This thesis describes how the human Alpha class glutathione transferase (GST) A1-1 can be reprogrammed either to function as a multipurpose biosensor for detection of small molecule analytes, or as a handle providing for more efficient protein purification. A novel, user-friendly, and efficient method for site-specific introduction of functional groups into the active site of hGST A1-1 is the platform for these achievements. The designed thioester reagents are glutathione-based and they are able to label one single nucleophile (Y9) and leave the other 50 nucleophiles (in hGST A1-1) intact. The modification reaction was tested with five classes of GSTs (Alpha, Mu, Pi, Theta and Omega) and was found to be specific for the Alpha class isoenzymes. The reaction was further refined to target a single lysine residue, K216 in the hGST A1-1 mutant A216K, providing a stable amide bond between the protein and the labeling group. To further improve the labeling process, biotinylated reagents that could deliver the acyl group to Y9 (wt hGST A1-1) or K216 in the lysine mutant, while attached to streptavidin-coated agarose beads, were designed and synthesized. A focused library of eleven A216K/M208X mutants was made via random mutagenesis to provide an array of proteins with altered micro-environments in the hydrophobic binding site, where M208 is situated. Through the invented route for site-specific labeling, a fluorescent probe (coumarin) was introduced on K216 in all double mutants, with the purpose of developing a protein-based biosensor, akin to the olfactory system. The array of coumarin-labeled proteins responded differently to the addition of different analytes, and the responses were analyzed through pattern recognition of the fluorescence signals. The labeled proteins could also be site-specifically immobilized on a PEG-based biosensor chip via the single C112 on the surface of the protein, enabling development of surface-based biosensing systems. Also, a refined system for efficient detection and purification of GST-fusion proteins is presented. Through a screening process involving A216K and all produced A216K/M208X mutants, two candidates (A216K and A216K/M208F) were singled out as scaffolds for the next generation of fusion proteins. In addition to the features present in commercially available GST fusion constructs, the new mutants can be site-specifically labeled with a fluorophore in bacterial lysates providing quick and sensitive monitoring of expression and purification. Furthermore, the proteins could be labeled with a unique aldehyde moiety providing for a novel protein purification scheme.