Not-XOR (NXOR) Logic Gate Realized with Enzyme-Catalyzed Reactions: Optical and Electrochemical Signal Transduction

The studied enzyme-based biocatalytic system mimics NXOR Boolean logic gate, which is a logical operator that corresponds to equality in Boolean algebra. It gives the functional value true ( 1 ) if both functional arguments (input signals) have the same logical value ( 0 , 0 or 1 , 1 ), and false ( 0 ) if they are different ( 0 , 1 or 1 , 0 ). The output signal producing reaction is catalyzed by pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQ-GDH), which is inhibited at acidic and basic pH values. Two other reactions catalyzed by esterase and urease produce acetic acid and ammonium hydroxide, respectively, shifting solution pH from the optimum pH for PQQ-GDH to acidic and basic values ( 1 , 0 and 0 , 1 input combinations, respectively), thus switching the enzyme activity off (output 0 ). When the input signals are not applied ( 0 , 0 combination) or both applied compensating each other ( 1 , 1 combination) the optimum pH is preserved, thus keeping PQQ-GDH running at the high rate (output 1 ). The biocatalytic cascade mimicking the NXOR gate was characterized optically and electrochemically. In the electrochemical experiments the PQQ-GDH enzyme communicated electronically with a conducting electrode support, thus resulting in the electrocatalytic current when signal combinations 0 , 0 and 1 , 1 were applied. The logic gate operation, when it was realized electrochemically, was also extended to the biomolecular release controlled by the gate. The release system included two electrodes, one performing the NXOR gate and another one activated for the release upon electrochemically stimulated alginate hydrogel dissolution. The studied system represents a general approach to the biocatalytic realization of the NXOR logic gate, which can be included in different catalytic cascades mimicking operation of concatenated gates in sophisticated logic circuitries.