Electron donor subunit of the cytochrome c nitrite reductase holocomplex NrfHA. Acquires electrons from the menaquinone pool and mediates their transfer to the catalytic subunit NrfA in an anaerobic respiratory process of nitrite (PubMed:11004582, PubMed:17139260, PubMed:18597779). The other biological function of the NrfHA holocomplex is to detoxify nitrite (PubMed:15547266, PubMed:25534748). This function is essential for the survival of this organism as it enables it to overcome inhibition by nitrite, which is produced by other organisms living in the same environment (Probable). Binds 4 heme groups per subunit. E(0) is -270 mV for the menaquinol-interacting methionine-coordinated heme 1, identified by the use of the inhibitor 2-heptyl-4-hydroxyquinoline N-oxide (HQNO), a structural analog of the physiological electron donor. Component of the NrfHA cytochrome c nitrite reductase complex composed of 4 NrfA catalytic subunits and 2 NrfH quinone-binding subunits (PubMed:17139260, PubMed:18597779, PubMed:22519292). Interacts with NrfA homodimer (PubMed:16754983, PubMed:17139260). Expression is induced by NrfR in response to nitrite stress. Not induced by nitrate or nitric oxide (NO) stress. Belongs to the NapC/NirT/NrfH family.