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 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).
3.10 Summary
De novo biosynthesis of ribonucleoside monophosphate, interconversion of nucleoside phosphates, conversion of nucleoside diphosphates to triphosphates, biosynthesis of deoxyribonucleotides, nucleic acid (DNA and RNA) biosynthesis, and supply of PRPP and amino acids for nucleotide biosynthesis are reviewed.
Figure 3.9 Outline of amino acids biosynthesis in plants. DAHP, 3-deoxy-D-arabino-heptulosonate-7-phosphate; E4P, erythrose-4-phosphate; F6P, fructose-6-phosphate; F1,6BP, fructose-1,6-bisphosphate; 6PG, 6-phosphogluconate; 3PGA, 3-phosphoglycerate; PEP, phosphoenolpyruvate.
References
1 Ashihara, H. (2016). Biosynthesis of 5-phosphoribosyl-1-pyrophosphate in plants: a review. Eur. Chem. Bull. 5: 314–323.
2 Baysdorfer, C. and Bassham, J.A. (1984). Spinach pyruvate kinase isoforms: partial purification and regulatory properties. Plant Physiol. 74: 374–379.
3 Becker, M.A. (2001). Phosphoribosylpyrophosphate synthetase and the regulation of phosphoribosylpyrophosphate production in human cells. In: Progress in Nucleic Acid Research and Molecular Biology., vol. 69 (ed. K. Moldave), 115–148. Academic Press.
4 Bowsher, C., Steer, M., and Tobin, A. (2012). Plant Biochemistry. New York: Garland Science.
5 Buchanan, B.B., Gruissen, W., and Jones, R.L. (2015). Biochemistry and Molecular Biology of Plants. Rockville: American Society of Plant Biologists.
6 Burgers, P.M.J., Koonin, E.V., Bruford, E. et al. (2001). Eukaryotic DNA polymerases: proposal for a revised nomenclature. J. Biol. Chem. 276: 43487–43490.
7 Coruzzi, G., Last, R., Dudareva, N., and Amrhein, N. (2015). Amino acids. In: Biochemistry and Molecular Biology of Plants, 2e (eds. B.B. Buchanan, W. Gruissem and R.L. Jones), 289–336. Wiley.
8 Dorion, S. and Rivoal, J. (2015). Clues to the functions of plant NDPK isoforms. Naunyn Schmiedeberg's Arch. Pharmacol. 388: 119–132.
9 Elledge, S.J., Zhou, Z., and Allen, J.B. (1992). Ribonucleotide reductase: regulation, regulation, regulation. Trends Biochem. Sci. 17: 119–123.
10 Fukami-Kobayashi, K., Nosaka, M., Nakazawa, A., and Gō, M. (1996). Ancient divergence of long and short isoforms of adenylate kinase molecular evolution of the nucleoside monophosphate kinase family. FEBS Lett. 385: 214–220.
11 Grummt, I. (1998). Regulation of mammalian ribosomal gene transcription by RNA polymerase I. In: Progress in Nucleic Acid Research and Molecular Biology, vol. 62 (ed. K. Moldave), 109–154. Academic Press.
12 Herr, A.J., Jensen, M.B., Dalmay, T., and Baulcombe, D.C. (2005). RNA polymerase IV directs silencing of endogenous DNA. Science 308: 118–120.
13 Johansson, M., Hammargren, J., Uppsäll, E. et al. (2008). The activities of nucleoside diphosphate kinase and adenylate kinase are influenced by their interaction. Plant Sci. 174: 192–199.
14 Johnson, J.D., Mehus, J.G., Tews, K. et al. (1998). Genetic evidence for the expression of ATP- and GTP-specific succinyl-CoA synthetases in multicellular eucaryotes. J. Biol. Chem. 273: 27580–27586.
15 Karp, G. (2013). Cell and Molecular Biology: Concepts
