**1. Introduction**

Membrane proteins constitute 20–30% of all proteins encoded by both prokaryotic and eukaryotic cells. They perform a wide variety of functions, including material transport, signal transduction, catalysis, proton and electron transport (Figure 1) [1]. They are also key to a number of earth's most fundamental reactions, such as respiration and photosynthesis [2,3]. Redox enzymes in the respiratory chain catalyse a variety of fundamental processes for energy conversion and fuel production, including H<sup>2</sup> oxidation, O<sup>2</sup> reduction, and carbon and nitrogen cycling. Membrane proteins that are involved in the light reaction of photosynthesis harvest light and facilitate electron transfer essential for solar energy conversion. The amphiphilic nature of membrane proteins makes them difficult to isolate, study and manipulate. Despite these challenges, membrane proteins have been widely advocated and studied for applications in bioelectrocatalysis, such as biofuel cells [4] and semi-artificial photosynthesis [5]. Here, we will review electrochemical studies of membrane proteins with the view to using these systems for bioelectrocatalysis. To aid discussion later on, we will briefly introduce a small selection of membrane enzymes active in bioenergy conversion, although this is far from a comprehensive list. We will then summarise the main strategies to immobilise membrane proteins on electrodes and discuss common techniques used to characterise these electrodes, including electrochemistry, spectroscopy, spectroelectrochemistry, microscopy and quartz crystal microbalance. Finally, some critical application challenges and potential future research directions will be highlighted that might find application in bioelectrocatalysis. Specifically, we will focus on two emerging directions. One is the reconstitution of membrane proteins into hybrid vesicles to extend their functional lifetime. The other is the use of microorganisms for microbial electrosynthesis and semi-artificial photosynthesis.

**Figure 1.** Representative functions of transmembrane proteins.
