that LMBD1 is a lysosomal membrane exporter for cobalamin.

Irradiation of the porphyrin causes unfolding of the protein in the protoporphyrin IX/beta-lactoglobulin noncovalent complex

Porphyrins such as protoporphyrin IX (PPIX) are known to occasionally cause conformational changes in proteins for which they are specific ligands. It has also been established that irradiation of porphyrins noncovalently intercalated between bases or bound to one of the grooves can cause conformational effects on DNA. Conversely, there is no evidence reported in the literature of conformational changes caused by noncovalently bound PPIX to globular proteins for which the porphyrin is not a specific ligand. This study shows that the irradiation of the porphyrin in the PPIX/lactoglobulin noncovalent complex indeed causes a local and limited (approximately 7%) unfolding of the protein near the location of Trp19. This event causes the intrinsic fluorescence spectrum of the protein to shift to the red by 2 nm and the average decay lifetime to lengthen by approximately 0.5 ns. The unfolding of lactoglobulin occurs only at pH >7 because of the increased instability of the protein at alkaline pH. The photoinduced unfolding does not depend on the presence of O2 in solution; therefore, it is not mediated by formation of singlet oxygen and is likely the result of electron transfer between the porphyrin and amino acid residues.

Characterization of secondary and tertiary conformational changes of beta-lactoglobulin adsorbed on silica nanoparticle surfaces

Nanoparticles possess unique properties as a result of their large surface area per unit volume and therefore can be functionalized by the immobilization of enzymes for a variety of biosensing applications. Changes in the tertiary conformation of beta-lactoglobulin adsorbed on 90 nm silica nanoparticles with time were inferred using tryptophan fluorescence and Fourier transform infrared spectroscopy (FTIR) for different surface concentrations, temperature, pH, ionic strength, and 2,2,2-trifluoroethanol (TFE) and dithiothreitol (DTT) concentrations. Rapid initial unfolding followed by a much slower rate at longer times was observed, with the extent of unfolding being higher at lower surface concentrations, higher ionic strengths, higher temperature, higher TFE and DTT concentrations, and pI. The effect of temperature on the unfolding of adsorbed protein on the nanoparticle surface was similar to that in the bulk even though the extent of unfolding was higher for adsorbed protein molecules. The results of the extent of change in tertiary conformation using FTIR as indicated by the change in the ratio of amide II'/amide I were consistent with those obtained by tryptophan fluorescence whereas the rates of conformational changes given by FTIR were found to be much faster. Circular dichroism (CD) spectra showed that altering the surface concentration by itself did not change the secondary structure of beta-lactoglobulin on the surface. TFE was found to increase the alpha helix content at the expense of the fraction of the beta sheet, whereas the beta sheet was converted to an unordered conformation in the presence of DTT.

Interaction of beta-lactoglobulin with resveratrol and its biological implications

Beta-lactoglobulin (beta-LG), the major whey protein in the milk of ruminants, has a high affinity for a wide range of compounds. Resveratrol (3,5,4'-trihydroxystilbene), a natural polyphenolic compound found in grapes and red wine, exhibits many physiological effects associated with health benefits. In this study, the interaction of resveratrol with beta-LG was investigated using circular dichroism, fluorescence and UV-vis absorbance. Self-association of resveratrol possibly occurs at high concentrations. Resveratrol interacts with beta-LG to form 1:1 complexes. Resveratrol is bound to the surface of the protein because beta-LG-bound polyphenol is in a weaker hydrophobic environment relative to 75% ethanol. The binding constant for the resveratrol-beta-LG interaction is between 10(4) and 10(6) M (-1), as determined by protein or polyphenol fluorescence. The beta-LG-resveratrol interaction may compete with self-association of both the polyphenol and the protein. It has no apparent influence on beta-LG secondary structure but partially disrupts tertiary structure. Complexing with beta-LG provides a slight increase in the photostability of resveratrol and a significant increase in its hydrosolubility.

Binding Properties of a New Anti-tumor Component (2,2′-bipyridin octylglycinato Pd(II) nitrate) with Bovine β-lactoglobulin-A and -B

An new water soluble palladium (II) complex of formula [Pd(bpy)(Oct-Gly)]NO(3), (where bpy is 2,2'-bipyridine and Oct-Gly is octylglycine) have been synthesised. The Pd(II) complex has been characterized by elemental analysis and conductivity measurements as well as spectroscopic methods such as infrared, (1)H NMR, and ultraviolet-visible. The interaction between the new Pd(II)-complex (2,2'-bipyridin octylglycinato Pd(II) nitrate), an anti-tumor component, with beta-lactoglobulin-A and -B (BLG-A and -B) was studied by fluorescence spectroscopy and far and near-UV circular dichroism (CD) spectrophotometric techniques. A strong fluorescence quenching interaction of Pd(II) complex with BLG-A and -B was observed. The quenching constant was determined using the modified Stern-Volmer equation. The calculated binding constants of Pd(II) complex with BLG-A and -B were 0.51 and 0.28 (x 10(6) M(-1)) and the corresponding average number of binding sites were 2.8 and 1.5, respectively. Far-UV CD studies showed that the Pd(II) complex can significantly change the secondary structure of BLG-A and -B via an increase in the content of alpha-helix structure, which stabilizes the secondary structure of the proteins. Near-UV CD data clearly indicate the alteration in the tertiary structure of BLG-A and -B due to the interaction with Pd(II) complex. Pd(II) complex can change and stabilize both the secondary and tertiary structures of BLG-A more than BLG-B. These conformational changes may be considered to be a deleterious effect of the designed ligand on the protein structures. The difference in the interaction properties observed for BLG-A and -B with Pd(II) complex is due to the difference in the amino acid sequences between these two variants.

Computational and experimental approaches assess the interactions between bovine beta-lactoglobulin and synthetic compounds of pharmacological interest

Extending a previous investigation, the ability of binding to the model calycin beta-lactoglobulin (BLG) was evaluated both in silico and in vitro for several fluorine-containing (semi-)synthetic molecules of pharmacological and pharmaceutical interest (antibiotics, vastatins, steroid drugs). Simulation procedures included molecular docking according to a Montecarlo-simulated annealing protocol and molecular dynamics; heteronuclear NMR and denaturant gradient gel electrophoresis were the selected experimental techniques. For the tested drugs, ranking of the binding affinity was consistently assessed by computation and by experiment. The affinity for BLG increased in the sequence: 5-fluorosalycilic acid<dexamethasone<<sulindac=norfloxacin<fluvastatin. The computed Ki for fluorosalycilate was in the order of 10(-4)M; accordingly, in a molecular dynamics simulation the chemical diffused out of the BLG calyx in less than 2ns, and no evidence of binding was found by NMR or electrophoresis. Conversely, the Ki for fluvastatin and norfloxacin were in the order of 10(-7) and 10(-6)M, similar to the affinity for BLG by natural ligands, such as retinoids and long-chain fatty acids. Moreover fluvastatin was found still bound to the protein after 5ns of molecular dynamics simulation. Interaction of fluvastatin and norfloxacin with BLG was made evident by changes in chemical shift and dynamic parameters in the 19F NMR spectra and in effective urea concentration and cooperativity features in denaturant gradient gel electrophoresis. Such findings prove BLG may act as a drug carrier accepting in its cavity molecules of different bulk, rigidity and hydrophobicity.

Binding of 2-nonanone and milk proteins in aqueous model systems

Interactions of the model flavor compound 2-nonanone with individual milk proteins, whey protein isolate (WPI), and sodium caseinate in aqueous solutions were investigated. A method to quantify the free 2-nonanone was developed using headspace solid-phase microextraction followed by gas chromatography with flame ionization detection. Binding constants (K) and numbers of binding sites (n) for 2-nonanone on the individual proteins were calculated. The 2-nonanone binding capacities decreased in the order bovine serum albumin > beta-lactoglobulin > alpha-lactalbumin > alpha s1-casein > beta-casein, and the binding to WPI was stronger than the binding to sodium caseinate. All proteins appeared to have one binding site for 2-nonanone per molecule of protein at the flavor concentrations investigated, except for bovine serum albumin, which possessed two classes of binding sites. The binding mechanism is believed to involve predominantly hydrophobic interactions.

Spectroscopic and cytotoxic studies of the novel designed palladium(II) complexes: β-Lactoglobulin and K562 as the targets

Since palladium complexes have been reported to show fewer side effects relative to other heavy metal anticancer compounds, in this study a new class of four structurally related anticancer Pd(II) complexes including 2,2'-bipyridin-n-butyl dithiocarbamato Pd(II) nitrate (Com-1), 2,2'-bipyridin-n-hexyl dithiocarbamato Pd(II) nitrate (Com-2), 2,2'-bipyridin glycinato Pd(II) nitrate (Com-3) and 2,2'-bipyridin octylglycinato Pd(II) nitrate (Com-4) was designed. The effect of four synthesized ligands on the protein structure and cell proliferation were investigated. Whey carrier proteins beta-lactoglobulin-A and-B (BLG-A and-B) and chronic myelogenous leukemia cell line K562 were the targets. Fluorescence and CD instruments were used to assess effect of the ligands on the protein structure. Growth inhibitory effect of the Pd(II) complexes towards the cancer cells was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. Results of fluorescence studies revealed that the complexes had no dithiocarbamate moiety (compounds 3 and 4) could quench the intrinsic fluorescence emission of the proteins at lower concentrations than those had such moiety (compounds 1 and 2). The far-UV-CD studies revealed that the regular secondary structure of BLG-A and -B did not show any noticeable alteration upon interaction with different of Pd(II)-complexes. The results of cell proliferation assay also displayed that Com-1 and Com-2 had more growth inhibitory activity against K562, than Com-3 and Com-4. Our results suggested that addition of dithiocarbamate moiety to structure of Pd(II) complexes probably has important role to improve the antiproliferative properties of the anticancer ligands and fewer effects on the carrier protein structure.

A Tryptophan Rotamer Located in a Polar Environment Probes pH-Dependent Conformational Changes in Bovine β-Lactoglobulin A

Bovine beta-lactoglobulin A (BLGA) is a well characterized globular protein whose tertiary structure has been investigated in detail. BLGA undergoes a pH-dependent conformational change which X-ray data described as involving mostly the loop connecting strands E and F and the deprotonation of a glutamic acid residue (Glu89). These structural changes have been investigated using, among other techniques, fluorescence spectroscopy. The intrinsic fluorescence of BLGA is dominated by two Trp residues. These residues are located far from the EF loop and would not be expected to probe the pH-induced conformational change of the protein. Trp19 is located at the bottom of the interior beta-barrel, whereas Trp61 is located at the aperture of the barrel near the CD loop and is "silent" in the emission of native BLGA because of the proximity of a disulfide moiety. Our study suggests that, surprisingly, the fluorescence of Trp19 has the characteristic of a more polar environment than structural models from X-ray data would suggest and that at least two distinct conformations (or rotamers) of Trp19 contribute to the fluorescence of the protein. The less populated rotamer (relative amplitude (alpha) approximately 20%, tau approximately 3 ns) probes a more polar environment and a pH-dependent conformational change of BLGA in the region of Trp19 which X-ray data do not detect. Finally, our study provides the estimate of the fluorescence lifetime of Trp61 in the "unquenched" form.

Computational and experimental approaches for assessing the interactions between the model calycin β-lactoglobulin and two antibacterial fluoroquinolones

Norfloxacin and levofloxacin, two fluoroquinolones of different bulk, rigidity and hydrophobicity taken as model ligands, were docked to one apo and two holo crystallographic structures of bovine beta-lactoglobulin (BLG) using different computational approaches. BLG is a member of the lipocalin superfamily. Lipocalins show a typical b-barrel structure encompassing an internal cavity where small hydrophobic molecules are usually bound. Our studies allowed the identification of two putative binding sites in addition to the calyx. The rigid docking approximation resulted in strong repulsive forces when the ligands were docked into the calyx of the apo form. On the contrary, hindrance was not experienced in flexible docking protocols whether on the apo or on the holo BLG forms, due to allowance for side chain rearrangement. K(i) between 10(-7) and 10(-6) M were estimated for norfloxacin at pH 7.4, smaller than 10(-5) M for levofloxacin. Spectroscopic and electrophoretic techniques experimentally validated the occurrence of an interaction between norfloxacin and BLG. Changes in chemical shift and dynamic parameters were observed between the (19)F NMR spectra of the complex and of the ligand. A K(i) (ca 10(-7) M) comparable with the docking results was estimated through a NMR relaxation titration. Stabilization against unfolding was demonstrated by denaturant gradient gel electrophoresis on the complex versus apo BLG. NMR experimental evidence points to a very loose interaction for ofloxacin, the racemic mixture containing levofloxacin. Furthermore, we were able to calculate in silico K(i)'s comparable to the published experimental values for the complexes of palmitic and retinoic acid with BLG.

Interactions between bovine beta-lactoglobulin A and various bioactive peptides as studied by front-face fluorescence spectroscopy

Front-face fluorescence spectroscopy was used for the first time to study the interactions between bovine beta-lactoglobulin variant A (beta-Lg A) and various beta-Lg-derived bioactive peptides. Fluorescence spectra were recorded for beta-Lg A-peptide mixtures at 25 degrees C and pH 6.8 with an excitation wavelength of 290 nm to characterize the molecular environment of tryptophan (Trp) residues present in the protein but absent in the peptides. Spectra remained unchanged following addition of peptides beta-Lg f92-100 and beta-Lg f125-135, while Phe-Phe interaction between beta-Lg f69-83 molecules interfered with analysis. Addition of beta-Lg f102-105 produced a blue shift (3 nm) and a significant increase in fluorescence intensity, while addition of beta-Lg f142-148 also caused a significant increase in fluorescence intensity but accompanied by a red shift (3 nm). These results indicate that the polarity of the Trp environment in the beta-Lg A structure may be modified differently depending on the peptide added.

Thermodynamics of Binding Interactions between Bovine β-Lactoglobulin A and the Antihypertensive Peptide β-Lg f142-148

The binding capacity of bovine beta-lactoglobulin variant A (beta-Lg A) for six peptides derived from beta-Lg was evaluated using an ultrafiltration method under the following conditions: pH 6.8, 40 degrees C, and a beta-Lg A/peptide molar ratio of 1:5. Only peptides beta-Lg f102-105, f142-148, and f69-83 bound in significant amounts to beta-Lg A corresponding to 1.5, 1.1, and 0.7 mol of peptide per mole of beta-Lg A, respectively. The interaction between beta-Lg A and the antihypertensive peptide beta-Lg f142-148 was investigated further by isothermal titration calorimetry. The binding isotherms at pH 6.8 and 25 degrees C confirmed that beta-Lg f142-148 bound to beta-Lg A and that the interaction followed a sequential three-site binding model with constants of association of 2 x 10(3), 1 x 10(3), and 0.4 x 10(3) M(-1) for the first, second, and third binding sites, respectively. The enthalpy of binding was exothermic for the first and second binding sites and endothermic for the third binding site. Binding of the peptide to all three sites was spontaneous as shown by the negative free energy values. These results show for the first time that beta-Lg A can bind bioactive peptides. This potential could be exploited to transport bioactive peptides and protect them in the gastrointestinal tract following their oral administration as nutraceuticals.

The pH-dependent conformational transition of β-lactoglobulin modulates the binding of protoporphyrin IX

We have investigated the interaction between PPIX and beta-lactoglobulin (beta-lg) as a function of the pH of the solution. beta-lg is a small globular protein (MW approximately 18 kDa) with a very well characterized structure that reveals several possible binding sites for ligands. The interaction with beta-lg affects the photophysical properties of PPIX. The shift of PPIX emission maximum, excitation maximum and the increase of the fluorescence intensity is an indicator that binding between the porphyrin and beta-lg occurs. The binding constant appears to be modulated by the pH of the solution. Spectroscopic measurements do not reveal any significant energy transfer between the Trp residues of beta-lg and PPIX, however, fluorescence anisotropy decay measurements confirm the binding and the modulation introduced by the pH of the solution. Since beta-lg has been shown to be stable within the range of pH adopted in our experiments (5.0-9.0), the results suggest that PPIX binds a site affected by the pH of the solution. Because of the crystallographic evidence an obvious site is near the aperture of the interior beta-barrel however an alternative (or concurrent) binding site may still be present.

Characterization of Local Polarity and Hydrophobic Binding Sites of β-Lactoglobulin by Using N-Terminal Specific Fluorescence Labeling

Because of wide ligand-binding ability and significant industrial interest of beta-lactoglobulin (beta-LG), its binding properties have been extensively studied. However, there still exists a controversy as to where a ligand binds, since at least two potential hydrophobic binding sites in beta-LG have been postulated for ligand binding: an internal one (calyx) and an external one (near the N-terminus). In this work, the local polarity and hydrophobic binding sites of beta-LG have been characterized by using N-terminal specific fluorescence labeling combined with a polarity-sensitive fluorescent probe 3-(4-chloro-6-hydrazino- 1,3,5-triazinylamino)-7-(dimethylamino)-2-methylphenazine (CHTDP). The polarity within the calyx is found to be extremely low, which is explained in terms of superhydrophobicity possibly resulting from its nanostructure, and the polarity is increased with the destruction of the calyx by heat treatment. However, the polarity of the N-terminal domain in native beta-LG is decreased after thermal denaturation. This polarity trend toward decreasing instead of increasing shows that beta-LG may have no definite external hydrophobic binding site. The hydrophobic binding of a ligand such as CHTDP at the surface of the protein is probably achieved via appropriate assembling of corresponding hydrophobic residues rather than via a fixed external hydrophobic binding site. Also, the ligand-binding location in beta-LG is found to be relevant to not only experimental conditions (pH < or = 6.2 or pH > 7.1) but also binding mechanisms (hydrophobic affinity or electrostatic interaction).

trans-Parinaric acid as a versatile spectroscopic label to study ligand binding properties of bovine beta-lactoglobulin

Advantageous spectroscopic properties of the plant derived polyunsaturated trans-parinaric acid (tPnA) was demonstrated in obtaining valuable data on the ligand binding characteristics of the lipocalin member bovine beta-lactoglobulin A (BLG-A). Titration of the protein with tPnA resulted in the appearance of an intense negative induced circular dichroism (CD) band and bathochromic shift of the ultraviolet (UV) peak of the ligand. The extrinsic optical activity was interpreted by the chiral contribution of the allylic axial CH bonds of tPnA to the pi-pi(*) transition of the planar tetraene chromophore. Analysis of the series of induced CD curves obtained by CD titration experiment indicated the complexation of a single ligand molecule to a uniform protein binding site. Additionally, the dramatic increase of fluorescence intensity of the lactoglobulin bound ligand suggested the hydrophobic nature of the binding site. CD and fluorescence titration data were utilized to calculate the binding constant (K(a)) of which high value ( approximately 10(6)M(-1)) refers to strong protein association of tPnA. pH dependent reversible dis- and reappearance of the induced CD signal unambigously proved the inclusion of tPnA into the central hydrophobic cavity of the lactoglobulin governed by the protonation induced conformational movement of the EF loop at the opening of the calyx. This conclusion was supported and complemented by molecular docking calculations.

Fur Seal Adaptations to Lactation: Insights into Mammary Gland Function

The fur seal (Arctocephalus spp. and Callorhinus spp., members of the pinniped family) is a mammal with the unusual capability to modulate its lactation cycle by turning milk production on and off without the typical mammalian regression and involution of the mammary gland. Lactation has evolved from constraints arising from the spatial and temporal separation of infant nursing and maternal foraging as the mother gives birth and feeds the pup on land while acquisition of nutrients for milk production occurs at sea. The lactation cycle begins with the female fur seal undergoing a perinatal fast of approximately 1 wk, after which time she departs the breeding colony to forage at sea. For the remainder of the long lactation period (116-540 days), the mother alternates between short periods ashore suckling the young with longer periods of up to 4 wk of foraging at sea. Milk production continues while foraging at sea, but at less than 20% the rate of production on land. Fur seals produce one of the richest milk reported, with a very high lipid content contributing up to 85% of total energy. This feature serves as an adaptation to the young's need to produce an insulating blubber layer against heat loss and to serve as an energy store when the mother is away foraging at sea. This atypical pattern of lactation means mothers have long periods with no suckling stimulus and can transfer high-energy milk rapidly while on land to minimize time away from foraging grounds. The absence of suckling stimulus and milk removal during foraging does not result in the onset of involution with associated apoptosis of mammary secretory cells and a subsequent progressive breakdown of the cellular structure of the mammary gland. The mechanisms controlling lactation in the fur seal mammary gland have been investigated using molecular and cellular techniques. These findings have shed light on the processes by which the unique features of lactation in the fur seal are regulated.