Protein-protein interactions are well known to regulate enzyme activity in cell signalling and metabolism. Here, we show that protein-protein interactions regulate the activity of a respiratory-chain enzyme, CymA, by changing the direction or bias of catalysis. CymA, a member of the widespread NapC/NirT superfamily, is a menaquinol-7 (MQ-7) dehydrogenase that donates electrons to several distinct terminal reductases in the versatile respiratory network of Shewanella oneidensis. Here we report the incorporation of CymA within solid-supported membranes that mimic the inner membrane architecture of S. oneidensis. Quartz-crystal microbalance with dissipation (QCM-D) resolved the formation of a stable complex between CymA and one of its native redox partners, flavocytochrome c3 (Fcc3) fumarate reductase. Cyclic voltammetry revealed that CymA alone could only reduce MQ-7, while the CymA-Fcc3 complex catalyzed the reaction required to support anaerobic respiration, namely, the oxidation of MQ-7.We propose that MQ-7 oxidation in CymA is limited by electron transfer to the hemes and that complex formation with Fcc3 facilitates the electron-transfer rate along the heme redox chain. These results reveal a yet unexplored mechanism by which bacteria can regulate multi-branched respiratory networks through protein-protein interactions.