Electron transfer subunit of the periplasmic nitrate reductase complex NapAB. Receives electrons from the membrane-anchored tetraheme c-type NapC protein and transfers these to NapA subunit, thus allowing electron flow between membrane and periplasm. Essential for periplasmic nitrate reduction with nitrate as the terminal electron acceptor. Component of the periplasmic nitrate reductase NapAB complex composed of NapA and NapB. Induced by anaerobic conditions, and further induced by presence of nitrate. Binds 2 heme C groups per subunit. Belongs to the NapB family. Truncated N-terminus.