Protein engineering by resurfacing is an efficient approach to provide new molecular toolkits for biotechnology and bioanalytical chemistry. H39GFP is a new variant of green fluorescent protein (GFP) containing 39 histidine residues in the primary sequence that was developed by protein resurfacing. Herein, taking H39GFP as the signal reporter, a label-free fluorometric sensor for Cu(2+) sensing was developed based on the unique multivalent metal ion-binding property of H39GFP and fluorescence quenching effect of Cu(2+) by electron transfer. The high affinity of H39GFP with Cu(2+) (Kd, 16.2 nM) leads to rapid detection of Cu(2+) in 5 min with a low detection limit (50 nM). Using acetylthiocholine (ATCh) as the substrate, this H39GFP/Cu(2+) complex-based sensor was further applied for the turn-on fluorescence detection of acetylcholinesterase (AChE) activity. The assay was based on the reaction between Cu(2+) and thiocholine, the hydrolysis product of ATCh by AChE. The proposed sensor is highly sensitive (limit of detection (LOD) = 0.015 mU mL(-1)) and is feasible for screening inhibitors of AChE. Furthermore, the practicability of this method was demonstrated by the detection of pesticide residue (carbaryl) in real food samples. Hence, the successful applications of H39GFP in the detection of metal ion and enzyme activity present the prospect of resurfaced proteins as versatile biosensing platforms.