Catalytic subunit of the periplasmic nitrate reductase (NAP). Only expressed at high levels during aerobic growth. NapAB complex receives electrons from the membrane-anchored tetraheme protein napC, thus allowing electron flow between membrane and periplasm. Essential function for nitrate assimilation and may have a role in anaerobic metabolism. Periplasmic nitrate reductase MTISRRDLLKAQAAGIAAMAANIPLSSQAPAVPGGVESLQITWSKAPCRFCGTGCGVMVGVKEGRVVATHGDLLAEVNRGLNCVKGYFLSKIMYGADRLTQPLLRKKDGVYAKDGEFTPVSWEEAFDTMAAQAKRVLRDKGPTALGMFGSGQWTIFEGYAATKLMRAGFRSNNLDPNARHCMASAAYAFMRTFGMDEPMGCYDDFEAADAFVLWGSNMAEMHPILWTRVADRRLGHPHVKVAVLSTFTHRSSDLADIPIVFKPGTDLAILNYIANHIIQTGRVNRDFVDRHTTFVAGATGIGYGLRDDDPREMAARTAEDPAATTPSTFEEFAELVSEYTLDKVSELSGVEPAFLEQLAELYADPDRKVMSLWTMGFNQHVRGVWANQMVYNLHLLTGKISEPGNSPFSLTGQASACGTARQVGTFRHRLPSDMTVTNPERRQDAEEIWRIPHGVIPEQPGLHAVAQDRALHDGTLNFYWIQVNNNLQASPNNDGEAWPGYRNPDNFIVVSDAYPTVTALAADLILPAAMWVEKEGAYGNAERRTHVWHQLVEAPGEARSDLWQMMEFSTRFTTDEVWPEEILAANPNYRGQSLFDVLFRNGSVDRFDLSELNPVTPTAESNAFGFYVQKGLFEEYAPFGRGHGHDLAPYDTYHEVRGLRWPVVDGKETLWRYREGLDPYVEPGAGVQFYGNPDGKARIIAVPYEPPAEPPDEEYNIWLVTGRVLEHWHSGSMTMRVPELYRAFPGARCFMNPEDARDMGFNQGAEVRIVSRRGEIRSRVETRGRNRMPRGVVFVPWFDASQLINKVTLDATDPISKQTDFKKCAVKILPV NAPA_PARPN napA 831