Thermodynamic, crystallographic and computational studies of non-mammalian fatty acid binding to bovine β-Lactoglobulin

Evaluation of the pH effect on complex formation between bovine β-lactoglobulin and aflatoxin M1: a molecular dynamic simulation and molecular docking study

The aim of this work was to evaluate interaction between aflatoxin M1 (AFM1) and structural models of β-lactoglobulin (β-LG) at pH 4.0 and 6.5. This information would provide an explanation of the variability in AFM1 during cheese production. Once β-LG models were optimized using molecular dynamic (MD) simulation, it was found that a region of the Calyx cavity underwent conformational changes, at the E-F loop, from the closed conformation at pH 6.5 to the open at pH 4.0. No differences in Site C conformation were observed at both pH. The binding free energy (ΔGb) of the β-LG-AFM1 complexes at the different pHs were determined by molecular docking. The ΔGb values obtained for the Calyx cavity showed that at pH 4.0 there is a more stable complex formation compared to pH 6.5 with values of -42.6 and -32.0 kJ mol-1, respectively. On the contrary, in the complexes formed in Site C at both pH´s there were no differences. Likewise, the ΔGb in the dimer interface was evaluated, obtaining a value of -29.3 kJ mol-1, like those obtained at Site C. In addition, by the MD simulations of the β-LG-AFM1 complexes, it was observed that at acidic pH the binding of AFM1 with β-LG is more stable. In conclusion, the computational tools showed that the most stable complex was formed at the Calyx cavity at pH 4.0. This suggests that during cheese production using acidic coagulation, the whey proteins show higher affinity toward AFM1 which may explain the observed variability of mycotoxin.Communicated by Ramaswamy H. Sarma.

β-Lactoglobulin variants as potential carriers of pramoxine: Comprehensive structural and biophysical studies

β-Lactoglobulin (BLG) is a member of the lipocalin family. As other proteins from this group, BLG can be modified to bind specifically compounds of medical interests. The aim of this study was to evaluate the role of two mutations, L39Y and L58F, in the binding of topical anesthetic pramoxine (PRM) to β-lactoglobulin. Circular dichroism spectroscopy, isothermal titration calorimetry (ITC), and X-ray crystallography were used to understand the mechanisms of BLG-PRM interactions. Studies were performed for three new BLG mutants: L39Y, L58F, and L39Y/L58F. ITC measurements indicated a significant increase in the affinity to the PRM of variants L58F and L39Y. Measurements taken for the double mutant L39Y/L58F showed the additivity of two mutations leading to about 80-fold increase in the affinity to PRM in comparison to natural protein BLG from bovine milk. The determined crystal structures revealed that pramoxine is accommodated in the β-barrel interior of BLG mutants and stabilized by hydrophobic interactions. The observed additive effect of two mutations on drug binding opens the possibility for further designing of new BLG variants with high affinity to selected drugs.

Thermodynamic and biophysical study of fatty acid effector binding to soybean lipoxygenase: implications for allostery driven by helix α2 dynamics

Previous comparative kinetic isotope effects have inferred an allosteric site for fatty acids and their derivatives that modulates substrate selectivity in 15-lipoxygenases. Hydrogen-deuterium exchange also previously revealed regionally defined enhanced protein flexibility, centred at helix α2 - a gate to the substrate entrance. Direct evidence for allosteric binding and a complete understanding of its mechanism remains elusive. In this study, we examine the binding thermodynamics of the fatty acid mimic, oleyl sulfate (OS), with the monomeric model plant 15-LOX, soybean lipoxygenase (SLO), using isothermal titration calorimetry. Dynamic light scattering and differential scanning calorimetry rule out OS-induced oligomerization or structural changes. These data provide evidence that the fatty acid allosteric regulation of SLO is controlled by the dynamics of helix α2.

Emerging artificial metalloenzymes for asymmetric hydrogenation reactions

Artificial metalloenzymes (ArMs) utilize the best properties of homogenous transition metal catalysts and naturally occurring proteins. While synthetic metal complexes offer high tunability and broad-scope reactivity with a variety of substrates, enzymes further endow these complexes with enhanced aqueous stability and stereoselectivity. For these reasons, dozens of ArMs have been designed to perform catalytic asymmetric hydrogenation reactions, and hydrogenase ArMs are, in fact, the oldest class of ArMs. Herein, we report recent advances in the design of hydrogenase ArMs, including (i) the modification of natural [Fe]-hydrogenase by insertion of artificial metallocofactors, (ii) design of a novel ArM system from the tractable and inexpensive protein β-lactoglobulin to afford a high-performing transfer hydrogenase, and (iii) the design of chimeric streptavidin scaffolds that drastically alter the secondary coordination sphere of previously reported streptavidin/biotin transfer hydrogenase ArMs.

Weak Binding of Epigallocatechin to α-Lactalbumin Greatly Improves Its Stability and Uptake by Caco-2 Cells

Improving the stability and bioavailability of catechins is of great importance. Epigallocatechin (EGC), the major catechin in green tea, is a potent antioxidant with numerous attributed health benefits. However, the low permeability and stability limit its enrichment in the diet for preventive medicine. In this study, we explored the interaction of EGC and α-lactalbumin by spectroscopic, thermodynamic, and crystallographic methods. The isothermal titration calorimetry experiments elucidated that α-lactalbumin binds to EGC at a ratio of 1:1 with a low affinity of (4.01 ± 0.11) × 10⁵ M^-1. A crystal structure solved at a high resolution (1.2 Å) provided direct evidence for the weak interaction between EGC and α-lactalbumin at an atomic level. The novel binding site was discovered at the exterior surface of α-lactalbumin for the first time, supporting a new binding behavior. Consequently, our results demonstrated that the binding of α-lactalbumin to EGC could protect EGC against light-induced, thermal-induced, and pH-induced damage. More importantly, the formed complex has better bioaccessibility than unbound EGC, which was approved by a cell absorption experiment. Such research is beneficial for designing protein-based nanocarriers for polyphenols.

l-Theanine improves emulsification stability and antioxidant capacity of diacylglycerol by hydrophobic binding β-lactoglobulin as emulsion surface stabilizer

Diacylglycerol (DAG) is commonly used as fat substitute in food manufacture due to its functional properties, but DAG has poor emulsification and oxidation stability, which limits its wide application in food industry. In this work, fluorescence quenching data and thermodynamic parameters were analyzed to investigate the interaction mechanism between l-theanine (L-Th) and β-lactoglobulin (β-LG). DAG emulsion was prepared by using β-lactoglobulin-theanine (β-LG-Th) as surface stabilizer, and its emulsification and oxidation stability were evaluated. The results showed that the hydrophobic interaction played an important role on the conjugate of β-LG and L-Th due to the negative values for ΔG, positive values for ΔH and ΔS at pH 4.0, pH 6.0 and pH 8.0. The DAG has been better embedded by using β-LG-Th as surface stabilizer, and the droplet size was about 0.2 µm to 1.5 µm when the pH was 6.0, the ratio of L-Th to β-LG was 1:1. β-LG-Th as surface stabilizer for DAG can increase the ζ-potential and emulsion index, make the emulsion droplet size distribution more uniform. The l-theanine was better to be used to improve the emulsification stability and antioxidant capacity of DAG by binding β-LG as surface stabilizer.

β-Lactoglobulin and Glycodelin: Two Sides of the Same Coin?

The two lipocalins, β-lactoglobulin (βLg) and glycodelin (Gd), are possibly the most closely related members of the large and widely distributed lipocalin family, yet their functions appear to be substantially different. Indeed, the function of β-lactoglobulin, a major component of ruminant milk, is still unclear although neonatal nutrition is clearly important. On the other hand, glycodelin has several specific functions in reproduction conferred through distinct, tissue specific glycosylation of the polypeptide backbone. It is also associated with some cancer outcomes. The glycodelin gene, PAEP, reflecting one of its names, progestagen-associated endometrial protein, is expressed in many though not all primates, but the name has now also been adopted for the β-lactoglobulin gene (HGNC, www.genenames.org). After a general overview of the two proteins in the context of the lipocalin family, this review considers the properties of each in the light of their physiological functional significance, supplementing earlier reviews to include studies from the past decade. While the biological function of glycodelin is reasonably well defined, that of β-lactoglobulin remains elusive.

Energetic and structural effects of the Tanford transition on ligand recognition of bovine β-lactoglobulin

Bovine β-lactoglobulin, an abundant protein in whey, is a promising nanocarrier for peroral administration of drug-like hydrophobic molecules, a process that involves transit through the different acidic conditions of the human digestive tract. Among the several pH-induced conformational rearrangements that this lipocalin undergoes, the Tanford transition is particularly relevant. This transition, which occurs with a midpoint around neutral pH, involves a conformational change of the E-F loop that regulates accessibility to the primary binding site. The effect of this transition on the ligand binding properties of this protein has scarcely been explored. In this study, we carried out an energetic and structural characterization of β-lactoglobulin molecular recognition at pH values above and below the zone in which the Tanford transition occurs. The combined analysis of crystallographic, calorimetric, and molecular dynamics data sheds new light on the interplay between self-association, ligand binding, and the Tanford pre- and post-transition conformational states, revealing novel aspects underlying the molecular recognition mechanism of this enigmatic lipocalin.