Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Metabolism of Nitrogen-Containing Compounds
N2 Fixation and Other Reactions of Inorganic Nitrogen Compounds

Nitrogen, which is a component of a vast number of compounds, undergoes complex metabolic transformations. Inorganic forms of nitrogen in the environment are highly diverse—ranging from the nitrate ion, in which the oxidation state of nitrogen is +5, to ammonia, where the oxidation state is -3. Living Cells are capable of both reducing and oxidizing these inorganic forms. Organic forms of nitrogen are most frequently formed by the incorporation of the ammonium ion into amino and amide groups. Once incorporated into an organic compound, nitrogen can subsequently be transferred to many other carbon-containing compounds. Compounds such as glutamic acid, aspartic acid, glutamine, asparagine, and carbamoyl phosphate participate particularly actively in such transfer reactions. Together, they form a common nitrogen pool from which nitrogen can be drawn for various metabolic needs and to which it can be replenished.

In addition to the Metabolic pathways of Synthesis and degradation of Amino Acids, NUCLEOTIDES, and other nitrogenous substances, many organisms possess a specialized METABOLISM that channels excess nitrogen into relatively nontoxic excretion products. All of these aspects of Nitrogen metabolism will be examined in this chapter, but due to the exceptional complexity of the subject, the Structure/133.html">Discussion will be concise. First, we will examine the reactions by which organic nitrogen compounds are formed from inorganic precursors, and then we will turn to the Reactions Involving the nitrogen pool. Next, we will discuss the specific Synthesis and Catabolism reactions of individual nitrogenous compounds.

The vast majority of the biosphere's nitrogen exists as chemically inert N₂, which accounts for up to 80% of all atmospheric molecules. Nitrogen "fixation" occurs primarily either through the action of lightning (leading to The formation of nitrogen oxides, which are subsequently converted into nitrate and nitrite) or As a result of bacterial activity [1]. Human activity also makes a certain contribution to Nitrogen Fixation through The production of chemical fertilizers. The interconversions between nitrate and nitrite, on the one hand, and ammonia and organic nitrogen compounds, on the other, belong to active biological processes. Some of these reactions have already been discussed in Chapter 10. For example, we examined the bacterial oxidation of NH₃ to N^-₂ and NO^-₃ (Chapter 10, Section E, 1) and the reduction of NO^-₃ to NO^-₂ [Equation (10-32)]. For many Bacteria and higher plants, this reduction of nitrate serves as the initial stage of an important multi-step assimilation process, during which nitrite is ultimately reduced to NH₃. However, the chemical mechanisms of nitrite reduction to NH₃ [Equation (10-36)] are not yet fully understood.