Denitrification enzyme activity

Denitrification enzyme activity Catabolic nitrate redaction involves several enzymes including dissimilatory nitrate rednctase (nitrate to nitrite), nitrite rednctase (nitrite to nitric oxide), nitric oxide reductase (nitric oxide to nitrons oxide), and nitrons oxide reductase (nitrous oxide to nitrogen gas). The combined effect of all these enzymes is operationally dehned as denitrification enzyme activity (DEA), which is now rontinely measnred under... 

FIGURE 8.46 Denitrification enzyme activity as a function of distance from inflow of a wetland. (From White and Reddy, 1999.)... 

White, J. R. and K. R. Reddy. 1999. Influence of nitrate and phosphorus loading on denitrification enzyme activity in Everglades wetland soils. Soil Sci. Soc. Am. J. 63 1945-1954. 

Measurement of the enzyme activity of the respiratory electron transport system (ETS activity) has also been used to estimate denitrification rates in the Arabian Sea (Naqvi and Shail a, 1993) and eastern tropical South Pacific (Codispoti and Packard, 1980) denitrification zones. To measure the ETS activity a crude enzyme extract is made by grinding filtered seawater samples in a buffered medium to liberate the enzymes, after which the ETS substrates NADT1+ and NADPT1+ along with tetrazo-hum salt are added as an artificial electron acceptor. Samples are then incubated for a... 

Peterjohn W. T. (1991) Denitrification enzyme content and activity in desert soils. Soil Biol. Biochem. 23, 845 -855. 

In a system with active denitrification, nitrate levels are usually low, thus, measurement of nitrate in soil and water column does not provide a reliable indication of this process. As denitrification is mediated by heterotrophic microorganisms, its rate may be regulated by nitrate concentration (electron acceptor) and available C (electron donor). Significant correlations were observed between denitrification rates and available organic C (mineralizable organic C) (Gale et al., 1993 D Angelo and Reddy, 1999). Similarly, MBC and denitrification enzyme assay (DEA) can also serve as potential indicators of denitrification rates (White and Reddy, 1999). 

Level II and III indicators may include more comprehensive measurements such as enzyme activity, denitrification, microbial and algal diversity, microbial biomass, primary production, and acid volatile sulfide and gaseous flux measurements to name a few. 

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

The nitrite formed is either excreted directly or reduced by non-ATP-yielding reactions to ammonia. The enzyme machinery for both processes, nitrate/nitrite respiration and denitrification, is formed only under anaerobic conditions or conditions of low oxygen tension. In fact, the activities of the enzymes involved in dissimila-tory nitrate reduction are strongly inhibited by oxygen. Thus, denitrification and nitrate/nitrite respiration take place only when oxygen is absent or available in insufficient amounts. 

Copper Enzymes in Denitrification Copper Hemo-cyanin/Tyrosinase Models Copper OrganometalUc Chemistry Copper Proteins Oxidases Copper Proteins with Dinuclear Active Sites Copper Proteins with Type 1 Sites Superconductivity. 

Active Sites, Copper Proteins Oxidases, Copper Proteins with Type 1 Sites, Copper Proteins with Type 2 Sites, Copper Enzymes in Denitrification, Iron-Sulfur Models of Protein Active Sites, Iron-Sulfur Proteins Nickel Enzymes Cofactors and Nickel Models of Protein Active Sites). However, since many metalloenzymes have been found or postulated to incorporate metal-sulfur bonding, it is appropriate that a very short sununary be included here. 

Chalcogenides Solid-state Chemistry Copper Enzymes in Denitrification Copper Hemocyanin/Tyrosinase Models Copper Proteins Oxidases Copper Proteins with Dinuclear Active Sites Copper Proteins with Type 1 Sites Copper Proteins with Type 2 Sites Iron Sulfitf Models of Protein Active Sites Iron-Snlfiir Proteins Nickel Enzymes Cofactors Nickel Models of Protein Active Sites Polynuclear Organometallic Cluster Complexes. 

Typical assessments of denitrification activity only assess the activity of the first three enzymes and do not consider the sensitivity of nitrous oxide reductase to toxic compounds. However, as hypothesized by others [14,15], it appears that nitrous oxide reductase is much more sensitive to TNT than the other three enzymes based on corresponding EC50 values of 400 mg kg-1 for the first three enzymes and 26 mg kg-1 for nitrous oxide reductase [10],... 

Copper- and heme-containing NiRs are both key enzymes in denitrification. They are both homooligomers and their subunits contain two distinct redox-active metal centers, an electron accepting site and a catalytic electron delivery center where the single electron reduction of nitrite to NO takes place. Thus, PR studies providing comparison of the two enzyme families are helping to resolve the different mechanisms of control of intramolecular ET reactivity. Internal electron transfer could be a rate-determining step in the catalytic cycle of both enzymes. 

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