9-23-08 - Nitrogen cycles Nitrate/nitrite Reductase Ammonia...

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Nitrogen cycles Nitrate/nitrite Reductase Ammonia monooxygenase Hydroxylamine oxidoreductase
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N 2 + 3H 2 ---> 2NH 3 ∆G’º = -33.5 kJ/mol However, N-N triple bond is a significant kinetic barrier Bond energy = 930 kJ/mol N 2 + 10 H + + 8 e - + 16 ATP ---> 2 NH 4 + + 16 ADP + 16 P i + H 2 Nitrogen is fixed by anaerobic bacteria 0 -3
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QuickTimeª and a TIFF (Uncompressed) decompressor are needed to see this picture. FeMo Cofactor
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Proposed mechanism for dinitrogen reduction Mo FeS N N Mo N N FeS H + Mo N NH FeS H + Mo N NH 2 + H + Mo N NH 3 + NH 3 Mo N Mo N + Mo N + Mo N + Mo N + Mo N + H + Mo NH H + Mo NH 2 H + Mo NH 3
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Nitrate:ferredoxin reductase S Mo 4+ O O - N + O O - S Mo 6+ O O - N O O - S Mo 6+ O O 2- S Mo 6+ O OH - H + H - S Mo 4+ O H 2 O
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Dissimilatory/Respiratory Nitrite Reductase Siroheme and ferredoxin dependent looks and behaves a lot like cytochrome c oxidase NO 2 - ----> NO ----> NH 2 OH ----> NH 4 + Allows anaerobic organisms to respire electrons onto nitrite instead of oxygen +3 +1 -1 -3
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Dissimilatory Nitrite Reductase NO 2 - ----> NO Produces NO and N 2 O - potent greenhouse gasses +3 +2 NO ---> N 2 O +2 +1 Nitrite reductase, multicopper enzyme Nitric oxide reductase, heme dependent N 2 O ---> N 2 +1 0 Nitrous oxide reductase, multicopper enzyme
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Biological Nitrification NH 3 ---> NO 2 - ---> NO 3 - -3 +3 +5 NH 3 + O 2 + 2 e - + 2 H + ---> NH 2 OH + H 2 O Ammonia monooxygenase (related to methane monooxygenase) NO 2 - ---> NO 3 - is non-enzymatic
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NH 2 OH + H 2 O ---> NO 2 - + 5 H+ + 4 e - Hydroxylamine oxidoreductase (heme dependent) 2 of these electrons are transferred to cytochrome bc 1 Ultimately they land on O 2 and generate proton gradient 2 electrons are returned to AMO to complete the cycle
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Assimilation of ammonia Use glutamate and glutamine as reservoirs
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Glutamine synthase glutamate + NH 4 + + ATP ---> glutamine + ADP + P i + H + Glutamate synthase glutamine + α KG + NADPH + H + ---> 2glutamate + NADP + Net reaction α KG + NADPH + NH 4 + + ATP ---> glutamate + NADP + + H 2 O + ADP + P i Need catalytic amounts of glutamate! There has to be another way!
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H COO - CH 2 CH 2 NH 3 + O O - ATP H COO - CH 2 2 NH 3 + O OPO 3 2- H NH 2 B H - 2 2 NH 3 + O NH 2 Glutamine synthase
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Glutamate synthase COO - C + H 3 N H CH 2 CH 2 C NH 2 O E S H B H A COO - C + H 3 N H CH 2 CH 2 C O S E +NH 3 H 2 O COO - C + H 3 N H CH 2 CH 2 C O - O COO - C O CH 2 CH 2 C O - O COO - C + H 2 N CH 2 CH 2 C O - O H - H A COO - C + H 3 N H CH 2 CH 2 C O - O
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What about the reverse reaction of glutamate dehydrogenase?
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9-23-08 - Nitrogen cycles Nitrate/nitrite Reductase Ammonia...

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