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Purine and Pyrimidine Nucleotide Synthesis and Metabolism

Purine and pyrimidine nucleotides are major energy carriers, subunits of nucleic acids and precursors for the synthesis of nucleotide cofactors such as NAD and SAM. Despite the obvious importance of these molecules, we still have much to learn about how these nucleotides are synthesized and metabolized by plants. Moreover, of the research that has been done in this area relatively little has used genetic analysis to evaluate the function(s) of specific enzymes. 
 
The pathways for the synthesis of nucleotides in plant cells are similar to those found in animals and microorganisms. This conclusion is based primarily on the results of studies using in vivo radiotracers, specific inhibitors of nucleotide synthesis and on analyses of the kinetic parameters of purified enzymes involved in nucleotide synthesis that are unlikely to have similar demands for purine and pyrimidine nucleotides have been used in this research. A more comprehensive understanding of the role(s) of specific nucleotide biosynthetic enzymes throughout plant development and factors that regulate their activity/expression is still lacking. Ultimately this information will explain how the requirements of different plants are met, such as those of ureide-producing legumes (Schubert and Boland, 1990) or those synthesizing caffeine (Suzuki et al., 1992; Ashihara and Crozier, 1999). 
 
There are two principal routes for the synthesis of nucleotides: the de novo and the salvage pathways (Figures 1 and ​and2,2, Figures 3 and ​and4,4, respectively). Using 5-phosphoribosyl-1-pyrophosphate (PRPP), the de novo pathway enzymes build purine and pyrimidine nucleotides from “scratch” using simple molecules such as CO2, amino acids and tetrahydrofolate. This route of nucleotide synthesis has a high requirement for energy as compared that of the salvage pathway. For example, five of the 12 steps of de novo purine synthesis require hydrolysis of ATP or GTP but only one salvage cycle reaction uses ATP. The enzymes of both of these biosynthetic routes are classified as “housekeeping” enzymes because they perform basic, cellular activities and are assumed to be present in low, constitutive levels in all cells. Whereas the de novo pathway is thought to reside in plastids, salvage cycle enzymes may be localized in more than one compartment. 
 
 
 
 
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Figure 1. 
 
 
De novo biosynthetic pathway of purine nucleotides in plants. Enzymes shown are: amido phosphoribosyltransferase, (2) GAR synthetase, (3) GAR formyl transferase, (4) FGAM synthetase, (5) AIR synthetase, (6) AIR carboxylase, (7) SAICAR synthetase, (8) adenylosuccinate lyase, (9) AICAR formyl transferase, (10) IMP cyclohydrolase, (11) SAMP synthetase, (12) adenylosuccinase, (13) IMP dehydrogenase, (14) GMP synthetase.