**2. Bioelectrochemistry and Bioelectrocatalysts**

Oxidoreductases undergo electron transfer with the surface of an electrode via two mechanisms [24]; mediated [25–28] and direct electron transfer [29–31] (Figure 1). As the redox active centres of

oxidoreductases can be placed deep within the enzymes, the rate of electron transfer between the redox centre and the electrode can be restricted and only a limited number of redox enzymes can undergo direct electron transfer (DET) [32]. Direct electron transfer relies on the appropriate orientation of an enzyme on the electrode surface, the location of the active site within the enzyme and the distance between the redox centre and the electrode. Efficient rates of electron transfer require that the redox active site in the enzyme be close to the surface of the electrode, with the appropriate orientation of the enzyme on the electrode. When the redox site is inaccessible, mediators can be used as electron shuttles between the electrode and the active site of enzymes (Figure 1a) [33,34].

**Figure 1.** Schematic diagram of (**a**) mediated and (**b**) direct electron transfer between an enzyme and an electrode.
