metabolism_lecture_13 - Chapter 22: Amino acid and...

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Chapter 22: Amino acid and nucleotide synthesis Nitrogen fixation and incorporation of ammonia into biomolecules Biosynthesis of amino acids Biosynthesis of nucleotides Catabolism of purines
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Importance of nitrogen in biochemistry Nitrogen, carbon, hydrogen and oxygen are the main elemental constituents of living organisms Nitrogen is important for the peptide backbone of proteins present in: - several amino acid side chains (His, Lys, Arg, Trp, Asn, Gln) - nucleobases in DNA and RNA - several cofactors (NAD, FAD, Biotin … ) - many small hormones (epinephrine), neurotransmitters (serotonin), pigments (chlorophyll, heme)
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Biochemistry of molecular nitrogen Our atmosphere is rich in nitrogen but its chemical inertness prevents use of N 2 by most organisms Atmospheric nitrogen is converted to nitrogen compounds by a few non-biological processes NO from N 2 and O during lightning NH 3 from N 2 and H 2 in the industrial Haber process (iron oxide as catalyst) Atmospheric nitrogen is fixed directly by certain bacteria and archaea Most are free-living single-celled prokaryotes (e.g. archaea) root nodules of legumes N fixation as one of the crucial aspects of termite gut symbiosis
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Assimilation and production of nitrogen compounds Most organisms can readily assimilate nitrogen from ammonia (NH 4 + ) Soil bacteria quickly oxidize ammonia into nitrite (NO 2 - ) and nitrate (NO 3 - ) (nitrification reaction) Plants and microorganisms can assimilate nitrogen from nitrite and nitrate Some bacteria can use nitrate as the ultimate electron acceptor when establishing a proton gradient for ATP synthesis (denitrification reaction) Few bacteria can convert ammonia and nitrite into molecular nitrogen ( anammox bacteria )
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Nitrogen Fixation The reaction N 2 + 3 H 2 2 NH 3 is exothermic ( H 0 = -92.4 kJ/mol) and exergonic ( G 0 = -33.5 kJ/mol) but very slow due to the stability of the N 2 triple bond (high activation energy) Only few prokaryotes produce the enzymes of the nitrogenase complex for nitrogen fixation: Dinitrogenase reductase: dimer, contains one 4Fe-4S redox center Dinitrogenase: tetramer, 2 Mo (in iron-molybdenum cofactor),
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metabolism_lecture_13 - Chapter 22: Amino acid and...

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