Nitrogen metabolism purine biosynthesis

Although both purine and pyrimidine rings have one 6-mem-bered component with two nitrogens and four carbons, the purines and pyrimidnes are not related metabolically. Distinct pathways for purine biosynthesis and degradation and for pyrimidine biosynthesis and degradation, exist in all organisms. 

See also Coenzymes in Nitrogen Metabolism, De Novo Biosynthesis of Purine Nucleotides, N-10-Formyltetrahydrofolate, 5-Methyltetrahydrofolate, 7,8-Dihydrofolate (DHF)... 

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. 

Purine bases are a group of compounds found in plants and animals —they include nucleic acids. Their biosynthesis is complex with numerous non-amino-acid precursors (Samuelsson 1992). Xanthine, an oxidised purine that occurs as a breakdown product of nucleic acid metabolism, is itself oxidised in the body to uric acid. Xanthine consists of two fused ring systems each containing two nitrogen atoms. 

The pattern of histidine biosynthesis began to emerge when certain metabolites accumulated by histidineless mutants of Nevrospora crasaa and E. coli were isolated and identified. Other clues were an interrelationship between histidine and purine metabolism shown by a sparing effect by histidine of the purine requirement of Lactobacillua casei [276, 277). k little later it was established that the purines were the source of at least a portion of the carbons and nitrogen of the imidazole ring 277-f 9). 

In addition to their role as components of nucleoproteins, purines and pyrimidines are vital to the proper functioning of the cell. The bases are constituents of various coenzymes, such as coenzyme A (CoA), adenosine triphosphate (ATP), guanosine triphosphate (GTP), cytidine triphosphate (CTP), diphosphopyridine nucleotide (DPN), triphosphopyridine nucleotide (TPN), and flavin adenine dinucleotide (FAD). A pyrimidine derivative, cytidine diphosphate choline, is involved in phospholipid synthe another pyrimidine compound, uridine diphosphate glucose, is an important substance in carbohydrate metabolism. Cytidine diphosphate ribitol functions in the biosynthesis of a new group of bacterial cell-wall components, the teichoic acids. While mammals excrete nitrogen derived from protein catabolism in the form of urea, birds eliminate their nitrogen by synthesizing it into the purine compound, uric acid. 

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