Downsizing effect on direct electron transfer-type bioelectrocatalysis by <scp>d</scp>-fructose dehydrogenase with structural insight

Membrane-bound heterotrimeric d-fructose dehydrogenase (FDH) from Gluconobacter japonicus exhibits distinct direct electron transfer (DET)-type bioelectrocatalytic activity. FDH contains 3 heme c moieties (heme 1c, 2c, and 3c), and the two downsized variants lacking heme 1c moiety (Δ1c FDH) or heme 1c and 2c moieties (Δ1c2c FDH) were constructed in our previous research. Recently, we elucidated the entire structure of FDH, enabling a structural perspective in mechanistic discussions. In this study, we quantitatively analyzed the downsizing effect of Δ1c- and Δ1c2c-deletion on DET-type bioelectrocatalysis. Non-catalytic redox signals of adsorbed enzymes were successfully obtained, providing a basis for independently evaluating kinetic parameters from DET-type catalytic waves. Analytical results revealed that the downsizing enhanced the electron transfer rate constant by 7-fold and 4-fold for Δ1c FDH and Δ1c2c FDH, respectively. Furthermore, the acceleration factors were discussed using structural predictions of the variants.