Cyanobacteriochromes are members of the phytochrome superfamily. In contrast to classical phytochromes, these small photosensors display a considerable variability of electronic absorption maxima. We have studied the light-induced conversions of the second GAF domain of AnPixJ, AnPixJg2, a phycocyanobilin-binding protein from the cyanobacterium Anabaena PCC 7120, using low-temperature resonance Raman spectroscopy combined with molecular dynamics simulations. AnPixJg2 is formed biosynthetically as a red-absorbing form (Pr) and can be photoconverted into a green-absorbing form (Pg). Forward and backward phototransformations involve the same reaction sequences and intermediates of similar cofactor structures as the corresponding processes in canonical phytochromes, including a transient cofactor deprotonation. Whereas the cofactor of the Pr state shows far-reaching similarities to the Pr states of classical phytochromes, the Pg form displays significant upshifts of the methine bridge stretching frequencies concomitant to the hypsochromically shifted absorption maximum. However, the cofactor in Pg is protonated and adopts a conformation very similar to the Pfr state of classical phytochromes. The spectral differences are probably related to an increased solvent accessibility of the chromophore which may reduce the π-electron delocalization in the phycocyanobilin and thus raise the energies of the first electronic transition and the methine bridge stretching modes. Molecular dynamics simulations suggest that the Z → E photoisomerization of the chromophore at the C-D methine bridge alters the interactions with the nearby Trp90 which in turn may act as a gate, allowing the influx of water molecules into the chromophore pocket. Such a mechanism of color tuning AnPixJg2 is unique among the cyanobacteriochromes studied so far.

Human sulfite oxidase (hSO) was immobilised on SAM-coated silver electrodes under preservation of the native heme pocket structure of the cytochrome b5 (Cyt b5) domain and the functionality of the enzyme. The redox properties and catalytic activity of the entire enzyme were studied by surface enhanced resonance Raman (SERR) spectroscopy and cyclic voltammetry (CV) and compared to the isolated heme domain when possible. It is shown that heterogeneous electron transfer and catalytic activity of hSO sensitively depend on the local environment of the enzyme. Increasing the ionic strength of the buffer solution leads to an increase of the heterogeneous electron transfer rate from 17 s(-1) to 440 s(-1) for hSO as determined by SERR spectroscopy. CV measurements demonstrate an increase of the apparent turnover rate for the immobilised hSO from 0.85 s(-1) in 100 mM buffer to 5.26 s(-1) in 750 mM buffer. We suggest that both effects originate from the increased mobility of the surface-bound enzyme with increasing ionic strength. In agreement with surface potential calculations we propose that at high ionic strength the enzyme is immobilised via the dimerisation domain to the SAM surface. The flexible loop region connecting the Moco and the Cyt b5 domain allows alternating contact with the Moco interaction site and the SAM surface, thereby promoting the sequential intramolecular and heterogeneous electron transfer from Moco via Cyt b5 to the electrode. At lower ionic strength, the contact time of the Cyt b5 domain with the SAM surface is longer, corresponding to a slower overall electron transfer process.

AbbreviationsIAA, iodoacetamide, FDH, formate dehydrogenase, Moco, molybdenum cofactor, MGD, molybdopterin guanine dinucleotide, MV, methyl viologen, SeCys, selenocysteine, XAS, X-ray absorption spectroscopy 3 AbstractFormate dehydrogenases (FDHs) are capable to perform the reversible oxidation of formate and are enzymes of high interest for fuel cell applications and for the production of reduced carbon compounds as energy source from CO 2 . Metalcontaining FDHs in general contain a highly conserved active site, comprising a molybdenum (or tungsten) center coordinated by two molybdopterin guanine dinucleotide molecules, a sulfido and a (seleno-)cysteine ligand, in addition to a histidine and arginine residue in the second coordination sphere. So far, the role of these amino acids for catalysis has not been studied in detail, due to the lack of suitable expression systems and the lability or oxygen sensitivity of the enzymes.Here, the roles of these active-site residues is revealed using the Mo-containing FDH from Rhodobacter capsulatus. Our results show that the cysteine ligand at the Mo ion is displaced by the formate substrate during the reaction, the arginine has a direct role in substrate binding and stabilization and the histidine elevates the pK a of the active-site cysteine. We further found that in addition to reversible formate oxidation, the enzyme is further capable to reduce nitrate to nitrite. We propose a mechanistic scheme, which combines both functionalities and provides important insights into the distinct mechanisms of C-H bond cleavage and oxygen atom transfer catalyzed by formate dehydrogenase. 4A specific enzyme system of increasing interest is formate dehydrogenase (FDH), being involved in the reversible conversion of CO 2 in biological systems (1,2 Desulfovibrio gigas (FdhAB) have shown that the metal ion in the oxidized enzyme is coordinated by four sulfur ligands of two dithiolene groups of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor, a selenocycteine (SeCys), and by a sixth ligand, which is now established as a sulfido group (5-9) ( Fig. 1). After reduction with formate, the structure of E. coli FdhF showed that the SeCys ligand was displaced from the Mo ion (5). In the second coordination sphere, a highly conserved histidine and arginine are present in all FDH enzymes described so far. Experimental evidence in which these residues are replaced by other amino acids is lacking, due 5 to the high oxygen sensitivity of the E. coli and D. gigas enzymes and the lack of suitable overexpression systems.Closely related to FDHs are periplasmic nitrate reductases (e.g. Cupriavidus necator NapA), which similarly belong to the DMSO reductase family of molybdoenzymes.Periplasmic nitrate reductases comprise an identical Mo coordination sphere containing six sulfur ligands in the oxidized state (including a Cys) and show a remarkable structural homology to FDHs (10-12) (Fig. 1). A conserved threonine and methionine are found close to the Mo ion in periplasmic nitrate reductases ( Fi...

Bone morphogenetic protein-2 (BMP-2) plays a crucial role in osteoblast differentiation and proliferation. Its effective therapeutic use for ectopic bone and cartilage regeneration depends, among other factors, on the interaction with the carrier at the implant site. In this study, we used classical molecular dynamics (MD) and a hybrid approach of steered molecular dynamics (SMD) combined with MD simulations to investigate the initial stages of the adsorption of BMP-2 when approaching two implant surfaces, hydrophobic graphite and hydrophilic titanium dioxide rutile. Surface adsorption was evaluated for six different orientations of the protein, two end-on and four side-on, in explicit water environment. On graphite, we observed a weak but stable adsorption. Depending on the initial orientation, hydrophobic patches as well as flexible loops of the protein were involved in the interaction with graphite. On the contrary, BMP-2 adsorbed only loosely to hydrophilic titanium dioxide. Despite a favorable interaction energy between protein and the TiO(2) surface, the rapid formation of a two-layer water structure prevented the direct interaction between protein and titanium dioxide. The first water adlayer had a strong repulsive effect on the protein, while the second attracted the protein toward the surface. For both surfaces, hydrophobic graphite and hydrophilic titanium dioxide, denaturation of BMP-2 induced by adsorption was not observed on the nanosecond time scale.

The catalytic cycle of the anaerobic [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F (DvMF) both in solution and immobilized on an Au electrode was studied by IR spectroscopic and electrochemical methods. IR spectroelectrochemistry in solution at different pH values allows the identification of the various redox-states of the active site and the determination of the midpoint potentials, as well as their acid-base equilibria. The spectroscopic characterization was based on the unique marker bands of the CN and CO stretching modes of the Ni-Fe center and served as reference for the surface-enhanced IR absorption (SEIRA) study of the immobilized enzyme. Using structural models of hydrogenases from DvMF and Desulfovibrio gigas , dipole moment calculations were carried out to guide the immobilization strategy. In view of the high dipole moment of about 1100 D pointing through the negatively charged area surrounding the distal [FeS] cluster, the Au electrode was coated by a self-assembled monolayer of amino-terminated mercaptanes which, due to the positively charged head groups, permit a durable electrostatic binding of the protein. SEIRA spectroscopy revealed a structurally and functionally intact active site as demonstrated by the reversible activation and inactivation under hydrogen and argon, respectively. Cyclic voltammetry on the immobilized enzyme demonstrate a reversible anaerobic inactivation upon changing the applied potential. The "switch" potential (E(switch)) associated with the reductive reactivation was determined to be -33 mV (vs normal hydrogen electrode). However, the catalytic current decreased on the time scale of hours during continuous cycling. SEIRA experiments demonstrate that the loss of catalytic activity is not due to protein desorption but is rather related to a slow degradation of the active site, possibly initiated by the attack of reactive species electrochemically generated from residual traces of oxygen in solution.

A comprehensive understanding of physical and chemical processes at biological membranes requires the knowledge of the interfacial electric field which is a key parameter for controlling molecular structures and reaction dynamics. An appropriate approach is based on the vibrational Stark effect (VSE) that exploits the electric-field dependent perturbation of localized vibrational modes. In this work, 6-mercaptohexanenitrile (C5CN) and 7-mercaptoheptanenitrile (C6CN) were used to form self-assembled monolayers (SAMs) on a nanostructured Au electrode as a simple mimic for biomembranes. The CN stretching mode was probed by surface enhanced infrared absorption (SEIRA) spectroscopy to determine the frequency and intensity as a function of the electrode potential. The intensity variations were related to potential-dependent changes of the nitrile orientation with respect to the electric field. Supported by electrochemical impedance spectroscopy, molecular dynamics simulations, and quantum chemical calculations the frequency changes were translated into profiles of the interfacial electric field, affording field strengths up to 4 × 108 V/m (C6CN) and 1.3 × 109 V/m (C5CN) between +0.4 and −0.4 V (vs Ag/AgCl). These profiles compare very well with the predictions of a simple electrostatic model developed in this work. This model is shown to be applicable to different types of electrode/SAM systems and allows for a quick estimate of interfacial electric fields. Finally, the implications for electric-field dependent processes at biomembranes are discussed.

The voltage-sensitive phosphatase Ci-VSP consists of an intracellular phosphatase domain (PD) coupled to a transmembrane voltage-sensor domain (VSD). Depolarization triggers the selective dephosphorylation of phosphoinositides. However, the molecular mechanisms of coupling are still elusive. To clarify the role of the VSD-PD linker as a putative partner for electrostatic interactions with the membrane, we carried out a cysteine-scanning mutagenesis of the whole motif M240-K257. Upon coexpression with PI(4,5)P(2)-sensitive KCNQ2/KCNQ3 channels in Xenopus oocytes, we identified four positions (A242C, R245C, K252C, and Y255C) with a completely abrogated PD activity. Because the mutation effect occurred periodically, we hypothesize that α-helical elements exist within the linker, with a gap near position S249. The combination of these results with the analysis of transient sensing currents of the VSD revealed distinct roles for the N-terminal (M240-S249) and C-terminal (Q250-K257) linker motifs in the VSD-PD coupling. According to our functional results, the computational structure prediction of the Q239-D258 fragment confirmed α-helical structures within the linker, with a short β-turn around S249 in the activated conformation. Remarkably, the position K252 may be a candidate for interacting with the PD rather than for binding to the membrane. This provides the first insight (to our knowledge) into the direct intervention of the linker in the VSD-PD coupling process.

Protein immobilization on electrodes is a key concept in exploiting enzymatic processes for bioelectronic devices. For optimum performance, an in-depth understanding of the enzyme-surface interactions is required. Here, we introduce an integral approach of experimental and theoretical methods that provides detailed insights into the adsorption of an oxygen-tolerant [NiFe] hydrogenase on a biocompatible gold electrode. Using atomic force microscopy, ellipsometry, surface-enhanced IR spectroscopy, and protein film voltammetry, we explore enzyme coverage, integrity, and activity, thereby probing both structure and catalytic H2 conversion of the enzyme. Electrocatalytic efficiencies can be correlated with the mode of protein adsorption on the electrode as estimated theoretically by molecular dynamics simulations. Our results reveal that pre-activation at low potentials results in increased current densities, which can be rationalized in terms of a potential-induced re-orientation of the immobilized enzyme.