Jonathan O. Babalola

Development of an assay for the intermembrane transfer of cholesterol by Niemann-Pick C2 protein.

Abstract Niemann-Pick type C disease is an inherited fatal disorder characterized by the accumulation of unesterified cholesterol and other lipids in the endosomal/lysosomal compartment. Two independent genes responsible for this neurodegenerative disorder have been identified, but the precise functions of the encoded Niemann-Pick C1 (NPC1) and C2 (NPC2) proteins are not yet known. We developed a cell-free assay for measuring intermembrane lipid transport and examined the ability of bovine NPC2 (bNPC2) for intermembrane cholesterol transfer. NPC2 specifically extracts cholesterol from phospholipid bilayers and catalyzes intermembrane transfer to acceptor vesicles in a dose- and time-dependent manner. This transfer activity is dependent on temperature, pH, ionic strength, lipid composition of the model membranes, and the ratio of donor to acceptor vesicles. In model membranes, the presence of the lysosomal anionic phospholipids bis(monooleoylglycero)phosphate and phosphatidyl inositol significantly stimulated cholesterol transfer by NPC2, whereas bis(monomyristoylglycero)phosphate, phosphatidyl serine, and phosphatidic acid had no effect. Moreover, ceramide stimulated cholesterol transfer slightly, whereas sphingomyelin reduced cholesterol transfer rates. With our assay system we identified for the first time the ability of other lysosomal proteins, most notably the GM2-activator protein, to mediate intermembrane cholesterol transfer. This assay system promises to be a valuable tool for further quantitative and mechanistic studies of protein-mediated lipid transfer.

Role of endosomal membrane lipids and NPC2 in cholesterol transfer and membrane fusion

We examined the effect of Niemann-Pick disease type 2 (NPC2) protein and some late endosomal lipids [sphingomyelin, ceramide and bis(monoacylglycero)phosphate (BMP)] on cholesterol transfer and membrane fusion. Of all lipid-binding proteins tested, only NPC2 transferred cholesterol at a substantial rate, with no transfer of ceramide, GM3, galactosylceramide, sulfatide, phosphatidylethanolamine, or phosphatidylserine. Cholesterol transfer was greatly stimulated by BMP, little by ceramide, and strongly inhibited by sphingomyelin. Cholesterol and ceramide were also significantly transferred in the absence of protein. This spontaneous transfer of cholesterol was greatly enhanced by ceramide, slightly by BMP, and strongly inhibited by sphingomyelin. In our transfer assay, biotinylated donor liposomes were separated from fluorescent acceptor liposomes by streptavidin-coated magnetic beads. Thus, the loss of fluorescence indicated membrane fusion. Ceramide induced spontaneous fusion of lipid vesicles even at very low concentrations, while BMP and sphingomyelin did so at about 20 mol% and 10 mol% concentrations, respectively. In addition to transfer of cholesterol, NPC2 induced membrane fusion, although less than saposin-C. In this process, BMP and ceramide had a strong and mild stimulating effect, and sphingomyelin an inhibiting effect, respectively. Note that the effects of the lipids on cholesterol transfer mediated by NPC2 were similar to their effect on membrane fusion induced by NPC2 and saposin-C.

Solubilization of Membrane Proteins into Functional Lipid-Bilayer Nanodiscs Using a Diisobutylene/Maleic Acid Copolymer

Abstract Once removed from their natural environment, membrane proteins depend on membrane‐mimetic systems to retain their native structures and functions. To this end, lipid‐bilayer nanodiscs that are bounded by scaffold proteins or amphiphilic polymers such as styrene/maleic acid (SMA) copolymers have been introduced as alternatives to detergent micelles and liposomes for in vitro membrane‐protein research. Herein, we show that an alternating diisobutylene/maleic acid (DIBMA) copolymer shows equal performance to SMA in solubilizing phospholipids, stabilizes an integral membrane enzyme in functional bilayer nanodiscs, and extracts proteins of various sizes directly from cellular membranes. Unlike aromatic SMA, aliphatic DIBMA has only a mild effect on lipid acyl‐chain order, does not interfere with optical spectroscopy in the far‐UV range, and does not precipitate in the presence of low millimolar concentrations of divalent cations.

Acid sphingomyelinase activity is regulated by membrane lipids and facilitates cholesterol transfer by NPC2

During endocytosis, membrane components move to intraluminal vesicles of the endolysosomal compartment for digestion. At the late endosomes, cholesterol is sorted out mainly by two sterol-binding proteins, Niemann-Pick protein type C (NPC)1 and NPC2. To study the NPC2-mediated intervesicular cholesterol transfer, we developed a liposomal assay system. (Abdul-Hammed, M., B. Breiden, M. A. Adebayo, J. O. Babalola, G. Schwarzmann, and K. Sandhoff. 2010. Role of endosomal membrane lipids and NPC2 in cholesterol transfer and membrane fusion. J. Lipid Res. 51: 1747–1760.) Anionic lipids stimulate cholesterol transfer between liposomes while SM inhibits it, even in the presence of anionic bis(monoacylglycero)phosphate (BMP). Preincubation of vesicles containing SM with acid sphingomyelinase (ASM) (SM phosphodiesterase, EC 3.1.4.12) results in hydrolysis of SM to ceramide (Cer), which enhances cholesterol transfer. Besides SM, ASM also cleaves liposomal phosphatidylcholine. Anionic phospholipids derived from the plasma membrane (phosphatidylglycerol and phosphatidic acid) stimulate SM and phosphatidylcholine hydrolysis by ASM more effectively than BMP, which is generated during endocytosis. ASM-mediated hydrolysis of liposomal SM was also stimulated by incorporation of diacylglycerol (DAG), Cer, and free fatty acids into the liposomal membranes. Conversely, phosphatidylcholine hydrolysis was inhibited by incorporation of cholesterol, Cer, DAG, monoacylglycerol, and fatty acids. Our data suggest that SM degradation by ASM is required for physiological secretion of cholesterol from the late endosomal compartment, and is a key regulator of endolysosomal lipid digestion.

Hybrid materials from agro-waste and nanoparticles: implications on the kinetics of the adsorption of inorganic pollutants

This study is a first-hand report of the immobilization of Nauclea diderrichii seed waste biomass (ND) (an agro-waste) with eco-friendly mesoporous silica (MS) and graphene oxide–MS (GO+MS ) nanoparticles, producing two new hybrid materials namely: MND adsorbent for agro-waste modified with MS and GND adsorbent for agro-waste modified with GO+MS nanoparticles showed improved surface area, pore size and pore volume over those of the agro-waste. The abstractive potential of the new hybrid materials was explored for uptake of Cr(III) and Pb(II) ions. Analysis of experimental data from these new hybrid materials showed increased initial sorption rate of Cr(III) and Pb(II) ions uptake. The amounts of Cr(III) and Pb(II) ions adsorbed by MND and GND adsorbents were greater than those of ND. Modification of N. diderrichii seed waste significantly improved its rate of adsorption and diffusion coefficient for Cr(III) and Pb(II) more than its adsorption capacity. The rate of adsorption of the heavy metal ions was higher with GO+MS nanoparticles than for other adsorbents. Kinetic data were found to fit well the pseudo-second-order and the diffusion–chemisorption kinetic models suggesting that the adsorption of Cr(III) and Pb(II) onto these adsorbents is mainly through chemisorption mechanism. Analysis of kinetic data with the homogeneous particle diffusion kinetic model suggests that particle diffusion (diffusion of ions through the adsorbent) is the rate-limiting step for the adsorption process.

Regeneration strategies for spent solid matrices used in adsorption of organic pollutants from surface water: a critical review

Adsorption is a very important physicochemical process used for several purposes including separation and purification of proteins and in water treatment. This article reviews the various strategies that have been employed in the regeneration of spent adsorbents used in the adsorption of organic pollutants from aqueous solutions. The principles, advantages, disadvantages, and factors influencing each of these techniques are discussed. Future perspectives on the use of these techniques in regeneration process are provided for further studies with the view to make the regeneration process of spent solids more efficient and sustainable.

Role of the Subunit Interactions in the Conformational Transitions in Adult Human Hemoglobin: An Explicit Solvent Molecular Dynamics Study

Hemoglobin exhibits allosteric structural changes upon ligand binding due to the dynamic interactions between the ligand binding sites, the amino acids residues and some other solutes present under physiological conditions. In the present study, the dynamical and quaternary structural changes occurring in two unligated (deoxy-) T structures and two fully ligated (oxy-) R, R2 structures of adult human hemoglobin were investigated with molecular dynamics. It is shown that, in the submicrosecond time scale, there is no marked difference in the global dynamics of the amino acid residues in both the oxy- and the deoxy-forms of the individual structures. In addition, the R, R2 are relatively stable and do not present quaternary conformational changes within the time scale of our simulations, while the T structure is dynamically more flexible and exhibited the T → R quaternary conformational transition, which is propagated by the relative rotation of the residues at the α(1)β(2) and α(2)β(1) interface.

Photoelectrocatalytic water treatment systems: degradation, kinetics and intermediate products studies of sulfamethoxazole on a TiO2–exfoliated graphite electrode

Sulfamethoxazole is an antibacterial agent which is commonly prescribed for the treatment of infections in humans and animals. The detection of this drug in the aqueous environment has raised considerable health concerns. Herein, we report the photoelectrocatalytic degradation of sulfamethoxazole at a TiO2–exfoliated graphite (TiO2–EG) anode. The TiO2–EG nanocomposite, synthesised by sol–gel and microwave methods, was characterised by XRD, Raman and FTIR spectroscopies, SEM and TEM. The cyclic voltammograms of the fabricated electrodes were obtained in [Fe(CN)6]3− redox probe. Concentration abatement of the antibiotic was monitored using a UV-vis spectrophotometer and the possible intermediates were investigated using LCMS. After 6 h of the photoelectrocatalytic process, almost 100% of the drug had been degraded and a 90% COD decay was achieved. The photoelectrocatalytic degradation of sulfamethoxazole entailed γ-, β-, δ- and e-cleavages, hydroxylation and rings opening. The outcome of this study shows that the EG–TiO2 anode can be applied for the photoelectrocatalytic remediation of water contaminated by pharmaceuticals.

Formation of Lipid-Bilayer Nanodiscs by Diisobutylene/Maleic Acid (DIBMA) Copolymer

Membrane proteins usually need to be extracted from their native environment and separated from other membrane components for in-depth in vitro characterization. The use of styrene/maleic acid (SMA) copolymers to solubilize membrane proteins and their surrounding lipids into bilayer nanodiscs is an attractive approach toward this goal. We have recently shown that a diisobutylene/maleic acid (DIBMA) copolymer similarly solubilizes model and cellular membranes but, unlike SMA(3:1), has a mild impact on lipid acyl-chain order and thermotropic phase behavior. Here, we used fluorescence spectroscopy, small-angle X-ray scattering, size-exclusion chromatography, dynamic light scattering, and 31P nuclear magnetic resonance spectroscopy to examine the self-association of DIBMA and its membrane-solubilization properties against lipids differing in acyl-chain length and saturation. Although DIBMA is less hydrophobic than commonly used SMA(3:1) and SMA(2:1) copolymers, it efficiently formed lipid-bilayer nanodiscs that decreased in size with increasing polymer/lipid ratio while maintaining the overall thickness of the membrane. DIBMA fractions of different molar masses were similarly efficient in solubilizing a saturated lipid. Coulomb screening at elevated ionic strength or reduced charge density on the polymer at low pH enhanced the solubilization efficiency of DIBMA. The free-energy penalty for transferring phospholipids from vesicular bilayers into nanodiscs became more unfavorable with increasing acyl-chain length and unsaturation. Altogether, these findings provide a rational framework for using DIBMA in membrane-protein research by shedding light on the effects of polymer and lipid properties as well as experimental conditions on membrane solubilization.

Engineering and modeling the effect of Mg doping in TiO2 for enhanced photocatalytic reduction of CO2 to fuels

Mg-Doped TiO2 nanoparticles were prepared via a modified sonothermal method, and their photocatalytic activities were investigated for the reduction of CO2 with H2O. The structural properties of the prepared catalysts with varying Mg doping levels were studied by UV-vis spectroscopy, N2 adsorption–desorption, XRD, SEM, TEM, and XPS. CO, H2, CH3OH, and CH4 were the major products observed with a maximum production rate of 29.2, 28.7, 5910.0 and 2.3 μmol g−1 h−1, respectively. Preferable Mg doping sites in TiO2 nanoparticles and interaction of CO2 with Mg-doped TiO2 were studied computationally. Modeling revealed that (101) facets and junctions of (101)/(101) and (001)/(101) facets are the preferred locations of surface Mg atoms. Adsorption of CO2 proceeds in the bent carbonate and hydrocarbonate forms. The increased activity of Mg-doped TiO2 is explained by the close proximity of surface Mg reaction sites to the positions of photogenerated electrons on (101) facets.