Functionalisation of Graphene Sensor Surfaces for the Specific Detection of Biomarkers

We report on a controllable and specific functionalisation route for graphene field-effect transistors (GFETs) for the recognition of small physiologically active molecules. Key element is the noncovalent functionalisation of the graphene surface with perylene bisimide (PBI) molecules directly on the growth substrate. This Functional Layer Transfer enables the homogeneous self-assembly of PBI molecules on graphene, onto which antibodies are subsequently immobilised. The sensor surface was characterised by atomic force microscopy, Raman spectroscopy and electrical measurements, showing superior performance over conventional functionalisation after transfer. Specific sensing of small molecules was realised by monitoring the electrical property changes of functionalised GFET devices upon the application of methamphetamine and cortisol. The concentration dependent electrical response of our sensors was determined down to a concentration of 300 ng/ml for methamphetamine.

Design of a pH-dependent cellulose-binding domain

Protein-carbohydrate interactions typically rely on aromatic stacking interactions of tyrosine, phenylalanine and tryptophan side chains with the sugar rings whereas histidine residues are rarely involved. The small cellulose-binding domain of the Cel7A cellobiohydrolase (formerly CBHI) from Trichoderma reesei binds to crystalline cellulose primarily using a planar strip of three tyrosine side chains. Binding of the wild-type Cel7A CBD is practically insensitive to pH. Here we have investigated how histidine residues mediate the binding interaction and whether the protonation of a histidine side chain makes the binding sensitive to pH. Protein engineering of the Cel7A CBD was thus used to replace the tyrosine residues in two different positions with histidine residues. All of the mutants exhibited a clear pH-dependency of the binding, in clear contrast to the wild-type. Although the binding of the mutants at optimal pH was less than for the wild-type, in one case, Y31H, this binding almost reached the wild-type level.

Improved immobilization of fusion proteins via cellulose-binding domains

Cellulose-binding domains (CBDs) are structurally and functionally independent, noncatalytic modules found in many cellulose or hemicellulose degrading enzymes. Recent biotechnological applications of the CBDs include facilitated protein immobilization on cellulose supports. In some occasions there have been concerns about the stability of the CBD driven immobilization. Here we have studied the chromatographic behavior of variants of the Trichoderma reesei cellobiohydrolase I CBD belonging to family I. Both CBDs fused to antibody fragments and isolated CBDs were studied and compared. Tritium labeling by reductive methylation was used as a sensitive detection method. The fusion protein as well as the isolated CBD was found to leak from the column at a rate of 0.3-0.5% of the immobilized protein per column volume. However, the leakage could be overcome by using two CBDs instead of a single CBD for the immobilization. In this way leakage was reduced to less than 0.01% per column volume. The improved immobilization could also be seen as a decreased migration of the protein down the column in extended washes.

Investigation of the function of mutated cellulose-binding domains of Trichoderma reesei cellobiohydrolase I

The function of the cellulose-binding domain (CBD) of the cellobiohydrolase I of Trichoderma reesei was studied by site-directed mutagenesis of two amino acid residues identified by analyzing the 3D structure of this domain. The mutant enzymes were produced in yeast and tested for binding and activity on crystalline cellulose. Mutagenesis of the tyrosine residue (Y492) located at the tip of the wedge-shaped domain to alanine or aspartate reduced the binding and activity on crystalline cellulose to the level of the core protein lacking the CBD. However, there was no effect on the activity toward small oligosaccharide (4-methylumbelliferyl beta-D-lactoside). The mutation tyrosine to histidine (Y492H) lowered but did not destroy the cellulose binding, suggesting that the interaction of the pyranose ring of the substrate with an aromatic side chain is important. However, the catalytic activity of this mutant on crystalline cellulose was identical to the other two mutants. The mutation P477R on the edge of the other face of the domain reduces both binding and activity of CBHI. These results support the hypothesis that both surfaces of the CBD are involved in the interaction of the binding domain with crystalline cellulose.