Enzymes in nitrogen fixation

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). 

In nitrogen fixation, the difficult reduction of N2 to NHJ is effected by the nitrogenase enzyme, which contains iron and molybdenum. An iron - -vanadium form of nitrogenase and an iron-only form are also known, but their presence in marine phytoplankton has not yet been established... 

The function of molybdenum in enzymic mechanisms has been reviewed and the role of molybdenum in nitrogen fixation examined. ... 

QUESTION 14.Z What are the substrates and enzyme components in nitrogen fixation Arrange... 

In a perspective provided by structure of the Periodic Table, molybdenum and tungsten are distinct in being the only 4d- and 5d-transition metals that are required for the normal metabolism of biological systems. These metals play a vital role as the catalytic centres of a wide variety of enzymes. Mo was first identified as an essential trace element in the 1930s, because of its role in nitrogen fixation this metal is now known to be the catalytic centre of over 50 enzymes. Evidence for the involvement of W in biological systems has been obtained only relatively recently, especially for enzymes of hyperthermophilic archea that thrive near lOOX. ... 

Reducing nitrogen (as occurs in nitrogen fixation), requires energy and electrons with low-potential (electron carriers with very low EO ). The enzymes involved are very sensitive to oxygen and must be studied only under anaerobic conditions. In root nodules of plants, the anaerobic environment is provided by the protein leghemoglobin, which binds any 02 that makes its way into the nodules. 

The nitrogenase enzyme found in root nodules of leguminous plants catalyzes crucial reactions in nitrogen fixation. 

Iron and sulfur can be extracted from F. the resulting apoferredoxin is reactivated by iron(II) salts and sulfides. The synthesis of the iron-free protein has been achieved by the Merrifield technique. On account of their properties as redox systems (Fe +e" Fe ") the F. effect electron transport between enzyme systems but do not exhibit any enzymatic activity. They transport electrons in the respiratory chain, in photosynthesis, and in nitrogen fixation. The iron-sulfur protein P439 of the Fc4S4-type (Mr 11600) plays a role in photosynthesis. Conclusions can be drawn about the evolutionary histories of plants from the similarities and differences in the amino acid sequences. For the evolutionary history of F. in photosynthesis, see Lit.K F. with FejSj- and FejSg-clusters also occur in bacteria Lit. TrendsBiochem.Sci. 13,30-33(1988). FEMSMicrobiol. Rev. 54,155-176 (1988) Trends Biochem. Sci. 13,369 f. (1988). 

Cobalt, Co an essential bioelement present in traces in plants, animals and microorganisms. It is important as a constituent of vitamin B12. Traces of Co are required for microbial growth. It is a cofactor or prosthetic group of several enzymes, e.g. pyrophosphatases, peptidases, arginase, as well as certain enzymes involved in nitrogen fixation. 

Molybdenum is an essential element for several enzymes important in plant and animal metabolism. It has a unique role in nitrogenase, an enzyme that converts molecular nitrogen into ammonia in nitrogen fixation. Molybdenum enzymes provide the key steps in the fixation of nitrogen by microorganisms and its utilization by higher plants (Table 1). 

All the systems described in this chapter are organometallic in character. Coenzyme has several forms with M—C or M—H bonds. In nitrogen fixation, CO binds competitively at the active site. The nickel enzymes are believed to operate via intermediates with M—H (H2ase) or M—C bonds (CODH and MeCoM reductase). 

In the following period of fifty years litde progress was made towards elucidating the mechanism of the process. Many attempts were made to obtain cell-fi ee preparations containing the enzymes presumably involved in nitrogen fixation, but these were unsuccessful until 1960. The intracellular orgeinisation of enzyme systems is known to be complex, groups of enzymes involved in sequential reactions... 

Hydroxylamine is known to be highly toxic to plants so the possibility that it is an obligatory intermediate in nitrate reduction must be considered with caution. Interest in hydroxylamine arises from the lengthy controversy, briefly mentioned here, concerning the rela-j five merits of hydroxylamine or ammonia as the key intermediate in nitrogen fixation. The toxicity of hydroxylamine may well be due to its action as an enzyme inhibitor. However, in spite of its toxicity, there are still some claims that hydroxylamine may be an intermediate in the formation of amino acids by oxime formation with such carbonyl compounds as glyoxilic acid (formed in photosynthesis), I pyruvate, a-ketoglutarate or oxaloacetate (intermediates in carbo-... 

Biological systems that participate in nitrogen fixation contain a composite metal-requiring enzyme system which assures a source of electrons of high reductive efficiency, and another enzyme which makes use of these electrons to reduce N2 to NH4 ... 

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. 

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