Nitrogen metabolism amino acid biosynthesis

In the preceding sections, attention was focused on amino acid metabolism in the intact animal. We now examine the metabolic pathways of individual amino acids, which take place in the cells of various human tissues. The first reaction in the metabolic pathways of many amino acids is the loss of nitrogen through transamination or deamination. Conversely, the biosynthesis of many non-essential amino acids involves the addition of nitrogen to amino acid precursors amination and transamination. Decarboxylation, or loss of C02, is another reaction shared by many amino acids. 

Nitrogen Metabolism "" " i i (ammonia incorporation, amino acid biosynthesis, nitrate/nitrite reduction, proline oxidation, polyamine biosynthesis)... 

Semifermentation Process. In this process, the metabolic intermediate in the amino acid biosynthesis or the precursor thereof is added to the medium, which contains carbon and nitrogen sources, and other nutrients required for growth and production, and the metaboHte is converted to the... 

The topic of nitrogen metabolism includes the biosynthesis and breakdown of amino acids, purines, and pyrimidines also, the metabolism of porphyrins is related to that of amino acids. Many of these pathways, particularly the anabolic ones, are long and complex. In discussing pathways in which the amount of material is large and highly detailed, we shall concentrate on the most important points. Specifically, we shall concentrate on overall patterns and on interesting reactions of wide applicability. We shall also be interested in health-related aspects of this material. Other reactions will be found at the BiochemistryNow Interactive website for this text. It can be considered a repository of supplementary material for this chapter, and we shall refer to it a number of times. 

In addition to transamination reactions, one-carbon transfer reactions occur frequently in amino acid biosynthesis. A good example of a one-carbon transfer can be found in the reactions that produce the amino acids of the serine family. This family also includes glycine and cysteine. Serine and glycine themselves are frequently precursors in other biosynthetic pathways. A discussion of the synthesis of cysteine will give us some insight into the metabolism of sulfur, as well as that of nitrogen. 

Effect on Protein Metabolism, Corticoid hormones affect various steps of protein metabolism amino acid penetration in the cells, intracellular biosynthesis of amino acids from small precursors, protein synthesis, and protein catabolism. In discussing the effect of corticoid hormones on protein synthesis, it is necessary to distinguish between the effects of the glucocorticoid on muscle and liver. The injection of Cl 1-oxygenated corticosteroid increases the excretion of urinary nitrogen, with loss of tissue nitrogen (e.g., in heart and kidney) [51]. 

L. Fowden, I. K. Smith and P. M. Dunhill. Some observations on the specificity of amino acid biosynthesis and incorporation into plant proteins. In Recent Aspects of Nitrogen Metabolism in Plants, edited by E. J. Hewitt and C. V. Cutting, Academic Press, London, 1968. 

Biosynthesis of Protein. The dynamic equilibrium of body protein was confirmed by animal experiments using A/-labeled amino acids in 1939 (104). The human body is maintained by a continuous equilibrium between the biosynthesis of proteins and their degradative metabolism where the nitrogen lost as urea (about 85% of total excreted nitrogen) and other nitrogen compounds is about 12 g/d under ordinary conditions. The details of protein biosynthesis in living cells have been described (2,6) (see also Proteins). 

Regulation is crucial in the biosynthesis of the nitrogen-containing compounds. Because each amino acid and each nucleotide is required in relatively small amounts, the metabolic flow through most of these pathways is not nearly as great as the biosynthetic flow leading to carbohydrate or fat in animal tissues. Because the different amino acids and nucleotides must be made in... 

Amino Acids Amino acids that enter the liver follow several important metabolic routes (Fig. 23-14). (1) They are precursors for protein synthesis, a process discussed in Chapter 27. The liver constantly renews its own proteins, which have a relatively high turnover rate (average half-life of only a few days), and is also the site of biosynthesis of most plasma proteins. (2) Alternatively amino acids pass in the bloodstream to other organs, to be used in the synthesis of tissue proteins. (3) Other amino acids are precursors in the biosynthesis of nucleotides, hormones, and other nitrogenous compounds in the liver and other tissues. 

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