Stitching together a nm thick peptide-based semiconductor sheet using UV light.

Langmuir monolayer allows for a two-dimensional nano-scale organization of amphiphilic molecules. We have adapted this technique to measure lateral and transverse conductivity in confined peptide nanosheets for the first time. We reported that two retro-isomers amphipathic peptides form stable monolayers showing a semiconductor-like behavior. Both peptides exhibit the same hydrophobicity and surface stability. They differ in the lateral conductivity and current-voltage due to the asymmetric peptide bond backbone orientation at the interface. Both peptides contain several tyrosines allowing the lateral crosslinking in neighboring molecules induced by UVB. UVB-light induces changes in the lateral conductivity and current-voltage behavior as well as monolayer heterogeneity monitored by Brewster Angle Microscopy. The semiconductor properties depend on the peptide bond backbone orientation and tyrosine crosslinking. Our results indicate that one may design extended nano-sheets with particular electric properties, reminiscent of semiconductors. We propose to exploit such properties for biosensing and neural interfaces.

Secretory Phospholipases A2 in Plants.

Secreted phospholipases (sPLA2s) in plants are a growing group of enzymes that catalyze the hydrolysis of sn-2 glycerophospholipids to lysophospholipids and free fatty acids. Until today, around only 20 sPLA2s were reported from plants. This review discusses the newly acquired information on plant sPLA2s including molecular, biochemical, catalytic, and functional aspects. The comparative analysis also includes phylogenetic, evolutionary, and tridimensional structure. The observations with emphasis in Glycine max sPLA2 are compared with the available data reported for all plants sPLA2s and with those described for animals (mainly from pancreatic juice and venoms sources).

Detecting phospholipase activity with the amphipathic lipid packing sensor motif of ArfGAP1.

The amphipathic lipid packing sensor (ALPS) motif of ArfGAP1 brings this GTPase activating protein to membranes of high curvature. Phospholipases are phospholipid-hydrolyzing enzymes that generate different lipid products that alter the lateral organization of membranes. Here, we evaluate by fluorescence microscopy how in-situ changes of membrane lipid composition driven by the activity of different phospholipases promotes the binding of ALPS. We show that the activity of phospholipase A2, phospholipase C and phospholipase D drastically enhances the binding of ALPS to the weakly-curved membrane of giant liposomes. Our results suggest that the enzymatic activity of phospholipases can modulate the ArfGAP1-mediated intracellular traffic and that amphiphilic peptides such as the ALPS motif can be used to study lipolytic activities at lipid membranes.

The rheological properties of beta amyloid Langmuir monolayers: Comparative studies with melittin peptide.

We determined the rheological properties of ß-amyloid Langmuir films at the air/water interface, a peptide whose interfacial structure is extended ß-sheet, and compared them with those of films composed of Melittin (Mel), which adopts an α-helical conformation at neutral pH. To determine the dilatational and shear moduli we evaluated the response of pure peptide monolayers to an oscillatory anisotropic compressive work. Additionally, a micro-rheological characterization was performed by tracking the diffusion of micrometer sized latex beads onto the interface. This technique allowed us the detection of different rheological behaviour between monolayers presenting a low shear response. Monolayers of the ß-sheet structure-adopting peptides, such as ß-amyloid peptides, exhibited a marked shear (elastic) modulus even at low surface pressures. In contrast, Mel monolayers exhibited negligible shear modulus and the micro-rheological shear response was markedly lower than that observed for either Aß1-40 or Aß1-42 amyloid peptides. When Mel monolayers were formed at the interface of an aqueous solution at pH 11, we observed an increase in both the lateral stability and film viscosity as detected by a slower diffusion of the latex beads, in keeping with an increase in ß-sheet structure at this high pH (verified by ATR and FT-IR measurements). We suggest that the interactions responsible for the marked response upon shear observed for ß-amyloid peptide monolayers are the hydrogen bonds of the ß-sheet structure that can form an infinite planar network at the interface. Conversely, α-helical Mel peptide lack of these inter-molecular interactions and, therefore the shear contribution was negligible. We propose that the secondary structure is important for modulating the rheological behavior of short peptide monolayers regardless of the mass density or surface charge at the surface.

A constant area monolayer method to assess optimal lipid packing for lipolysis tested with several secreted phospholipase A2.

We present an analysis of lipid monolayer hydrolysis at a constant area to assess the optimal lateral surface pressure value (Πopt) and thus, the surface packing density of the lipid, at which the activity of a given lipolytic enzyme is maximal. This isochoric method consists of a measurement of the decrease down to zero of the Πopt of phospholipid substrate monolayer due to continuous hydrolysis using only one reaction compartment. We performed the comparison of both approaches using several commercially available and literature-evaluated sPLA2s. Also, we characterized for the first time the profile of hydrolysis of DLPC monolayers catalyzed by a sPLA2 from Streptomyces violaceoruber and isoenzymes purified from Bothrops diporus venom. One of these viper venom enzymes is a new isoenzyme, partially sequenced by a mass spectrometry approach. We also included the basic myotoxin sPLA2-III from Bothrops asper. Results obtained with the isochoric method and the standard isobaric one produced quite similar values of Πopt, validating the proposal. In addition, we propose a new classification parameter, a lipolytic ratio of hydrolysis at two lateral pressures, 20 mN·m(-1) and 10 mN·m(-1), termed here as LR20/10 index. This index differentiates quite well "high surface pressure" from "low surface pressure" sPLA2s and, by extension; it can be used as a functional criterion for the quality of a certain enzyme. Also, this index could be added to the grouping systematic criteria for the superfamily proposed for phospholipase A2.

Auxins action on Glycine max secretory phospholipase A2 is mediated by the interfacial properties imposed by the phytohormones.

Secretory phospholipase A2 (sPLA2) are soluble enzymes that catalyze the conversion of phospholipids to lysophospholipids and free fatty acids at membrane interfaces. The effect of IAA and IPA auxins over the activity of recombinant sPLA2 isoforms from Glycine max was studied using membrane model systems including mixed micelles and Langmuir lipid monolayers. Both phytohormones stimulate the activity of both plant sPLA2 using DLPC/Triton mixed micelles as substrate. To elucidate the mechanism of action of the phytohormones, we showed that both auxins are able to self-penetrate lipid monolayers and cause an increment in surface pressure and an expansion of lipid/phytohormone mixed interfaces. The stimulating effect of auxins over phospholipase A2 activity was still present when using Langmuir mixed monolayers as organized substrate regardless of sPLA2 source (plant or animal). All the data suggest that the stimulating effect of auxins over sPLA2 is due to a more favorable interfacial environment rather to a direct effect over the enzyme.

Kinetic characterization, optimum conditions for catalysis and substrate preference of secretory phospholipase A2 from Glycine max in model membrane systems.

Two secretory phospholipase A2 (sPLA2s) from Glycine max, GmsPLA2-IXA-1 and GmsPLA2-XIB-2, have been purified as recombinant proteins and the activity was evaluated in order to obtain the optimum conditions for catalysis using mixed micelles and lipid monolayers as substrate. Both sPLA2s showed a maximum enzyme activity at pH 7 and a requirement of Ca(2+) in the micromolar range. These parameters were similar to those found for animal sPLA2s but a surprising optimum temperature for catalysis at 60 °C was observed. The effect of negative interfacial charges on the hydrolysis of organized substrates was evaluated through initial rate measurements using short chain phospholipids with different head groups. The enzymes showed subtle differences in the specificity for phospholipids with different head groups (DLPC, DLPG, DLPE, DLPA) in presence or absence of NaCl. Both recombinant enzymes showed lower activity toward anionic phospholipids and a preference for the zwitterionic ones. The values of the apparent kinetic parameters (Vmax and KM) demonstrated that these enzymes have more affinity for phosphatidylcholine compared with phosphatidylglycerol, in contrast with the results observed for pancreatic sPLA2. A hopping mode of catalysis was proposed for the action of these sPLA2 on mixed phospholipid/triton micelles. On the other hand, Langmuir-monolayers assays indicated an optimum lateral surface pressure for activity in between 13 and 16 mN/m for both recombinant enzymes.

Thyroid hormones-membrane interaction: reversible association of hormones with organized phospholipids with changes in fluidity and dipole potential.

Differential scanning calorimetry (DSC), mixed monomolecular layers and fluorescence spectroscopy techniques were applied to investigate the effect of thyroid hormones (THs) on the biophysical properties of model membranes. We found that both 3,3',5-triiodo-L-thyronine (T3) and 3,5,3',5'-tetraiodo-L-thyronine (T4) induce a broadening of the calorimetric main phase transition profile and reduce the transition enthalpy in liquid-crystalline state of dipalmitoylphosphatylcholine (DPPC) multilamellar vesicles. Tm changes from 41 °C to 40 °C compared to pure DPPC. When the experiments were done by adding THs to preformed multilamellar vesicles a second broader component in the DSC scan also appears at 20 min of incubation and becomes gradually more prominent with time, indicating a progressive alteration of lipid phase induced by THs. Analysis of surface pressure-molecular area isotherms in mixed monolayers of THs with either DPPC or 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) at air-water interface indicated a reduction in molecular area for THs/lipid mixtures at all surface pressures. A substantial decrease in surface potential in mixed lipid/THs monolayers at all surface pressures were observed for both phospholipids without affecting the mixed monolayer integrity. The data of mixed lipid/THs behavior support the establishment of lateral miscibility. Alterations of bidimensional liquid expanded→liquid condensed phase transition observed for DPPC/THs mixed monolayers are compatible with the changes observed in DSC. The transverse movement of THs and the decrease of dipole potential were also observed in single unilamellar vesicles by using appropriate fluorescent probes.

Maintenance and thermal stabilization of NADH dehydrogenase-2 conformation upon elimination of its C-terminal region.

Development of an artificial enzyme with activity and structure comparable to that of natural enzymes is an important goal in biological chemistry. Respiratory NADH dehydrogenase-2 (NDH-2) of Escherichia coli is a peripheral membrane-bound flavoprotein, belonging to a group of enzymes with scarce structural information. By eliminating the C-terminal region of NDH-2, a water soluble version with significant enzymatic activity was previously obtained. Here, NDH-2 structural features were established, in comparison to those of the truncated version. Far-UV circular dichroism, Fourier transform infrared spectroscopy and limited proteolysis analysis showed that the overall structure of both proteins was similar at 30 °C. Experimental data agree with the predicted NDH-2 structure (PDB: 1OZK). The absence of C-terminal region stabilized in ∼5-10 °C the truncated protein conformation. However, truncation impaired enzymatic activity at low temperatures, probably due to the weak interaction of the mutant protein with FAD cofactor.

Cloning and functional expression of secreted phospholipases A(2) from Bothrops diporus (Yarará Chica).

Bothrops diporus is a very common viper in Argentina. At present, no complete sequence of secreted phospholipase A(2) (sPLA(2)) from this snake has been reported. We have cloned two sPLA(2) isoenzymes as well as a putative sPLA(2)-like myotoxin from venom gland. The two sPLA(2) were expressed as inclusion bodies in Escherichia coli with an N-terminal tag of ubiquitin. After in vitro renaturation and cleavage step, using an ubiquitin specific peptidase, the recombinants exhibited sPLA(2) activity when analyzed by means of Langmuir dilauroylphosphatidylcholine monolayers as substrate. Both enzymes have a similar surface pressure-activity profile when compared with non-recombinant purified isoforms. To our knowledge, this is the first time that analysis of optimal lateral pressure of substrate monolayers by using the surface barostat technique is performed on recombinant sPLA(2)s.