Читать книгу Plant Nucleotide Metabolism - Hiroshi Ashihara - Страница 48

As described in Section 3.1, amino acids are the precursors of ribonucleotide monophosphates involved in the de novo biosynthesis of purine, pyrimidine, and pyridine nucleotides. Plants absorb nitrogen from the environment in the form of nitrate (NO₃⁻) and ammonium (NH₄⁺). Nitrate assimilation is performed by two enzymes: nitrate reductase (EC 1.7.1.1) and nitrite reductase (EC 1.7.7.1). In many plants, nitrate reductase occurs in the cytosol and catalyses the reaction: Nitrate + NADH + H⁺ → Nitrite + NAD⁺ + H₂O (step 1 in Figure 3.8). In contrast, nitrite reductase occurs in the chloroplast and other plastids. This reduction requires six electrons donated by reduced ferredoxin. The reaction catalysed is: nitrite + 6 reduced ferredoxin + 7H⁺ → NH₃ + 2H₂O + 6 oxidized ferredoxin (step 2 in Figure 3.8).

Figure 3.8 Nitrate reduction and assimilation of ammonia in plants. Enzymes shown are: (1) nitrate reductase (NR); (2) nitrite reductase (NiR); (3) glutamine synthetase (GS); (4) glutamate synthase (GOGAT).

Ammonium is assimilated by glutamine synthetase (GS, EC 6.3.1.2) and glutamate synthase (L-glutamine: 2-oxoglutarate aminotransferase, GOGAT, EC 1.4.1.13) and glutamate is formed (Figure 3.8). These two enzymes catalyse the following reactions:

and

Plants have a high potential for nitrate assimilation in leaves and/or roots. In plants, unlike animals, all protein constituent amino acids are synthesized from the intermediates of the glycolysis, pentose phosphate pathway, and the TCA cycle (Figure 3.9).

Aspartate is used as a substrate in the biosynthesis of IMP, OMP, and NaMN nucleotides of the initial nucleotide products of purine, pyrimidine, and pyridine biosynthesis (see Figure 3.1). In addition, glutamine and glycine are used for purine ring synthesis. The genes and enzymes of nitrogen assimilation and amino acid biosynthesis have been well studied in plants (Coruzzi et al. 2015).