Physiological role for the membrane bound ascorbate-TMPD oxidase in pseudomonas putida

Identification of cyclic intermediates in Azorhizobium caulinodans nicotinate catabolism

In wild-type Azorhizobium caulinodans ORS571, nicotinate served both as anabolic substrate for NAD+ production and as catabolic substrate for use as the N source. Catabolic enzyme activities were greatest from cultures grown with nicotinate as the N source and least when cultures were grown with ammonium as the N source. Vector insertion mutants unable to catabolize nicotinate (nic::Vi mutants) still required micromolar quantities of this compound for growth. Therefore, A. caulinodans wild type is NAD+ auxotrophic. As the first two intermediates in A. caulinodans nicotinate catabolism, two cyclic compounds, 6-hydroxynicotinate and 1,4,5,6-tetrahydro-6-oxonicotinate, were identified. These compounds were purified from the growth medium of strain 61009 (a nic::Vi mutant) by high-performance liquid chromatography; their identities were subsequently confirmed by UV absorbance, nuclear magnetic resonance, and mass spectra. The conversion of 1 mol of nicotinate to 6-hydroxynicotinate consumed 0.5 mol of O2. From 18O isotopic incorporation experiments, water was the hydroxyl-equivalent source. A nicotinate hydroxylase activity proved to be cell wall-membrane associated; this activity served as direct electron donor (not indirect via NADP+) to O2 via membrane electron transport. These catabolic reactions have not previously been witnessed together in the same organism. A. caulinodans nicotinate catabolism seems coupled to N2 fixation, although the explicit mechanism of this coupling remains to be determined.

Tetramethyl-p-phenylenediamine oxidase of Pseudomonas aeruginosa

An oxidase complex has been solubilized and partially purified from the membrane particle of Pseudomonas aeruginosa grown under limited oxygen condition. The oxidase consists of two major cytochrome components, cytochrome c554 and cytochrome o (b561), with a molar ratio of about 9:1 in terms of c-heme to protoheme content. Ninety percent of the cytochrome c+o complex, corresponding to all of the cytochrome c554, is reducible by reduced N,N,N',N'-tetramethyl-p-phenylenediamine. This partially purified oxidase exhibited a maximal specific activity about 5 mumol O2 uptake x min-1 x mg protein-1, with a Km (of reduced N,N,N',N'-tetramethyl-p-phenylenediamine) = 7.2 x 10(-4) M at 30 degrees C. The oxidase is sensitive to KCN, NaN3 and NaNO2. Oxidation-reduction potentiometric titration shows that cytochrome c554 has a midpoint potential of 289 mV and cytochrome o of + 25 mV at pH 7.2 in the partially purified oxidase preparation. The purity of the preparation has been estimated to be about 85--90% by gel electrophoresis.

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.