Plant Nucleotide Metabolism. Hiroshi Ashihara
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a) This activity is usually not detected in plants.
Figure 3.5 Photosynthetic Calvin–Benson–Bassham cycle. (1) Ribulose bisphosphate carboxylase/oxygenase; (2) phosphoglycerate kinase; (3) glyceraldehyde-3-phosphate dehydrogenase; (4) aldolase; (5) fructose-bisphosphatase; (6) transketolase; (7) aldolase; (8) sedoheptulose diphosphatase; (9) transketolase; (10) erythrose-4-phosphate isomerase; (11) phosphoribulokinase.
Figure 3.6 Oxidative pentose phosphate pathway. (1) Glucose-6-phosphate dehydrogenase; (2) 6-phosphogluconolactonase; (3) 6-phosphogluconate dehydrogenase (decarboxylating); (4) ribose-5-phosphate isomerase; (5) ribulose-5-phosphate 3-epimerase; (6) transketolase; (7) transaldolase; (8) glucose-6-phosphate isomerase.
Plant PRPP synthetase has been characterized using native enzymes and it has been shown that the Pi-independent properties are different from bacterial and mammalian PRPP synthetase for which Pi is essential for activity. Recent recombinant enzyme studies revealed that both Pi-dependent (class I) and Pi-independent PRPP synthetase (class II) occur in plants (see Ashihara 2016).
3.8 Supply of Amino Acids for Nucleotide Biosynthesis
For the nucleotide biosynthesis, certain amino acids, namely, glutamine, glycine, and aspartate are utilized for nucleotide biosynthesis. Interconversion of nucleoside monophosphates, nucleoside diphosphates, and triphosphates are summarized in Figure 3.7.
Figure 3.7 Conversion of nucleoside mono-, di- and triphosphates. (1) ATP synthase; (2) nucleoside-monophosphate kinase; (3) nucleoside-diphosphate kinase; (4) adenylate kinase; (5) guanylate kinase; (6) UMP/CMP kinase; (7) various kinases; [8] CTP synthetase (see Part III).
3.9 Nitrogen Metabolism and Amino Acid Biosynthesis in Plants
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