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 limited. Unusually, only a few prokaryotes, including E. coli and Salmonella typhimurium, contain IGK as opposed to AK activity (Nygaard 1983).
IGK has been detected in plants (Deng and Ashihara 2010; Katahira and Ashihara 2006) (Table 5.1). Partial purification of IGK from mitochondria of Jerusalem artichoke was reported by Combés et al. (1989) (Table 5.3). The enzyme appears to be located in the intermembrane space of mitochondria. The Km values for guanosine (14 μM) are lower than for inosine (70 μM). There are no reports of the plant IGK being cloned, but molecular studies with bacteria and animals indicate that AK and IGK belong to the ribokinase family of proteins that share a number of unique primary and tertiary structural elements (Park and Gupta 2008).
5.4.3 Deoxyribonucleoside Kinases
Deoxyribonucleoside kinases salvage deoxyribonucleosides by transfer of a phosphate group to the 5′ position of a deoxyribonucleoside. This salvage pathway is well characterized in mammals, but little is known about deoxyribonucleoside salvage enzymes in plants.
Relatively high activity of deoxyadenosine kinase (dAK, EC 2.7.1.76) and deoxyguanosine kinase (dGK, EC 2.7.1.113) was detected in extracts of potato tubers (Katahira and Ashihara 2006) (see Table 5.1) and A. thaliana (Clausen et al. 2012). There are two types of genes that encode deoxyribonucleoside salvage enzymes in the A. thaliana genome. One is a single AtdNK gene which codes deoxynucleoside kinase (dNK, EC 2.7.1.145). The enzyme is able to use deoxyadenosine, deoxyguanosine, and deoxycytidine as substrates. Another is a thymidine kinase-like enzyme which participates in pyrimidine salvage (Clausen et al. 2012; Clausen et al. 2008) (see Part III). These findings suggest that the activity of dAK and dGK detected in plant extracts may be due to dNK which possesses broad substrate specificity.
5.5 Properties of Nucleoside Phosphotransferase
In contrast to nucleoside kinases (see Section 5.4, reaction 2 in Figure 5.2), NPTs use nucleoside monophosphates as phosphate donors (reaction 3 in Figure 5.2). The non-specific nucleoside phosphotransferase (NPT, nucleotide: nucleoside 5′-phosphotransferase, EC 2.7.1.77) catalyses the conversion of nucleosides to nucleoside monophosphate using 5′-nucleoside monophosphate. For example, guanosine + AMP → GMP + adenosine. In potato tubers, in vitro activity of NPT is substantial and at a similar or slightly lower level than the respective nucleoside kinases. Neither NPT nor IGK can convert xanthosine to xanthosine monophosphate (XMP) (see Table 5.1).
NPT is widely distributed in plants (Brawerman and Chargaff 1955). Highly purified, native NPT has been obtained from carrot roots (Brunngraber and Chargaff 1967, 1970), cotyledons of yellow lupin seedlings (Guranowski 1979a), and barley seedlings (Prasher et al. 1982) (Table 5.4). The lupin enzyme is similar to the enzyme purified from the bacterium, Erwinia herbicola (Chao 1976). With the yellow lupin NPT, purine and pyrimidine nucleosides are good phosphate acceptors and 5′-nucleotides are more effective phosphate donors than adenosine-3′-monophosphate (3′-AMP). The high Km values (>0.4 mM) indicate that NPT binds purine nucleosides but with much lower affinity than purine nucleoside kinases (Km: <10 μM) (Table 5.3). Therefore, if the cellular concentration of purine nucleosides is low, purine nucleoside kinases (AK and IGK) are preferentially involved in purine nucleoside salvage. In the A. thaliana database, no matches have been found for search string non-specific nucleotide triphosphate (NTP) (EC 2.7.1.77).
Table 5.4 Properties of the native nucleoside phosphotransferase (NPT) from plants.
| Km values (μM) | |||||||||||
| Enzyme | Enzyme source | Isozyme | Optimum pH | AR | IR | GR | AdR | AMP | Gene | TAIR Locus | References |
| NPT | Carrot roota) | 5.0 | b) | b) | b) | b) | b) | Brunngraber and Chargaff (1967) | |||
| Yellow lupin cotyledonsa) | 8.0 | 400 | 400 |
400
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