Different effects of 2-n-heptyl-4-hydroxyquinoline-N-oxide on oxygen and nitrate respiration in Klebsiella aerogenes

Terminal branching of the respiratory electron transport chain in Neisseria meningitidis

The respiratory components of the envelope membrane preparation of Neisseria meningitidis were investigated. Oxidase activities were demonstrated in this fraction in the presence of succinic acid, reduced nicotinamide adenine dinucleotide, and ascorbate-N,N,N',N'-tetramethyl-p-phenylene-diamine (TMPD). Differences in the kinetics of inhibition by terminal oxidase inhibitors on the three oxidase activities indicated that ascorbate-TMPD oxidation involved only an azide-sensitive oxidase, whereas oxidation of the physiological substrates involved two oxidases, one of which was relatively azide resistant. Spectrophotometric studies revealed that ascorbate-TMPD donated its electrons exclusively to cytochrome o, whereas the physiological substrates were oxidized via both cytochromes o and a. The effects of class II inhibitors on the oxidases suggest terminal branching of the electron transport chain at the cytochrome b level. A model of the respiratory system in N. meningitidis is proposed.

Evidence for involvement of the electron transport system at a late step of anaerobic microbial heme synthesis

The penultimate step in heme biosynthesis, the oxidation of protoporphyrinogen to protoporphyrin, can be anaerobically coupled to the reduction of fumarate in extracts of anaerobically-grown Escherichia coli. This coupling is approximately 90% inhibied by 2-heptyl-4-hydroxy quinoline-N-oxide (HQNO), a known inhibitor of the electron transport chain. This observation suggests that the mechanism of the anaerobic oxidation of protoporphyrinogen in E. coli involves a coupling into the anaerobic electron transport system. In contrast, the aerobic oxidation of protoporphyrinogen, which occurs in mammalian and yeast mitochondria, is known to be linked directly to oxygen without the mediation of an electron transport system.

Fumarate reduction in Proteus mirabilis

1. Proteus mirabilis formed fumarate reductase under anaerobic growth conditions. The formation of this reductase was repressed under conditions of growth during which electron transport to oxygen or to nitrate is possible. In two of three tested chlorate-resistant mutant strains of the wild type, fumarate reductase appeared to be affected. 2. Cytoplasmic membrane suspensions isolated from anaerobically grown P. mirabilis oxidized formate and NADH with oxygen and with fumarate, too. 3. Spectral investigation of the cytoplasmic membrane preparation revealed the presence of (probably at least two types of) cytochrome b, cytochrome a1 and cytochrome d. Cytochrome b was reduced by NADH as well as by formate to approximately 80%. 4. 2-n-Heptyl-4-hydroxyquinilone-N-oxide and antimycin A inhibited oxidation of both formate and NADH by oxygen and fumarate. Both inhibitors increased the level of the formate/oxygen steady state and the formate/fumarate steady state. 5. The site of inhibition of the respiratory activity by both HQNO and antimycin A was located at the oxidation side of cytochrome b. 6. The effect of ultraviolet-irradiation of cytoplasmic membrane suspensions on oxidation/reduction phenomena suggested that the role of menaquinone is more exclusive in the formate/fumarate pathway than in the electron transport route to oxygen. 7. Finally, the conclusion has been drawn that the preferential route for electron transport from formate and from NADH to fumarate (and to oxygen) includes cytochrome b as a directly involved carrier. A hypothetical scheme for the electron transport in anaerobically grown P. mirabilis is presented.