<p>The cytochrome bd type terminal oxidases catalyse quinol dependent, Na<sup>+</sup> independent oxygen uptake [<cite idref="PUB00008048"/>]. Members of this family are integral membrane proteins and contain a protoheame IX centre B558. </p><p>Cytochrome bd may play an important role in microaerobic nitrogen fixation in the enteric bacterium <taxon tax_id="573">Klebsiella pneumoniae</taxon>, where it is expressed under all conditions that permit diazotrophy [<cite idref="PUB00003598"/>]. </p><p>The 14 kDa (or VI) subunit of the complex is not directly involved in electron transfer, but has a role in assembly of the complex [<cite idref="PUB00006325"/>].</p>