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3 General Aspects of Nucleotide Biosynthesis and Interconversions
3.1 Introduction
This chapter focuses on (i) an outline of the formation of purine, pyrimidine, and pyridine ribonucleoside monophosphates from small molecular weight precursors, the initial metabolites of de novo nucleotide biosynthesis, (ii) conversion of these initial metabolites to nucleoside di- and triphosphates, and (iii) synthesis of deoxyribonucleotides from ribonucleotides. The biosynthesis of purine, pyrimidine, and pyridine monophosphate utilizes three distinct pathways. The individual pathways are presented in more detail in Part II (purine), Part III (pyrimidine), and Part VI (pyridine). In contrast, pathways ii and iii both involve common interconversions of purine and pyrimidine nucleotides, and are therefore, described in this chapter.
3.2 De Novo Biosynthesis of Ribonucleoside Monophosphates
In many organisms including plants, ribonucleotides are synthesized from precursors derived from amino acids, sugar phosphates, and carbon dioxide. De novo synthesis of purine, pyrimidines, and the ring of pyridine (nicotinamide adenine) nucleotides are synthesized via three distinct pathways. It is noteworthy that the initial products are nucleotides, namely inosine-5′-monophosphate (IMP), orotidine-5′-monophosphate (OMP) and nicotinate mononucleotide (NaMN), but not nucleosides or nucleobases. The de novo pathways of nucleotide biosynthesis are summarized in Figure 3.1. In the case of purine nucleotides, IMP is synthesized from 5-phosphoribosyl-1-pyrophosphate (PRPP) (Figure 3.1a). The carbon and nitrogen atoms of the purine ring come from multiple sources. Glycine contributes all its carbon and nitrogen atoms (positions 4, 5, and 7 of a purine ring), with additional nitrogen atoms from glutamine (positions 3 and 9) and aspartate (position 1), and further carbon atoms from formyl groups (positions 2 and 8), which are transferred from the coenzyme tetrahydrofolate as 10-formyltetrahydrofolate, and a carbon atom from bicarbonate (position 6). A detailed biochemical and molecular analysis of purine biosynthesis in plants has yet to be carried out, but the enzymatic steps are similar to those occurring in microorganisms and animals. More details of the 14 enzymatic steps involved in adenosine monophosphate (AMP) and GMP production from PRPP are presented in Part II.