BackgroundThis study provides fundamental information on the influence of graphene oxide (GO) nanosheets and glycans on protein catalytic activity, dynamics, and thermal stability. We provide evidence of protein stabilization by glycans and how this strategy could be implemented when GO nanosheets is used as protein immobilization matrix. A series of bioconjugates was constructed using two different strategies: adsorbing or covalently attaching native and glycosylated bilirubin oxidase (BOD) to GO.ResultsBioconjugate formation was followed by FT-IR, zeta-potential, and X-ray photoelectron spectroscopy measurements. Enzyme kinetic parameters (km and kcat) revealed that the substrate binding affinity was not affected by glycosylation and immobilization on GO, but the rate of enzyme catalysis was reduced. Structural analysis by circular dichroism showed that glycosylation did not affect the tertiary or the secondary structure of BOD. However, GO produced slight changes in the secondary structure. To shed light into the biophysical consequence of protein glycosylation and protein immobilization on GO nanosheets, we studied structural protein dynamical changes by FT-IR H/D exchange and thermal inactivation.ConclusionsIt was found that glycosylation caused a reduction in structural dynamics that resulted in an increase in thermostability and a decrease in the catalytic activity for both, glycoconjugate and immobilized enzyme. These results establish the usefulness of chemical glycosylation to modulate protein structural dynamics and stability to develop a more stable GO-protein matrix.Electronic supplementary materialThe online version of this article (doi:10.1186/s12951-015-0134-0) contains supplementary material, which is available to authorized users.

Edited by Mike Shipston Ras family small GTPases undergo prenylation (such as farnesylation) for proper localization to the plasma membrane, where they can initiate oncogenic signaling pathways. Small GTP-binding protein GDP-dissociation stimulator (SmgGDS) proteins are chaperones that bind and traffic small GTPases, although their exact cellular function is unknown. Initially, SmgGDS proteins were classified as guanine nucleotide exchange factors, but recent findings suggest that Smg-GDS proteins also regulate prenylation of small GTPases in vivo in a substrate-selective manner. SmgGDS-607 recognizes the polybasic region and the CAAX box of several small GTPases and inhibits prenylation by impeding their entry into the geranylgeranylation pathway. Here, using recombinant and purified enzymes for prenylation and protein-binding assays, we demonstrate that SmgGDS-607 differentially regulates farnesylation of several small GTPases. SmgGDS-607 inhibited farnesylation of some proteins, such as DiRas1, by sequestering the protein and limiting modification catalyzed by protein farnesyltransferase (FTase). We found that the competitive binding affinities of the small GTPase for SmgGDS-607 and FTase dictate the extent of this inhibition. Additionally, we discovered that SmgGDS-607 increases the rate of farnesylation of HRas by enhancing product release from FTase. Our work indicates that SmgGDS-607 binds to a broad range of small GTPases and does not require a PBR for recognition. Together, these results provide mechanistic insight into SmgGDS-607-mediated regulation of farnesylation of small GTPases and suggest that SmgGDS-607 has multiple modes of substrate recognition.

Graphene possesses excellent mechanical strength and chemical inertness with high intrinsic carrier mobility and superior flexibility making them exceptional candidates for optoelectronic applications. Graphene quantum dots (GQDs) derived from graphene domains have been widely explored to study their photoluminescence properties which can be tuned by size. GQDs are biocompatible, low cytotoxic, strongly luminescent and disperse well in polar and non-polar solvents showing bright promise for the integration into devices for bioimaging, light emitting and photovoltaic applications. In the present study, graphene quantum dots were synthesized by an electrochemical cyclic voltammetry technique using reduced graphene oxide (rGO). GQDs have been incorporated into binder free TiO2 paste and studied as a photoelectrode material fabricated on ITO/PEN substrates for flexible dye sensitized solar cells (DSSCs). DSSC based on GQDs-TiO2 exhibited open circuit output potential difference (Voc) of 0.73 V, and short circuit current density (Jsc) of 11.54 mA cm-2 with an increment in power conversion efficiency by 5.48 %, when compared with those with DSSC build with just a TiO2 photoanode (open-circuit output potential difference (Voc) of 0.68 V and short circuit density (Jsc) of 10.67 mA cm-2). The results have been understood in terms of increased charge extraction and reduced recombination losses upon GQDs incorporation.

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