Enzymes involved in the metabolism of thiosulfate by Thiobacillus thioparus. 3. Properties of thiosulfate-oxidizing enzyme and proposed pathway of thiosulfate oxidation

Thiosulfate-oxidizing enzyme was purified from Thiobacillus thioparus extracts 120- to 160-fold and the properties were studied. The enzyme had a molecular weight of 115 000 and contained 2 moles of non-heme iron. Ferricyanide was a much better electron acceptor than cytochrome c, but with cytochrome c the Km for thiosulfate was lowered from 0.1 mM to 5 μM and the pH response of the enzyme changed. Sulfite was a very strong inhibitor destroying 50% of the activity at 5 μM. The inhibition was time-dependent and essentially irreversible. Properties of the T. thioparus enzyme were compared to those of thiosulfate-oxidizing enzyme isolated from Thiobacillus neapolitanus and Ferrobacillus ferrooxidans. A pathway of thiosulfate oxidation is proposed, and metabolic roles of various enzymes studied in T. thioparus are discussed.

Oxidation of ammonia by spheroplasts of Nitrosomonas europaea

The ammonia-oxidizing activity of Nitrosomonas europaea spheroplasts was restored by addition of hydroxylamine or by preincubation with Mg(2+).

Pyruvate carboxylase of Aspergillus niger: kinetic study of a biotin-containing carboxylase

Pyruvate carboxylase was partially purified from Aspergillus niger and the properties were studied. The enzyme was found to be cold-labile and protected by 25% glycerol. The pH optimum was determined to be 7.9–8.0. The enzyme was shown to be a biotin-containing enzyme by its inactivation by avidin and protection against such inactivation by biotin. The enzyme activity was stimulated by K+ ions and inhibited by Na+ ions. Acetyl-CoA had no effect on enzyme activity, but L-aspartate was inhibitory. Apparent Michaelis constants were determined for the substrates and metal cofactor involved, i.e. pyruvate, ATP, bicarbonate, and Mg2+.Initial-velocity studies were carried out at varied concentrations of substrates in order to determine the true Michaelis constants and to elucidate the kinetic mechanism of reaction. Product-inhibition studies were carried out with each product (ADP, Pi, and oxalacetate) in combination with every substrate (ATP, bicarbonate, and pyruvate). From these kinetic studies and the existing knowledge on biotin-containing carboxylases, a mechanism was proposed for the action of pyruvate carboxylase which involves three independently active sites on the enzyme, one for each substrate. The interactions between the sites were visualized as being mediated by carboxybiotin formed on the enzyme. A steady-state rate equation was derived that satisfied kinetic results obtained.

The thermodynamics of iron oxidation by the ferrobacilli

The free-energy changes involved in the oxidation of ferrous iron by the ferrobacilli have been recalculated. Microcalorimetric measurements of the heat output of the oxidation have shown that the recalculated free-energy changes are in harmony with these measurements in that the measured heat output differs from the calculated free energy by an amount approximating to the theoretical entropy change involved.

Carbon dioxide fixation and phosphoenolpyruvate carboxylase in Ferrobacillus ferrooxidans

Carbon dioxide fixation was studied in intact cells and cell-free extracts of Ferrobacillus ferrooxidans. The major pathways of fixation were found to be the carboxydismutase system and the phosphoenolpyruvate (PEP) carboxylase system. PEP carboxylase activity was shown to be under metabolic regulation, similar to the regulation established in heterotrophic microorganisms.Acetyl-CoA stimulated PEP carboxylase activity, while aspartate was inhibitory. The F. ferrooxidans enzyme appeared to have a neutral or acidic pH optimum, in contrast to the same enzyme isolated from heterotrophs.

Mechanism of Fe++-cytochrome c reductase of Ferrobacillus ferrooxidans

The mechanism of Fe++-cytochrome c reductase was investigated. Kinetic studies on initial velocity and product inhibition, as well as spectrofluorometric studies, were consistent with a Ping Pong Bi Bi mechanism with two stable forms of enzyme. The Kmvalues for the substrates were found to be 0.59 mM for Fe++and 0.085 mM for cytochrome c. The inhibition constants for Fe+++and reduced cytochrome c were 0.137 mM and 0.0135 mM, respectively. The oxidation–reduction of non-heme iron bound to the enzyme protein was implicated as the responsible factor for the oscillation of the enzyme between its two forms. NaCl was a dead-end inhibitor binding the ferrous form of the enzyme, showing an uncompetitive inhibition with Fe++as a variable substrate.

Ferrous iron oxidation by Ferrobacillus ferrooxidans. Purification and properties of Fe++-cytochrome c reductase

The electron transport chain and Fe++-cytochrome c reductase of Ferrobacillus ferrooxidans were studied to elucidate the mechanism of iron oxidation by this autotrophic bacterium.The iron oxidation involved the cytochrome c and a type cytochrome of F. ferrooxidans in the electron transport system. The initial enzyme of the oxidation system was found to be Fe++-cytochrome c reductase. The iron oxidase system was labile to freezing or sonication; either treatment disrupted some link between the cellular cytochromes c and a. Fe++-cytochrome c reductase and cytochrome oxidase retained their individual activities after either treatment.Fe++-cytochrome c reductase was purified 60-fold to 70-fold. The enzyme was judged to be approximately 90% pure by disc electrophoresis, sedimentation, and DEAE-cellulose chromatography. A suitable assay system with a veronal–acetate buffer was developed for the determination of enzyme activity. The effects of inhibitors and potential activators were studied. No specific inhibitor or cofactor was found, although the enzyme was inhibited by various ionic compounds.Fe++-cytochrome c reductase was dissociated into two subunits, one protein and the other ribonucleic acid (RNA). Neither of the subunits had enzymatic activity and efforts to reconstitute the holoenzyme from the two subunits were unsuccessful. The molecular weights of the holoenzyme, protein subunit, and RNA subunit were determined as 100,000–110,000, 27,000–30,000, and 315,000–330,000, respectively. The protein subunit contained one non-heme iron atom per protein molecule. It was concluded that RNA is an essential component of the enzyme and the failure to recover the activity from subunits is due to the aggregation of RNA after dissociation.