Dynamic organization of mixed Langmuir films of glucose oxidase and stearylamine at the air-water interface.

The structure and the dynamic organization of a mixed Langmuir film of glucose oxidase and stearylamine at the air-water interface have been studied. The film has been first characterized at the air-water interface by surface pressure/area isotherms. The dynamics of the mixed film was studied by following the evolution of the film area at a constant pressure and the evolution of the pressure at a constant area. After transfer of the films on solid substrates, the chemical composition of the mixed film has been quantified by UV-vis and IR spectroscopies. These characterizations were carried out in order to study the incorporation of glucose oxidase into the stearylamine film, and its influence on the structural evolution of the film. From these results, the dynamic organization of this mixed film may be described. For short times, glucose oxidase molecules interact with stearylamine molecules in solution or at the interface; these interactions would lead to the formation of a complex between stearylamine and glucose oxidase molecules. For long times (at least 3 h), a homogeneous mixed film constituted essentially of this complex is obtained at the air-water interface. A detailed analysis by atomic force microscopy allowed us to support this model and the existence of the glucose oxidase/stearylamine complex.

Hypothesis: hyperstructures regulate bacterial structure and the cell cycle.

A myriad different constituents or elements (genes, proteins, lipids, ions, small molecules etc.) participate in numerous physico-chemical processes to create bacteria that can adapt to their environments to survive, grow and, via the cell cycle, reproduce. We explore the possibility that it is too difficult to explain cell cycle progression in terms of these elements and that an intermediate level of explanation is needed. This level is that of hyperstructures. A hyperstructure is large, has usually one particular function, and contains many elements. Non-equilibrium, or even dissipative, hyperstructures that, for example, assemble to transport and metabolize nutrients may comprise membrane domains of transporters plus cytoplasmic metabolons plus the genes that encode the hyperstructure's enzymes. The processes involved in the putative formation of hyperstructures include: metabolite-induced changes to protein affinities that result in metabolon formation, lipid-organizing forces that result in lateral and transverse asymmetries, post-translational modifications, equilibration of water structures that may alter distributions of other molecules, transertion, ion currents, emission of electromagnetic radiation and long range mechanical vibrations. Equilibrium hyperstructures may also exist such as topological arrays of DNA in the form of cholesteric liquid crystals. We present here the beginning of a picture of the bacterial cell in which hyperstructures form to maximize efficiency and in which the properties of hyperstructures drive the cell cycle.

Hypothesis: the meeting place model for prion disease.

Prions are responsible for spongiform diseases such as scrapie and bovine spongiform encephalopathy. It is now generally accepted that the disease mechanism involves the conversion from the normal form, PrPC, to the pathogenic form, PrPSc, and that this isoform is infectious. In the case of scrapie, 15 different forms of the disease have been described and some of these different phenotypes can be conferred by infectious prions that are themselves encoded by normal genes. We propose here that a prion with an altered structure has a correspondingly altered preference for lipids; this altered preference creates a proteolipid domain containing different lipids and other factors such as chaperonins and enzymes responsible for post-translational modifications. Normal prions associated with this abnormal domain adopt the conformation dictated by its lipidic composition (and by the other factors present) and so acquire the lipidic preference of the original pathogenic prions. These transformed prions could then create new proteolipid domains. This process may be considered as semi-conservative replication in which prion and lipids are analogous to the Watson and Crick strands and the proteolipid domain to the double helix itself.

Structure and supramolecular architecture of membrane channel-forming peptides.

Peptides gathering together to induce channels in lipid bilayers may be classified in several categories according to the spatial structures involved. For example, gramicidin A forms intramolecular tubes, alamethicin, bundles of helical rods with intermolecular pores, porins (being proteins, properly speaking) are rich in beta-sheets that may form barrels, whereas cyclic peptides might stack together resulting in the formation of pores. The chemical structure of these compounds is now well characterized. The transmembrane electrical signals that they transmit are also typical of the particular supramolecular configurations (or architecture). Investigations in this field are thus relevant to structure-function relationship studies due to the availability of natural or synthetic analogues allowing the measurement of the influence of physico-chemical parameters upon the energy profiles of the pores. Consequently, questions such as the existence and probabilities of conductance substrates, their voltage-dependence and their ion or molecular selectivity can be tackled. Today, the loosest aspect of these studies lies in the actual molecular conformations and architecture in the membranes of the peptide aggregates, the knowledge of which remains imprecise, even 'at rest' in the best-studied cases. This review attempts to point out still unresolved questions and to propose some plausible approaches concerning, for example: 1) the configurations of the molecular aggregates responsible for ion transfer; 2) the mechanisms for channel-opening and closing (gating); 3) the eventual cooperative phenomena between channels, via the bilayer or interfacial components. Possible applications of these structures will be tentatively outlined.

Theoretical study of mechano-chemical couplings in a compartmental enzyme system. I. Analytical treatment.

This paper deals with theoretical aspects of the volume changes of a system in which diffusion, convection and reaction processes are coupled. This study involves a material able to swell in the presence of a chemical effector produced by an enzyme reaction. Three limiting factors of volume change rate were considered: fluid flow, diffusion or reaction limitations. Dimensionless diffusion-reaction and diffusion-convection parameters were introduced to allow quantitative predictions in limit cases. The steady states appear to be independent of convection processes; however, the transient states depend on diffusion, convection and reaction processes.

Conductivity-mediated regulation in a compartmental enzyme system. Superactivation and conductance bistability.

This paper deals with the 'conductivity effect' which results from the action of an electric current in a compartmental system involving a chemical reaction producing ionic species. This effect, due to the difference between internal and external conductivities, leads to the accumulation of ionic species inside the reactive compartment. Different results are obtained depending on the considered kinetics: amplification of reaction rate increase, and chemical and conductance bistabilities.

[Organizing role of electric fields in structured enzymatic systems]. / Etude du rôle organisateur des champs électriques dans les systèmes enzymatiques structurés.

The organizing role of direct electric fields in structured enzyme systems is studied theoretically. Membrane or compartmented structures are considered with Michaelian proton producing kinetics. Periodic and aperiodic instabilities may develop with the action of electric fields.

pH feedback control of enzyme membranes.

pH feedback on immobilized enzymes is theoretically examined with respect to substrate and pH levels, strength of acids produced by the reaction, buffering and asymmetry of the system. All the productions of proton by the different reactions are taken into account by using a 'symbolic species' H. The system of differential diffusion-reaction equations is then integrated using numerical methods. The local 'effective enzyme activity' modulated by an acidity factor enables us to predict and quantify evolutions of the systems: NonMichaelian behavior of an immobilized Michaelis-Menten-type enzyme is shown, even when pH back-actions are excluded; the analysis of intramembrane pH profiles shows that the shift of the optimal pH is a complex function of the substrate and pH levels, the intrinsic pH dependence of the enzyme, and the membrane characteristics. This study may easily be transposed to other types of effector such as divalent cations and used in examining self-regulations of multienzyme systems where pH-active reactions are involved.

Monitoring of artificial enzyme membrane systems by electric currents. I. Analytical treatment with direct current and buffered conductivity.

Continuous electric fields (E) modify the transport flows and the intramembrane concentration profiles of protons or of ionic substrates or cofactors (inhibitors). These "mediators' induce variations in enzyme activity, quantifiable by a generalized Damköhler group II psi distinguishing electrotransport reactions from diffusion reactions. For three typical reaction schemas, using only one mediator, the steady-state equations have been established. Depending on boundary conditions, the direction of electric current (for asymmetrical systems) and the value of psi, activations, inhibitions or activations followed by inactivations have been found. With buffered conductivity (supporting electrolyte), the limiting concentration profiles (E leads to infinity) are uniformly equal to the boundary values; i.e., diffusion constraints are suppressed and the regime is controlled by the reaction. The calculations give the relative activity variations for partially suppressed transport controls.