Mikrobiologisches Institut der Eidgeno¨ssischen Technischen Hochschule, ETH-Zentrum, Zu¨rich, Switzerland The membrane-bound b subunit of the oxaloacetate decarboxylase Na+ pump of Klebsiella pneumoniae catalyzes the decarboxylation of enzyme-bound biotin. This event is coupled to the transport of 2 Na+ ions into the periplasm and consumes a periplasmically derived proton. The connecting fragment IIIa and transmembrane helices IV and VIII of the b subunit are highly conserved, harboring residues D203, Y229, N373, G377, S382, and R389 that play a profound role in catalysis. .
