Enzyme nitrogen fixation

Some of the critical enzymes in our cells are metalloproteins, large organic molecules made up of folded polymerized chains of amino acids that also include at least one metal atom. These metalloproteins are intensely studied by biochemists, because they control life and protect against disease. They have also been used to trace evolutionary paths. The d-block metals catalyze redox reactions, form components of membrane, muscle, skin, and bone, catalyze acid-base reactions, control the flow of energy and oxygen, and carry out nitrogen fixation. 

Nitrogen fixation is any process in which N2 in the atmosphere reacts to form any nitrogen compound. Biological nitrogen fixation is the enzyme-catalyzed reduction of N2 to NH3, NH4, or any organic nitrogen compound. 

In nature, nitrogen fixation is accomplished by nitrogenase, an enzyme that binds N2 and weakens its bonding sufficiently to break the triple bond. Only a few algae and bacteria contain nitrogenase. Our Chemishy and Life Box describes what is known about this enzyme. 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

In Nature, nitrogen fixation is mediated by the enzyme nitrogenase according to Eq. (1) (6)... 

A class of plants, called legumes, has bacteria which extract N2 directly, converting it to NH3. This nitrogen fixation process, catalyzed by an enzyme produced by the bacteria, is highly efficient at usual temperatures and pressures. 

Bacterial ADPGPP enzymes, 12 491 Bacterial a-amylases, 10 280 Bacterial artificial chromosome (BAC) vectors, 12 508 Bacterial cellulose, 20 557 Bacterial genera, nitrogen fixation by, 17 295-296... 

Molybdate, although present in small amounts in soil, is an essential nutrient for nitrogen fixation, specifically in the enzyme nitrogenase. It does not move readily through soil and is therefore considered to be of limited mobility. 

Hille, R. (2005) Molybdenum-containing hydroxylases, Arch. Biochem. Biophys., 433, 107-116. Howard, J.B. and Rees, D.C. (2006) How many metals does it take to fix N2 A mechanistic overview of biological nitrogen fixation, Proc. Natl. Acad. Sci. U.S.A., 103, 17088-17093. Knowles, J.R. (1991) Enzyme catalysis not different, just better, Nature, 350, 121-124. 

Biochemically, Mo draws attention because it is an essential enzyme cofactor in nearly all organisms, with particular importance for nitrogen fixation, nitrate reduction and sulfite oxidation. Such biochemical ubiquity is surprising in view of the general scarcity of Mo at the Earth s surface. 

At the same time, this redox lability makes Mo well suited as a cofactor in enzymes that catalyze redox reactions. An example is the prominence of Mo in nitrogen fixation. This prokaryotic metabolism, the dominant pathway for conversion of atmospheric Nj to biologically-useful NH, utilizes Mo (along with Fe) in the active site of the nitrogenase enzyme that catalyzes Nj reduction. Alternative nitrogenases that do not incorporate Mo have been identified, but are markedly less efficient (Miller and Eady 1988 Eady 1996). 

Redox catalysis Zn, Fe, Cu, Mn, Mo, Co, V Se, Cd, Nl Enzymes (see Table 11.4 for more Information) Reactions with oxygen (Fe, Cu) Oxygen evolution (Mn) Nitrogen fixation (Fe, Mo) Inhibition of llpid peroxidation (Se) Carbonic anhydrase (Cd) Reduction of nucleotides (Co) Reactions with H2 (Nl) Bromoperoxidase activity (V)... 

In marked contrast to the conditions under which nitrogen is converted to ammonia in the laboratory, in nature atmospheric N2 is converted to ammonia under the mildest of conditions by enzymes called nitrogenases in a process termed nitrogen fixation. This example testifies to the power of enzymes, about which much more follows in chapter 9. 

The biological nitrogen fixation process is Introduced. Discussion focusses on the Dominant Hypothesis of nitrogenase composition and functioning. The enzyme system catalyzes the six-electron reduction of N2 to 2 NH3 concomitant with the evolution of H2. ATP hydrolysis drives the process. The two protein components of the enzyme,... 

The process of nitrogen fixation is an essential part of the nitrogen cycle on the planet earth(l). It is estimated that greater than 60Z of the N2 that is ultimately converted to NH4+ is done so by the nitrogenase enzyme system. The availability of fixed nitrogen is often the limiting factor in plant growth( ). To... 

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