Читать книгу Plant Nucleotide Metabolism - Hiroshi Ashihara - Страница 77
5.5 Properties of Nucleoside Phosphotransferase
ОглавлениеIn 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 | — | Guranowski (1979a) | |||||
| Barley seedlingsa) | 540 | 2130 | 87 | b) | — | Prasher et al. (1982) |
AR, Adenosine; IR, inosine; GR, guanosine; AdR, deoxyadenosine; AMP, adenosine-5′-monophosphate.
a) Native enzyme.
b) Activity was found but the Km value is not available.
