Plant Nucleotide Metabolism. Hiroshi Ashihara

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adk1 AT3G09820 Moffatt et al. (2000) ADK2 9.5 0.3 370 4.8 adk2 AT5G03300 Tobacco BY-2 cells^b) ADK1T 7.5 4.0 2.4 0.3 3.9 adk1t Kwade et al. (2005) ADK1S 7.0 1.9 2.5 6.1 3.9 adk1s ADK2T 8.5 7.3 3.2 8.7 2.6 adk2t ADK2S 8.8 3.9 2.2 1.7 3.8 adk2s Physcomitrella (moss)^{b) c)} von Schwartzenberg et al. (1998) IGK IR GR XR ATP Jerusalem artichoke^a) 7.6 70 14 1450 Combés et al. (1989) dNK AdR GdR A. thaliana ^b) 14 27 dnk AT1G72040 Clausen et al. (2012)

      AR, Adenosine; ATP, adenosine triphosphate. Cytokinin ribosides: DHZR, dihydrozeatin riboside; iPR, isopentenyladenosine; tZR, trans-zeatin riboside. IR, Inosine; GR, guanosine; XR, xanthosine; AdR, deoxyadenosine; GdR, deoxyguanosine.

      a) Native enzyme.

      b) Recombinant enzyme.

      c) Activity was detected but no data was published on the Km value.

Almost homogeneous AK was isolated from seeds of yellow lupin (Lupinus luteus) by Guranowski (1979b). The Km value for adenosine was low (1.5 μM), but was higher (300 μM) for ATP. This trend is also found with AK from other plant sources (Table 5.3). ATP is the main phosphate donor. dATP and guanosine triphosphate (GTP) can substitute for ATP, but less effectively. AK has been investigated in the context of phosphorylation of cytokinins (see Part V).

      Cloning and characterization of AK was first performed with the moss Physcomitrella patens (von Schwartzenberg et al. 1998). A gene adk was cloned from a cDNA library by functional complementation of an E. coli purine auxotrophic strain. The deduced amino acid sequence had a 52% homology with the human adk. Subsequently, cDNAs and genes encoding two isoforms of AK were isolated from A. thaliana (Moffatt et al. 2000). The adk1- and adk2-coding sequences were very similar sharing, respectively, 92% and 89% amino acid and nucleotide identity. Each cDNA was overexpressed in E. coli, and the catalytic activity of the two isoforms determined. Both AK isozymes have similar catalytic properties (Table 5.3). Four AK isoforms, designated 1S, 2S, 1T, and 2T have been identified in BY2 tobacco cells (Kwade et al. 2005). In contrast to AK from other plant sources, all four tobacco AK isoforms displayed a high affinity for adenosine and three cytokinin ribosides (see Part V).

      5.4.2 Inosine/Guanosine Kinase

      Inosine/guanosine kinase (IGK, ATP: inosine/guanosine 5′-phosphotransferase, EC 2.7.1.73) catalyses the phosphorylation of inosine to IMP and that of guanosine to GMP using ATP as a phosphate donor. This enzyme activity was first found in cell-free extracts of Ehrlich ascites tumour cells (Pierre and LePage 1968). In contrast to AK, the occurrence of IGK is

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