Sec_6 - 6-1 Nitrogen Metabolism - Amino Acid and Nucleotide...

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6-1 N i t r o g e n M e t a b o l i s m - A m i n o A c i d a n d N u c l e o t i d e S y n t h e s i s Nitrogen exists in nature in 9 different oxidation states ranging from +5 to -3 and living organisms participate in the interconversions among all of these oxidation states. NO 3 - [NO 2 ] NO 2 - [NO] N 2 O 2 -2 N 2 H-N=N-H H 2 NOH NH 3 +5 +4 +3 +2 +1 0 -1 -2 -3 nitrate nitrite hyponitrite diimide hydroxyl ammonia amine The following generalized diagram demonstrates the basic processes that we will be covering. N 2 denitrification nitrate reduction nitrogen fixation NH 3 NO 2 - NO 3 - nitrification excretion assimilation organic N Of these processes, the most important in a global sense is nitrogen fixation because it is the main source of all organic carbon. 1 . N i t r o g e n F i x a t i o n Non biological nitrogen fixation provides a clear picture of the large amounts of energy needed for reducing (or oxidizing) the triple bond of nitrogen gas N 2 . N 2 + 3 H 2 2NH 3 450 o C 200 atm N 2 + O 2 2 NO 2 NO 2 electrical storms Biological nitrogen fixation on the other hand occurs at 25 o C and at 1 atm pressure, providing a clear example of the catalytic properties of enzymes.
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6-2 Nitrogen fixation is carried out by bacteria, either free living such as Azotobacter and Clostridium species, or by symbiotic bacteria in symbiosis with plants, most commonly in root nodules formed by Rhizobium species. Effectively it can be viewed as a series of three two electron-transfers that reduce N 2 to 2 NH 3 . N N H-N=N-H H 2 N-NH 2 2 NH 3 energy 2e - 2H + energy 2e - 2H + energy 2e - 2H + Not unlike photosynthesis, nitrogen fixation can be broken down into two stages. (a) reduced electron carrier and ATP producing stage - phosphoroclastic reactions (b) nitrogen reducing stage - nitrogenase . The process must be carried out anaerobically. Free living bacteria that are fixing nitrogen are usually found in swampy anoxic locations where the process can proceed. Symbiotic bacteria in root nodules create an anoxic environment within the nodule by producing leghemoglobin that removes O 2 before it can interact with and inactivate the nitrogenase. ( a ) p h o s p h o r o c l a s t i c r e a c t i o n s CC O O H 3 C O pyruvate CS C o A H 3 C O acetyl-CoA CoASH CO 2 NAD + NADH + H + Pyruvate dehydrogenase CO P O 3 H 3 C O acetyl-phosphate Pi CoASH Phospho transacetylase H 3 C O acetate ADP ATP Acetate kinase ATP and NADH produced in these reactions are then used in the second stage by the nitrogenase system. Nitrogenase 2
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6-3 ( b ) n i t r o g e n a s e c o m p l e x The nitrogenase complex is made up of two proteins, dinitrogenase reductase (azoferridoxin) and dinitrogenase (molybdoferridoxin). Electrons are fed into the complex via ferridoxin. NADH + H + NAD + + 2H + ferridoxin ox ferridoxin red Fe-prot red Fe-prot ox Fe-prot red 16 ATP 8 e - Fe-prot ox 16ADP + 16 Pi MoFe-prot ox N N MoFe-prot ox HN=NH MoFe-prot red N N MoFe-prot ox N 2 16 ATP 16 ADP + 16 Pi 2 H + dinitrogenase reductase dinitrogenase A peculiarity with the nitrogenase is that for 1 N 2 to be reduced to 2 NH 3
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Sec_6 - 6-1 Nitrogen Metabolism - Amino Acid and Nucleotide...

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