Expanding PQQ's Biosensor Applications: The Effects of Illumination on the Reactivity of PQQ.
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2,7,9-Tricarboxyl-1H-pyrrolo[2,3-f]quinoline-4,5-dione, commonly known as PQQ or pyrroloquinoline quinone, functions as a redox cofactor in bacterial methanol and glucose dehydrogenases. These enzymes require a divalent metal ion for activity. This study focuses on PQQ found within the homodimeric enzyme soluble glucose dehydrogenase (sGDH) of bacterium Acinetobactor calcoaceticus. Within sGDH, PQQ is activated when it is bound to Ca<sup>2+</sup>. It can then oxidize glucose to d-gluconolactone. Knowing this, PQQ is currently being used as a biosensor in test strips to detect glucose levels in samples such as blood. As PQQ is structurally similar to photochemically reactive phenanthrene-9,10-dione, we expect that light can be used as a tool to alter PQQ's reactivity. The involvement of light in its enzymatic process, however, is not known. Thus far, using a model system lacking the apoprotein, we have found that under illumination, PQQ shows enhanced reactivity towards aldose sugars such as glucose. Moreover, PQQ effectively oxidizes ketose sugars such as D-fructose under light. This is significant since ketose sugars are not among PQQ's reported substrates for sGDH. This oxidation reaction is further enhanced in the presence of those divalent metal ions that can effectively bind PQQ within sGDH. Our aim is to isolate and characterize the product of PQQ's light reaction with D-fructose. By studying PQQ's reactivity under illumination, we aim to broaden its substrate specificity and in turn expand its biosensor applications.