The hemerythrin-like diiron protein from Mycobacterium kansasii is a nitric oxide peroxidase.

The hemerythrin-like protein from Mycobacterium kansasii (Mka HLP) is a member of a distinct class of oxo-bridged diiron proteins that are found only in mycobacterial species that cause respiratory disorders in humans. Because it had been shown to exhibit weak catalase activity and a change in absorbance on exposure to nitric oxide (NO), the reactivity of Mka HLP toward NO was examined under a variety of conditions. Under anaerobic conditions, we found that NO was converted to nitrite (NO2-) via an intermediate, which absorbed light at 520 nm. Under aerobic conditions NO was converted to nitrate (NO3-). In each of these two cases, the maximum amount of nitrite or nitrate formed was at best stoichiometric with the concentration of Mka HLP. When incubated with NO and H2O2, we observed NO peroxidase activity yielding nitrite and water as reaction products. Steady-state kinetic analysis of NO consumption during this reaction yielded a Km for NO of 0.44 µM and a kcat/Km of 2.3 × 105 M-1s-1. This high affinity for NO is consistent with a physiological role for Mka HLP in deterring nitrosative stress. This is the first example of a peroxidase that uses an oxo-bridged diiron center and a rare example of a peroxidase utilizing NO as an electron donor and cosubstrate. This activity provides a mechanism by which the infectious Mycobacterium may combat against the cocktail of NO and superoxide (O2•-) generated by macrophages to defend against bacteria, as well as to produce NO2- to adapt to hypoxic conditions.

Reduction of a Heme Cofactor Initiates N-Nitroglycine Degradation by NnlA.

Linear nitramines are potentially carcinogenic environmental contaminants. The NnlA enzyme from Variovorax sp. strain JS1663 degrades the nitramine N-nitroglycine (NNG)-a natural product produced by some bacteria-to glyoxylate and nitrite (NO2-). Ammonium (NH4+) was predicted as the third product of this reaction. A source of nonheme FeII was shown to be required for initiation of NnlA activity. However, the role of this FeII for NnlA activity was unclear. This study reveals that NnlA contains a b-type heme cofactor. Reduction of this heme-either by a nonheme iron source or dithionite-is required to initiate NnlA activity. Therefore, FeII is not an essential substrate for holoenzyme activity. Our data show that reduced NnlA (FeII-NnlA) catalyzes at least 100 turnovers and does not require O2. Finally, NH4+ was verified as the third product, accounting for the complete nitrogen mass balance. Size exclusion chromatography showed that NnlA is a dimer in solution. Additionally, FeII-NnlA is oxidized by O2 and NO2- and stably binds carbon monoxide (CO) and nitric oxide (NO). These are characteristics shared with heme-binding PAS domains. Furthermore, a structural homology model of NnlA was generated using the PAS domain from Pseudomonas aeruginosa Aer2 as a template. The structural homology model suggested His73 is the axial ligand of the NnlA heme. Site-directed mutagenesis of His73 to alanine decreased the heme occupancy of NnlA and eliminated NNG activity, validating the homology model. We conclude that NnlA forms a homodimeric heme-binding PAS domain protein that requires reduction for initiation of the activity. IMPORTANCE Linear nitramines are potential carcinogens. These compounds result from environmental degradation of high-energy cyclic nitramines and as by-products of carbon capture technologies. Mechanistic understanding of the biodegradation of these compounds is critical to inform strategies for their remediation. Biodegradation of NNG by NnlA from Variovorax sp. strain JS 1663 requires nonheme iron, but its role is unclear. This study shows that nonheme iron is unnecessary. Instead, our study reveals that NnlA contains a heme cofactor, the reduction of which is critical for activating NNG degradation activity. These studies constrain the proposals for NnlA reaction mechanisms, thereby informing mechanistic studies of degradation of anthropogenic nitramine contaminants. In addition, these results will inform future work to design biocatalysts to degrade these nitramine contaminants.

Structural contributions of different manganese oxide minerals on the redox transformations and proliferation of iodine.

The redox capabilities of birnessite minerals are contingent upon the physical characteristics of the solid, indicating that different allotropes have various reactivities. Here, the role of these structural differences on the oxidation of iodine, a risk driving environmental contaminant in several federal complexes, was investigated. The mechanism of which can be seen here, with one of the minerals of study, acid birnessite. The pH range chosen for this study was pH 5-6. Throughout the experiments it was seen that the average oxidation state (AOS) had the greatest contribution to the differences in redox capabilities of the various birnessite minerals. Several trends were observed throughout this study: as AOS decreased, oxidation of iodide (I-) increased; as specific surface area (SSA) increased, the sorption of iodate (IO3-) increased. Additional experiments were conducted at trace levels of iodine, to better model environmental conditions. In that case, a one-step conversion of I- to IO3- occurred, to a greater extent than under artificially elevated concentrations.

TcO2 oxidative dissolution by birnessite under anaerobic conditions: a solid-solid redox reaction impacting the environmental mobility of Tc-99.

Remediation efforts for the abatement of Tc-99 contamination in the environment have traditionally focused on the reduction of soluble pertechnetate (Tc(vii)O4-) to insoluble, and less mobile, technetium(iv) oxide (TcO2). Effectiveness of the reductive immobilization of Tc-99 depends on the susceptibility of TcO2 to oxidation to TcO4-in situ, as it is subject to dissolution by oxidizing agents, such as oxygen. Manganese minerals can be a liability for the long-term in situ immobilization of Tc-99, even in suboxic and anoxic systems due to their strong oxidizing capacity. This study presents for the first time the oxidative dissolution of TcO2 to pertechnetate by birnessite under anaerobic conditions. Oxidative dissolution of TcO2 was studied as a function of pH and birnessite:TcO2 ratios and in the presence of Ca2+ and Mn2+. As low as 5 mg of birnessite dissolved ∼65% of the original TcO2 in the suspensions and subsequently released TcO4- in the aqueous phase at both pH 6.5 and 8 in the absence of oxygen. On the other hand, the ability of birnessite to sequester calcium and manganese on its surface at pH 6.5 through sorption was shown to inhibit the oxidative capacity of birnessite. Maximum TcO4- release in the aqueous phase by Ca- and Mn-loaded birnessite was ∼50% less compared to pure birnessite, indicating that divalent cations sorb on active centers responsible for birnessite's oxidative capacity and potentially passivate the mineral. In summary, birnessite exerts strong geochemical controls over the mobility of Tc-99 in anoxic systems by oxidatively mobilizing the otherwise insoluble Tc(iv) to Tc(vii) and their presence in natural systems needs to be taken into account when long-term remediation strategies are being designed.