Biochem 2 - Ch. 25 notes

Biochem 2 - Ch. 25 notes - CHAPTER 25: Nitrogen acquisition...

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CHAPTER 25: Nitrogen acquisition and amino acid metabolism The nitrogen cycle Nitrogen in the environment exists primarily in an oxidized state, either as N 2 or nitrate (NO 3 - ). Biological systems acquire nitrogen by reducing this nitrogen to ammonium ions (NH 4 + ). Ammonium ions are then incorporated into organic molecules. All of these processes can occur in bacteria, but not in higher organisms. The oxidation of reduced nitrogen also occurs in nature. Hence there is a large nitrogen cycle (Fig 25.1) . Nitrate assimilation is the conversion of nitrate to ammonia. It occurs in plants, some fungi, and some bacteria. Nitrogen fixation is the conversion of N 2 to ammonia. It occurs only in bacteria. Sometimes these bacteria associate with plants. This is an anaerobic process. Animals can do neither, so they must acquire nitrogenous compounds from either plants or bacteria. Animals release nitrogen in a reduced form: ammonia, urea, or possibly uric acid. This release occurs to dispose some waste products or as decomposition. Nitrification . Bacteria then can take the reduced nitrogen, as ammonia, and oxidize it. Nitrifying bacteria convert ammonia to nitrite and nitrate, and use the energy released for growth. This process is called nitrification because it makes the nitrogen more readily available to plants. Nitrate is more soluble than ammonia, and plants absorb it more quickly. Denitrification is the conversion of nitrate to N 2 , which is released to the atmosphere. It is done by denitrifying bacteria, which are very efficient. The function of this process is to provide bacteria with an electron acceptor other than oxygen for energy generation. This is an anaerobic process. Oxygen is a preferred electron acceptor, and shuts this off in E. coli . Furthermore, oxygen interfers with this process. 25.1 Biological N acquisition. There are two routes of such acquisition: nitrate assimilation (over 99% of N incorporation) and nitrogen fixation (less than 1% of N incorporation). Both produce ammonia. Nitrate assimilation occurs in two steps (see equations on p 769 ). The first step is a two-electron reduction of nitrate to nitrite, catalyzed by nitrate reductase. 1
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The second step is a six-electron reduction of nitrite to ammonia, catalyzed by nitrite reductase. Nitrate reductase transfers a pair of electrons via four protein-bound intermediates to nitrate (see diagram on p 769 ). NADH to - SH to FAD to cyto b557 to MoCo and finally to nitrate. MoCo is a molybdenum-containing cofactor (see Fig 25.2a) MoCo is a cofactor for a variety of hydroxylase-type reactions: xanthine DH, aldehyde oxidase, sulfite oxidase. Nitrite reductase (plants) obtains electrons from photosynthetically reduced ferrodoxin. Electrons transferred to a 4Fe-4S cluster to siroheme (Fig 25.2b) and finally to nitrite, which binds the siroheme and accepts electrons from it. Microbial nitrite reductases have more complex electron transfer pathways.
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Biochem 2 - Ch. 25 notes - CHAPTER 25: Nitrogen acquisition...

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