PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Amino Acid Metabolism 1:Nitrogen fixation and assimilation, amino acid degradation, the urea cycleBioc 460 Spring 2008 - Lecture 38 (Miesfeld)Urea is a nitrogen-containing metabolite that efficiently removes toxic ammoniaRed clover is a leguminous plant that is often used in crop rotation strategiesGlutamine synthetase converts glutamate to glutamine through a nitrogen assimilation reaction•Certain types of bacteria can use nitrogen fixation reactions to convert atmospheric N2 into NH4+ which can then be incorporated into the amino acids glutamate and glutamine by plants. •The enzymes glutamate synthase, glutamine synthetase, glutamate dehydrogenase, and aminotransferases are responsible for the vast majority of nitrogen metabolizing reactions in most organisms.•Protein degradation by the proteasomal complex releases oligopeptides that are degraded into individual amino acids that are either recycled for protein synthesis, or deaminated to salvage reduced carbon for energy.•The urea cycle uses nitrogen from NH4+ and the amino acid aspartate to generate urea which is excreted to maintain daily nitrogen balance.Key Concepts in Amino Acid MetabolismAmino Acid MetabolismDigestion of dietary protein, and protein turnover within cells, especially muscle cells, provides amino acids that serve as metabolic intermediates in anabolic reactions requiring nitrogen-containing precursors. The carbon skeleton of amino acids can be harvested for energy converting reactions, whereas, the nitrogen is safely removed to avoid ammonia toxicity.Nitrogen fixation and assimilation by plants and bacteriaNitrogen is the second most abundant element in the biosphere after carbon, and in addition to its presence in amino acids and nucleotides, it is also found in some carbohydrates (glucosamine) and lipids (sphingosine), as well as, the enzyme cofactors thiamine, NAD+, and FAD. Nitrogen in biological compounds ultimately comes from nitrogen gas (N2) which constitutes 80% of our atmosphere. However, N2 must first be reduced to NH3 (the ionized form of ammonia in solution is NH4+) by the process of nitrogen fixation, or oxidized to nitrate (NO3-) by atmospheric lightning, before it can be used by other liver organisms.Plants cannot carry out nitrogen fixation on their own, but they can incorporate NH4+ they obtain from the environment into the amino acids glutamate and glutamine through a process called nitrogen assimilation. When animals eat plants, amino acids and nucleotides provide the nitrogen needed to synthesize a variety of biomolecules.Nitrogen fixation and assimilation by plants and bacteria1. What purpose does nitrogen fixation and assimilation serve in the biosphere?Nitrogen fixation takes place in bacteria and is the primary process by which atmospheric N2 gas is converted to ammonia (NH4+) and nitrogen oxides (NO2- and NO3-) in the biosphere. Nitrogen assimilation incorporates this ammonia into amino acids, primarily glutamate and glutamine. 2. What are the net reactions of nitrogen fixation and assimilation by plants and bacteria?Nitrogen fixation is mediated by the nitrogenase enzyme complex:N2 + 8 H+ 8 e- + 16 ATP + 16 H2O ----> 2 NH3 + H2 + 16 ADP + 16 PiNitrogen assimilation using glutamine synthetase and glutamate synthase: -ketoglutarate + NH4+ + ATP + NADPH + H+ --> Glutamate + ADP + Pi + NADP+Nitrogen fixation and assimilation by plants and bacteria3. What are the key enzymes in nitrogen fixation and assimilation?Bacterial nitrogenase complex – is the enzyme that uses redox reactions coupled to ATP hydrolysis to convert N2 gas into 2 NH3. Glutamine synthetase - is found in all organisms and it incorporates NH4+ into glutamate to form glutamine through an ATP coupled redox reaction. Glutamate synthase - is found in bacteria, plants, and some insects, and it works in concert with glutamine synthetase to replenish glutamate so that the glutamine synthetase reaction is not substrate limited. Glutamate dehydrogenase - is found in all organisms and it interconverts glutamate, NH4+, and -ketoglutarate in a redox reaction utilizing either NAD(P)+/NAD(P)H.Nitrogen fixation and assimilation by plants and bacteria4. What are examples of nitrogen fixation and assimilation in real life?Natural fertilizers can be used in organic farming to reduce the dependence on industrial sources of nitrogen. The two most common sources of natural fertilizers are manure, if livestock are readily available, and crop rotation practices in which leguminous plants such as soy bean or clover, are planted in alternate seasons with nonleguminous crop plants such as corn and wheat. By plowing under the leguminous plants, the nitrogen contained in the plants is released into the soil and processed by soil bacteria to provide nitrogenous compounds for the corn and wheat plants.Nitrogen fixationIn order to obtain nitrogen from the atmosphere for incorporation into biomolecules, the triple bond of N2 must be broken. However, this is not easily done considering that the bond energy of N2 is 930 kJ/mol. To overcome this high energy barrier, one of three processes are required, 1. Biological fixation by bacteria that reduce N2 to NH3 through an ATP-dependent process requiring the multisubunit enzyme complex nitrogenase. 2. Industrial fixation by the Haber process in which N2 and H2 gases are heated to ~500ºC under a pressure of ~250 atmospheres (~350 kilopascals) to produce liquid ammonia which is used commercially to make fertilizer. 3) Atmospheric fixation as a result of lightning which breaks the N2 triple bond and allows nitrogen to combine with oxygen to form nitrogen oxides that are dissolved in rain and fall to earth.Nitrogen fixationRhizobium bacteriumThe Haber processTucson lightningNitrogen fixation in bacteriaBiological nitrogen fixation by bacteria requires the activity of nitrogenase, a large protein complex consisting of two functional components. One component is called dinitrogenase reductase (Fe-protein) which consists of two identical subunits that each contain a binding site for ATP, and a single 4Fe-4S redox center liganded to cysteine residues in the two subunits.
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