Cell-Penetrating Peptide-Mediated Biomolecule Transportation in Artificial Lipid Vesicles and Living Cells.

Signal transduction and homeostasis are regulated by complex protein interactions in the intracellular environment. Therefore, the transportation of impermeable macromolecules (nucleic acids, proteins, and drugs) that control protein interactions is essential for modulating cell functions and therapeutic applications. However, macromolecule transportation across the cell membrane is not easy because the cell membrane separates the intra/extracellular environments, and the types of molecular transportation are regulated by membrane proteins. Cell-penetrating peptides (CPPs) are expected to be carriers for molecular transport. CPPs can transport macromolecules into cells through endocytosis and direct translocation. The transport mechanism remains largely unclear owing to several possibilities. In this review, we describe the methods for investigating CPP conformation, translocation, and cargo transportation using artificial membranes. We also investigated biomolecular transport across living cell membranes via CPPs. Subsequently, we show not only the biochemical applications but also the synthetic biological applications of CPPs. Finally, recent progress in biomolecule and nanoparticle transportation via CPPs into specific tissues is described from the viewpoint of drug delivery. This review provides the opportunity to discuss the mechanism of biomolecule transportation through these two platforms.

Probing Single-molecule Interfacial Electron Transfer Inside a Single Lipid Vesicle.

Inhomogeneity in single molecule electron transfer at the surface of lipid in a single vesicle has been explored by single molecule spectroscopic technique. In our study we took Di-methyl aniline (DMA), as the electron donor (D) and three different organic dyes as acceptor. These dyes are C153, C480 and C152 and they reside in different regions in the vesicle depending upon their preference of residence. For each probe, we found fluctuations in the single-molecule fluorescence decay, which are attributed to the variation in the reactivity of interfacial electron transfer. We found a non-exponential auto-correlation fluctuation of the intensity of the probe, which is ascribed to the kinetic disorder in the rate of electron transfer. We have also shown the power law distribution of the dark state (off time), which obeys the levy's statistics. We found a shift in lifetime distribution for the probe (C153) from 3.9 ns to 3.5 ns. This observed quenching is due to the dynamic electron transfer. We observed the kinetic disorderness in the electron transfer reaction for each dye. This source of fluctuation in electron transfer rate may be ascribed to the inherent fluctuation, occurring on the time scale of ~ 1.1 ms (for C153) of the vesicle, containing lipids.

Single-vesicle imaging reveals lipid-selective and stepwise membrane disruption by monomeric α-synuclein.

The interaction of the neuronal protein α-synuclein with lipid membranes appears crucial in the context of Parkinson's disease, but the underlying mechanistic details, including the roles of different lipids in pathogenic protein aggregation and membrane disruption, remain elusive. Here, we used single-vesicle resolution fluorescence and label-free scattering microscopy to investigate the interaction kinetics of monomeric α-synuclein with surface-tethered vesicles composed of different negatively charged lipids. Supported by a theoretical model to account for structural changes in scattering properties of surface-tethered lipid vesicles, the data demonstrate stepwise vesicle disruption and asymmetric membrane deformation upon α-synuclein binding to phosphatidylglycerol vesicles at protein concentrations down to 10 nM (∼100 proteins per vesicle). In contrast, phosphatidylserine vesicles were only marginally affected. These insights into structural consequences of α-synuclein interaction with lipid vesicles highlight the contrasting roles of different anionic lipids, which may be of mechanistic relevance for both normal protein function (e.g., synaptic vesicle binding) and dysfunction (e.g., mitochondrial membrane interaction).

Light-Triggered Cargo Loading and Division of DNA-Containing Giant Unilamellar Lipid Vesicles.

A minimal synthetic cell should contain a substrate for information storage and have the capability to divide. Notable efforts were made to assemble functional synthetic cells from the bottom up, however often lacking the capability to reproduce. Here, we develop a mechanism to fully control reversible cargo loading and division of DNA-containing giant unilamellar vesicles (GUVs) with light. We make use of the photosensitizer Chlorin e6 (Ce6) which self-assembles into lipid bilayers and leads to local lipid peroxidation upon illumination. On the time scale of minutes, illumination induces the formation of transient pores, which we exploit for cargo encapsulation or controlled release. In combination with osmosis, complete division of two daughter GUVs can be triggered within seconds of illumination due to a spontaneous curvature increase. We ultimately demonstrate the division of a selected DNA-containing GUV with full spatiotemporal control-proving the relevance of the division mechanism for bottom-up synthetic biology.

Membrane potential is vital for rapid permeabilization of plasma membranes and lipid bilayers by the antimicrobial peptide lactoferricin B.

Lactoferricin B (LfcinB) is a cationic antimicrobial peptide, and its capacity to damage the bacterial plasma membrane is suggested to be a main factor in LfcinB's antimicrobial activity. However, the specific processes and mechanisms in LfcinB-induced membrane damage are unclear. In this report, using confocal laser-scanning microscopy, we examined the interaction of LfcinB with single Escherichia coli cells and spheroplasts containing the water-soluble fluorescent probe calcein in the cytoplasm. LfcinB induced rapid calcein leakage from single E. coli cells and from single spheroplasts, indicating that LfcinB interacts directly with the plasma membrane and induces its rapid permeabilization. The proton ionophore carbonyl cyanide m-chlorophenylhydrazone suppressed this leakage. Next, we used the single giant unilamellar vesicle (GUV) method to examine LfcinB's interaction with GUVs comprising polar lipid extracts of E. coli containing a water-soluble fluorescent probe, Alexa Fluor 647 hydrazide (AF647). We observed that LfcinB stochastically induces local rupture in single GUVs, causing rapid AF647 leakage; however, higher LfcinB concentrations were required for AF647 leakage from GUVs than from E. coli cells and spheroplasts. To identify the reason for this difference, we examined the effect of membrane potential on LfcinB-induced pore formation, finding that the rate of LfcinB-induced local rupture in GUVs increases greatly with increasing negative membrane potential. These results indicate that membrane potential plays an important role in LfcinB-induced local rupture of lipid bilayers and rapid permeabilization of E. coli plasma membranes. On the basis of these results, we discuss the mode of action of LfcinB's antimicrobial activity.

Intrinsically disordered proteins in synaptic vesicle trafficking and release.

The past few years have resulted in an increased awareness and recognition of the prevalence and roles of intrinsically disordered proteins and protein regions (IDPs and IDRs, respectively) in synaptic vesicle trafficking and exocytosis and in overall synaptic organization. IDPs and IDRs constitute a class of proteins and protein regions that lack stable tertiary structure, but nevertheless retain biological function. Their significance in processes such as cell signaling is now well accepted, but their pervasiveness and importance in other areas of biology are not as widely appreciated. Here, we review the prevalence and functional roles of IDPs and IDRs associated with the release and recycling of synaptic vesicles at nerve terminals, as well as with the architecture of these terminals. We hope to promote awareness, especially among neuroscientists, of the importance of this class of proteins in these critical pathways and structures. The examples discussed illustrate some of the ways in which the structural flexibility conferred by intrinsic protein disorder can be functionally advantageous in the context of cellular trafficking and synaptic function.

In Vitro Assays: Friends or Foes of Cell-Penetrating Peptides.

The cell membrane is a complex and highly regulated system that is composed of lipid bilayer and proteins. One of the main functions of the cell membrane is the regulation of cell entry. Cell-penetrating peptides (CPPs) are defined as peptides that can cross the plasma membrane and deliver their cargo inside the cell. The uptake of a peptide is determined by its sequence and biophysicochemical properties. At the same time, the uptake mechanism and efficiency are shown to be dependent on local peptide concentration, cell membrane lipid composition, characteristics of the cargo, and experimental methodology, suggesting that a highly efficient CPP in one system might not be as productive in another. To better understand the dependence of CPPs on the experimental system, we present a review of the in vitro assays that have been employed in the literature to evaluate CPPs and CPP-cargos. Our comprehensive review suggests that utilization of orthogonal assays will be more effective for deciphering the true ability of CPPs to translocate through the membrane and enter the cell cytoplasm.

Docosahexaenoic acid lowers cardiac mitochondrial enzyme activity by replacing linoleic acid in the phospholipidome.

Cardiac mitochondrial phospholipid acyl chains regulate respiratory enzymatic activity. In several diseases, the rodent cardiac phospholipidome is extensively rearranged; however, whether specific acyl chains impair respiratory enzyme function is unknown. One unique remodeling event in the myocardium of obese and diabetic rodents is an increase in docosahexaenoic acid (DHA) levels. Here, we first confirmed that cardiac DHA levels are elevated in diabetic humans relative to controls. We then used dietary supplementation of a Western diet with DHA as a tool to promote cardiac acyl chain remodeling and to study its influence on respiratory enzyme function. DHA extensively remodeled the acyl chains of cardiolipin (CL), mono-lyso CL, phosphatidylcholine, and phosphatidylethanolamine. Moreover, DHA lowered enzyme activities of respiratory complexes I, IV, V, and I+III. Mechanistically, the reduction in enzymatic activities were not driven by a dramatic reduction in the abundance of supercomplexes. Instead, replacement of tetralinoleoyl-CL with tetradocosahexaenoyl-CL in biomimetic membranes prevented formation of phospholipid domains that regulate enzyme activity. Tetradocosahexaenoyl-CL inhibited domain organization due to favorable Gibbs free energy of phospholipid mixing. Furthermore, in vitro substitution of tetralinoleoyl-CL with tetradocosahexaenoyl-CL blocked complex-IV binding. Finally, reintroduction of linoleic acid, via fusion of phospholipid vesicles to mitochondria isolated from DHA-fed mice, rescued the major losses in the mitochondrial phospholipidome and complexes I, IV, and V activities. Altogether, our results show that replacing linoleic acid with DHA lowers select cardiac enzyme activities by potentially targeting domain organization and phospholipid-protein binding, which has implications for the ongoing debate about polyunsaturated fatty acids and cardiac health.

Proteolipid domains form in biomimetic and cardiac mitochondrial vesicles and are regulated by cardiolipin concentration but not monolyso-cardiolipin.

Cardiolipin (CL) is an anionic phospholipid mainly located in the inner mitochondrial membrane, where it helps regulate bioenergetics, membrane structure, and apoptosis. Localized, phase-segregated domains of CL are hypothesized to control mitochondrial inner membrane organization. However, the existence and underlying mechanisms regulating these mitochondrial domains are unclear. Here, we first isolated detergent-resistant cardiac mitochondrial membranes that have been reported to be CL-enriched domains. Experiments with different detergents yielded only nonspecific solubilization of mitochondrial phospholipids, suggesting that CL domains are not recoverable with detergents. Next, domain formation was investigated in biomimetic giant unilamellar vesicles (GUVs) and newly synthesized giant mitochondrial vesicles (GMVs) from mouse hearts. Confocal fluorescent imaging revealed that introduction of cytochrome c into membranes promotes macroscopic proteolipid domain formation associated with membrane morphological changes in both GUVs and GMVs. Domain organization was also investigated after lowering tetralinoleoyl-CL concentration and substitution with monolyso-CL, two common modifications observed in cardiac pathologies. Loss of tetralinoleoyl-CL decreased proteolipid domain formation in GUVs, because of a favorable Gibbs-free energy of lipid mixing, whereas addition of monolyso-CL had no effect on lipid mixing. Moreover, murine GMVs generated from cardiac acyl-CoA synthetase-1 knockouts, which have remodeled CL acyl chains, did not perturb proteolipid domains. Finally, lowering the tetralinoleoyl-CL content had a stronger influence on the oxidation status of cytochrome c than did incorporation of monolyso-CL. These results indicate that proteolipid domain formation in the cardiac mitochondrial inner membrane depends on tetralinoleoyl-CL concentration, driven by underlying lipid-mixing properties, but not the presence of monolyso-CL.

An intrinsically disordered domain in Polaribacter irgensii KOPRI 22228 CspB confers extraordinary freeze-tolerance.

Organisms living in extremely cold environments possess mechanisms to survive low temperatures. Among the known cold-induced genes, cold-shock proteins (Csps) are the most prominent. A csp-homologous gene, cspBPi, has been cloned from the Arctic bacterium Polaribacter irgensii KOPRI 22228, and overexpression of this gene greatly increased the freezing tolerance of its host. This protein consists of a unique N-terminal domain and a well conserved C-terminal cold shock domain. To elucidate the detailed mechanisms involved in the extraordinary freeze-tolerance conferred by CspBPi, we identified the responsible domain by mutational analysis. Changes of residues in the cold shock domain that are crucial for binding RNA or single-stranded DNA did not impair the ability of the host to survive freezing stress. All domain-shuffled CspBPi variants containing the N-terminal domain retained the ability to confer superior freeze-tolerance. Slow electrophoretic mobility and far-UV circular dichroism spectra of the N-terminal domain suggested an intrinsically disordered structure for this region. The N-terminal domain also bound to lipid vesicles in vitro. This lipid vesicle binding characteristic is shared with other intrinsically disordered proteins, such as α-synuclein and plant dehydrins, known to confer cold-tolerance when overexpressed, suggesting a mechanism for cold-survival through membrane binding.

Epsin N-terminal Homology Domain (ENTH) Activity as a Function of Membrane Tension.

The epsin N-terminal homology domain (ENTH) is a major player in clathrin-mediated endocytosis. To investigate the influence of initial membrane tension on ENTH binding and activity, we established a bilayer system based on adhered giant unilamellar vesicles (GUVs) to be able to control and adjust the membrane tension σ covering a broad regime. The shape of each individual adhered GUV as well as its adhesion area was monitored by spinning disc confocal laser microscopy. Control of σ in a range of 0.08-1.02 mN/m was achieved by altering the Mg(2+) concentration in solution, which changes the surface adhesion energy per unit area of the GUVs. Specific binding of ENTH to phosphatidylinositol 4,5-bisphosphate leads to a substantial increase in adhesion area of the sessile GUV. At low tension (<0.1 mN/m) binding of ENTH can induce tubular structures, whereas at higher membrane tension the ENTH interaction deflates the sessile GUV and thereby increases the adhesion area. The increase in adhesion area is mainly attributed to a decrease in the area compressibility modulus KA We propose that the insertion of the ENTH helix-0 into the membrane is largely responsible for the observed decrease in KA, which is supported by the observation that the mutant ENTH L6E shows a reduced increase in adhesion area. These results demonstrate that even in the absence of tubule formation, the area compressibility modulus and, as such, the bending rigidity of the membrane is considerably reduced upon ENTH binding. This renders membrane bending and tubule formation energetically less costly.

Lipid-anchored Synaptobrevin Provides Little or No Support for Exocytosis or Liposome Fusion.

SNARE proteins catalyze many forms of biological membrane fusion, including Ca(2+)-triggered exocytosis. Although fusion mediated by SNAREs generally involves proteins anchored to each fusing membrane by a transmembrane domain (TMD), the role of TMDs remains unclear, and previous studies diverge on whether SNAREs can drive fusion without a TMD. This issue is important because it relates to the question of the structure and composition of the initial fusion pore, as well as the question of whether SNAREs mediate fusion solely by creating close proximity between two membranes versus a more active role in transmitting force to the membrane to deform and reorganize lipid bilayer structure. To test the role of membrane attachment, we generated four variants of the synaptic v-SNARE synaptobrevin-2 (syb2) anchored to the membrane by lipid instead of protein. These constructs were tested for functional efficacy in three different systems as follows: Ca(2+)-triggered dense core vesicle exocytosis, spontaneous synaptic vesicle exocytosis, and Ca(2+)-synaptotagmin-enhanced SNARE-mediated liposome fusion. Lipid-anchoring motifs harboring one or two lipid acylation sites completely failed to support fusion in any of these assays. Only the lipid-anchoring motif from cysteine string protein-α, which harbors many lipid acylation sites, provided support for fusion but at levels well below that achieved with wild type syb2. Thus, lipid-anchored syb2 provides little or no support for exocytosis, and anchoring syb2 to a membrane by a TMD greatly improves its function. The low activity seen with syb2-cysteine string protein-α may reflect a slower alternative mode of SNARE-mediated membrane fusion.

Identification of a Membrane-bound Prepore Species Clarifies the Lytic Mechanism of Actinoporins.

Pore-forming toxins (PFTs) are cytolytic proteins belonging to the molecular warfare apparatus of living organisms. The assembly of the functional transmembrane pore requires several intermediate steps ranging from a water-soluble monomeric species to the multimeric ensemble inserted in the cell membrane. The non-lytic oligomeric intermediate known as prepore plays an essential role in the mechanism of insertion of the class of ß-PFTs. However, in the class of α-PFTs, like the actinoporins produced by sea anemones, evidence of membrane-bound prepores is still lacking. We have employed single-particle cryo-electron microscopy (cryo-EM) and atomic force microscopy to identify, for the first time, a prepore species of the actinoporin fragaceatoxin C bound to lipid vesicles. The size of the prepore coincides with that of the functional pore, except for the transmembrane region, which is absent in the prepore. Biochemical assays indicated that, in the prepore species, the N terminus is not inserted in the bilayer but is exposed to the aqueous solution. Our study reveals the structure of the prepore in actinoporins and highlights the role of structural intermediates for the formation of cytolytic pores by an α-PFT.

Entry of Bluetongue Virus Capsid Requires the Late Endosome-specific Lipid Lysobisphosphatidic Acid.

The entry of viruses into host cells is one of the key processes of infection. The mechanisms of cellular entry for enveloped virus have been well studied. The fusion proteins as well as the facilitating cellular lipid factors involved in the viral fusion entry process have been well characterized. The process of non-enveloped virus cell entry, in comparison, remains poorly defined, particularly for large complex capsid viruses of the family Reoviridae, which comprises a range of mammalian pathogens. These viruses enter cells without the aid of a limiting membrane and thus cannot fuse with host cell membranes to enter cells. Instead, these viruses are believed to penetrate membranes of the host cell during endocytosis. However, the molecular mechanism of this process is largely undefined. Here we show, utilizing an in vitro liposome penetration assay and cell biology, that bluetongue virus (BTV), an archetypal member of the Reoviridae, utilizes the late endosome-specific lipid lysobisphosphatidic acid for productive membrane penetration and viral entry. Further, we provide preliminary evidence that lipid lysobisphosphatidic acid facilitates pore expansion during membrane penetration, suggesting a mechanism for lipid factor requirement of BTV. This finding indicates that despite the lack of a membrane envelope, the entry process of BTV is similar in specific lipid requirements to enveloped viruses that enter cells through the late endosome. These results are the first, to our knowledge, to demonstrate that a large non-enveloped virus of the Reoviridae has specific lipid requirements for membrane penetration and host cell entry.

In Vitro Activity of a Purified Natural Anion Channelrhodopsin.

Natural anion channelrhodopsins (ACRs) recently discovered in cryptophyte algae are the most active rhodopsin channels known. They are of interest both because of their unique natural function of light-gated chloride conductance and because of their unprecedented efficiency of membrane hyperpolarization for optogenetic neuron silencing. Light-induced currents of ACRs have been studied in HEK cells and neurons, but light-gated channel conductance of ACRs in vitro has not been demonstrated. Here we report light-induced chloride channel activity of a purified ACR protein reconstituted in large unilamellar vesicles (LUVs). EPR measurements establish that the channels are inserted uniformly "inside-out" with their cytoplasmic surface facing the medium of the LUV suspension. We show by time-resolved flash spectroscopy that the photochemical reaction cycle of a functional purified ACR from Guillardia theta (GtACR1) in LUVs exhibits similar spectral shifts, indicating similar photocycle intermediates as GtACR1 in detergent micelles. Furthermore, the photocycle rate is dependent on electric potential generated by chloride gradients in the LUVs in the same manner as in voltage-clamped animal cells. We confirm with this system that, in contrast to cation-conducting channelrhodopsins, opening of the channel occurs prior to deprotonation of the Schiff base. However, the photointermediate transitions in the LUVs exhibit faster kinetics. The ACR-incorporated LUVs provide a purified defined system amenable to EPR, optical and vibrational spectroscopy, and fluorescence resonance energy transfer measurements of structural changes of ACRs with the molecules in a demonstrably functional state.

Diclofenac Loaded Lipid Nanovesicles Prepared by Double Solvent Displacement for Skin Drug Delivery.

PURPOSE: Herein, we detail a promising strategy of nanovesicle preparation based on control of phospholipid self-assembly: the Double Solvent Displacement. A systematic study was conducted and diclofenac as drug model encapsulated. In vitro skin studies were carried out to identify better formulation for dermal/transdermal delivery. METHODS: This method consists in two solvent displacements. The first one, made in a free water environment, has allowed triggering a phospholipid pre-organization. The second one, based on the diffusion into an aqueous phase has led to liposome formation. RESULTS: Homogeneous liposomes were obtained with a size close to 100 nm and a negative zeta potential around -40 mV. After incorporation of acid diclofenac, we obtained nanoliposomes with a size between 101 ± 45 and 133 ± 66 nm, a zeta potential between 34 ± 2 and 49 ± 3 mV, and the encapsulation efficiency (EE%) was between 58 ± 3 and 87 ± 5%. In vitro permeation studies showed that formulation with higher EE% dispayed the higher transdermal passage (18,4% of the applied dose) especially targeting dermis and beyond. CONCLUSIONS: Our results suggest that our diclofenac loaded lipid vesicles have significant potential as transdermal skin drug delivery system. Here, we produced cost effective lipid nanovesicles in a merely manner according to a process easily transposable to industrial scale. Graphical Abstract ᅟ.

Characterization of a lipid-based jumbo phage compartment as a hub for early phage infection.

Viral genomes are most vulnerable to cellular defenses at the start of the infection. A family of jumbo phages related to phage ΦKZ, which infects Pseudomonas aeruginosa, assembles a protein-based phage nucleus to protect replicating phage DNA, but how it is protected prior to phage nucleus assembly is unclear. We find that host proteins related to membrane and lipid biology interact with injected phage protein, clustering in an early phage infection (EPI) vesicle. The injected virion RNA polymerase (vRNAP) executes early gene expression until phage genome separation from the vRNAP and the EPI vesicle, moving into the nascent proteinaceous phage nucleus. Enzymes involved in DNA replication and CRISPR/restriction immune nucleases are excluded by the EPI vesicle. We propose that the EPI vesicle is rapidly constructed with injected phage proteins, phage DNA, host lipids, and host membrane proteins to enable genome protection, early transcription, localized translation, and to ensure faithful genome transfer to the proteinaceous nucleus.

NMR investigation of the equilibrium partitioning of a water-soluble bile salt protein carrier to phospholipid vesicles.

Membrane binding by cytosolic fatty acid binding proteins (FABP) appears to constitute a key step of intracellular lipid trafficking. We applied NMR spectroscopy to study the partitioning of a water-soluble bile acid binding protein (BABP), belonging to the FABP family, between its free and lipid-vesicle-bound states. As the lipid-bound protein was NMR-invisible, the signals of the free biomolecule were analyzed to obtain quantitative information on binding affinity and steady-state kinetics. The data indicated a reversible interaction of BABP with anionic vesicles occurring in a very slow exchange regime on the NMR time scale. The approximate binding epitope was demonstrated from results on BABP samples in which different positively charged lysine residues were mutated to neutral alanines. H/D exchange measurements indicated a higher exposure to solvent for the core amino acid residues in the liposome-bound state. Finally, the BABP-liposome interaction was also investigated for the first time through an MRI-chemical exchange saturation transfer experiment that has potential applications not only in the field of biology, but also in biomedicine, bioanalytical chemistry, and nanotechnology.

External surface area determination of lipid vesicles using trinitrobenzene sulfonate and ultraviolet/visible spectrophotometry.

The lamellarity of liposomes is an important parameter to be controlled in liposomal delivery-release applications. A practical estimate of the degree of liposome lamellarity can be obtained by measuring the relative external surface area of the liposomes using a chemical assay. All such assays are based on a signal change caused by exposed marker lipids on reaction with a specific externally added reagent. However, a quantitative determination is often distorted by background reactions and contributions of internal lipid labeling. In the so-called TNBS assay, the marker lipid is phosphatidylethanolamine (PE) and the externally added reagent is TNBS (2,4,6-trinotrobenzene sulfonate). Mechanistic aspects of the TNBS assay were considered for improving the assay. Internal lipid labeling via PE flip-flop and/or TNBS permeation was minimal not only in cholesterol-containing liposomes but also in cholesterol-free liposomes if in the latter case membrane fluidity was decreased by slightly increasing the PE content. Compared with earlier versions of the TNBS assay, the amount of marker lipid and the time for analysis could be reduced considerably. The elaborated protocol was also applied to liposomes prepared from lipidic egg yolk isolates, offering a simple and inexpensive method for the development and in-process control of new liposome formation technologies.

The Impact of Enzyme Orientation and Electrode Topology on the Catalytic Activity of Adsorbed Redox Enzymes.

It is well established that the structural details of electrodes and their interaction with adsorbed enzyme influences the interfacial electron transfer rate. However, for nanostructured electrodes, it is likely that the structure also impacts on substrate flux near the adsorbed enzymes and thus catalytic activity. Furthermore, for enzymes converting macro-molecular substrates it is possible that the enzyme orientation determines the nature of interactions between the adsorbed enzyme and substrate and therefore catalytic rates. In essence the electrode may impede substrate access to the active site of the enzyme. We have tested these possibilities through studies of the catalytic performance of two enzymes adsorbed on topologically distinct electrode materials. Escherichia coli NrfA, a nitrite reductase, was adsorbed on mesoporous, nanocrystalline SnO2 electrodes. CymA from Shewanella oneidensis MR-1 reduces menaquinone-7 within 200 nm sized liposomes and this reaction was studied with the enzyme adsorbed on SAM modified ultra-flat gold electrodes.