are interconverted (see Figure 3.5), coupled with the various reactions shown in Sections 3.3.1, 3.3.2 and 3.4.
3.3.1 Nucleoside-Monophosphate Kinase
Nucleoside monophosphates are converted to nucleoside diphosphates by nucleoside-monophosphate kinase (EC 2.7.4.4) which catalyses reaction 1 as shown in Table 3.1. The substrate specificity of this enzyme is broad; many nucleoside monophosphates can act as acceptors and other nucleoside triphosphates can also serve as phosphate donors instead of ATP. However, the cellular concentration of ATP is usually much higher than that of other nucleoside triphosphates (Table 2.1), so ATP is the principal phosphate donor.
Table 3.1 Enzymes involved in the conversion of nucleoside mono-, di- and triphosphate in plants.
| Enzymes | EC | Reaction | References | |
| 1 | Nucleoside-monophosphate kinase | 2.7.4.4 | ATP + Nucleoside monophosphate → ADP + Nucleoside diphosphate | Noda (1962) |
| 2 | Nucleoside-diphosphate kinase | 2.7.4.6 | ADP + Nucleoside triphosphate → ATP + Nucleoside diphosphate | Dorion and Rivoal (2015) |
| 3 | Adenylate kinase (= Myokinase) | 2.7.4.3 | AMP + ATP → 2 ADP | Johansson et al. (2008) |
| 4 | Guanylate kinase | 2.7.4.8 | GMP + ATP → GDP + ADP | Nomura et al. (2014) |
| 5 | UMP/CMP kinase (= Cytidylate kinase) | 2.7.4.14 | CMP + ATP → CDP + ADP UMP + ATP → UDP + ADP | Zhou et al. (1998) |
| 6 | ATP synthase (H+-transporting two-sector ATPase) | 3.6.3.14 | ADP + Pi + H+out → ATP + H2O + H+in | Li et al. (2012) |
| 7 | Phosphoglycerate kinase | 2.7.2.3 | ADP + 3-Phospho-D-glyceroyl phosphate → ATP + 3-Phospho-D-glycerate | Troncoso-Ponce et al. (2012) |
| 8 | Pyruvate kinase | 2.7.1.40 | ADP + Phosphoenolpyruvate → ATP + Pyruvate | Baysdorfer and Bassham (1984) |
| 9 | Succinate – CoA ligase (ADP-forming) | 6.2.1.5 | ADP + Pi + Succinyl-CoA → ATP + Succinate + CoA | Johnson et al. (1998) |
3.3.2 Specific Nucleoside-Monophosphate Kinases
In addition to the non-specific nucleoside-monophosphate kinase mentioned above, there are at least three nucleoside-monophosphate kinases which contribute to the conversion of NMP to NDP. They are adenylate (AMP) kinase (EC 2.7.4.3), guanylate (GMP) kinase (EC 2.7.4.8) and cytidylate (UMP/cytidine-5′-monophosphate [CMP]) kinase (EC 2.7.4.14). It has been reported that genes encoding adenylate kinase show high sequence homology with those encoding UMP-CMP kinase (Fukami-Kobayashi et al. 1996). The equilibrium constants of these enzymes are ∼1, so the reaction is reversible. Adenylate kinase is ubiquitous and is found in different subcellular locations, including the cytosol, mitochondria, and plastids of plants. The reactions of each enzyme, and associated references, are presented in Table 3.1.
3.4 Conversion of Nucleoside Diphosphates to Nucleoside Triphosphates
Phosphorylation of nucleoside diphosphates, especially ADP to ATP, is performed by two different types of reaction. One, ADP + Pi → ATP, is the electron transfer system of respiration and photosynthesis. The other is reactions involving substrate level phosphorylation, which is the formation of ATP or GTP by the direct transfer of a phosphoryl group to ADP or GDP from another phosphorylated metabolite, such as phosphoenolpyruvate.
3.4.1 ATP Synthesis by Electron Transfer Systems
The conversion of ADP to ATP is performed by phosphorylation linked to the electron transport chains of respiration and photosynthesis (Karp 2013; Niyogi et al. 2015). Briefly, in oxidative phosphorylation, electrons are derived from substrates that enter the TCA (tricarboxylic acid) cycle (aka the citrate or Krebs cycle) (Figure 3.2), whereas in photophosphorylation, they are furnished by chlorophyll in the presence of light. ATP synthesis is coupled to the subsequent successive oxidation–reduction of members of electron transport chains, which pass electron pairs through stages of successively lower potential energy until they reach oxygen, the terminal electron sink. ATP synthase (H+-transporting two-sector ATPase, EC 3.6.3.14) is an enzyme that creates ATP. It is formed from ADP and Pi (Table 3.1).
Figure 3.2 The tricarboxylic acid (Krebs) cycle in plants. (1) Pyruvate dehydrogenase; (2) citrate synthase; (3) aconitase; (4) isocitrate dehydrogenase; (5) 2-oxoglutarate dehydrogenase; (6) succinyl-CoA synthetase; (7) succinate dehydrogenase; (8) fumarase; (9) malate dehydrogenase.
3.4.2 Substrate-Level ATP Synthesis
Substrate-level phosphorylation of ADP is performed by the direct transfer of a phosphoryl group to ADP from another phosphorylated compound (Table 3.1). In the TCA cycle in plants, substrate-level phosphorylation occurs mainly in the cytosol or chloroplasts during glycolysis and in mitochondria.
Substrate-level phosphorylation in glycolysis occurs via two steps, 7 and 10, in Figure 3.3.
