The oxygen affinity of cytochrome bo9 in Escherichia coli determined by the deoxygenation of oxyleghemoglobin and oxymyoglobin: Km values for oxygen are in the submicromolar range

Apparent oxygen affinities for Escherichia coli cells and membranes containing a terminal oxidase with only one type of ligand-binding heme, cytochrome o*, were measured with oxyleghemoglobin and oxymyoglobin as sensitive oxygen reporters. Two Km values (0.15 to 0.35 mM and 0.016 to 0.085 mM) were detected, well below values determined for the purified oxidase by insensitive electrode methods. Escherichia coli contains two terminal quinol oxidases (21, 22, 28). One of these, cytochrome bo9, contains the ligand-binding heme O and a second low-spin heme of either the B or O type (29). This oxidase has also been called cytochrome b562o (16), bo3, and oo3 (29), but the bo9 nomenclature is in accordance with IUB recommendations (25). Cytochrome bo9 is a member of the heme-copper superfamily of terminal oxi-dases (7) and is a proton pump (28). The alternative oxidase, cytochrome bd, is structurally unrelated to this family (13) and appears not to pump protons (28). Cytochome bo9 is synthe-sized under conditions of high aeration (8, 15), whereas max